This chapter describes how to create and work with  and  objects, which are used to encode graphics rendering commands into a command buffer. MTLRenderCommandEncoder commands describe a graphics rendering pipeline, as seen in Figure 5-1.

A MTLRenderCommandEncoder object represents a single rendering command encoder. A MTLParallelRenderCommandEncoder object enables a single rendering pass to be broken into a number of separate MTLRenderCommandEncoder objects, each of which may be assigned to a different thread. The commands from the different render command encoders are then chained together and executed in a consistent, predictable order, as described in .

Creating and Using a Render Command Encoder

To create, initialize, and use a single render command encoder:

1. Create a  object to define a collection of attachments that serve as the rendering destination for the graphics commands in the command buffer for that rendering pass. Typically, you create a MTLRenderPassDescriptorobject once and reuse it each time your app renders a frame. See .

2. Create a MTLRenderCommandEncoder object by calling the  method of  with the specified render pass descriptor. See .

3. Create a  object to define the state of the graphics rendering pipeline (including shaders, blending, multisampling, and visibility testing) for one or more draw calls. To use this render pipeline state for drawing primitives, call the  method of MTLRenderCommandEncoder. For details, see .

4. Set textures, buffers, and samplers to be used by the render command encoder, as described in .

5. Call MTLRenderCommandEncoder methods to specify additional fixed-function state, including the depth and stencil state, as explained in .

6. Finally, call MTLRenderCommandEncoder methods to draw graphics primitives, as described in .

Creating a Render Pass Descriptor

A  object represents the destination for the encoded rendering commands, which is a collection of attachments. The properties of a render pass descriptor may include an array of up to four attachments for color pixel data, one attachment for depth pixel data, and one attachment for stencil pixel data. The  convenience method creates a MTLRenderPassDescriptor object with color, depth, and stencil attachment properties with default attachment state. The  property specifies a buffer where the device can update to indicate whether any samples pass the depth and stencil tests—for details, see .

Each individual attachment, including the texture that will be written to, is represented by an attachment descriptor. For an attachment descriptor, the pixel format of the associated texture must be chosen appropriately to store color, depth, or stencil data. For a color attachment descriptor, , use a color-renderable pixel format. For a depth attachment descriptor, , use a depth-renderable pixel format, such as. For a stencil attachment descriptor, , use a stencil-renderable pixel format, such as .

The amount of memory the texture actually uses per pixel on the device does not always match the size of the texture’s pixel format in the Metal framework code, because the device adds padding for alignment or other purposes. See the  chapter for how much memory is actually used for each pixel format, as well limitations on the size and number of attachments.

Load and Store Actions

The  and  properties of an attachment descriptor specify an action that is performed at either the start or end of a rendering pass. (For MTLParallelRenderCommandEncoder, the load and store actions occur at the boundaries of the overall command, not for each of its MTLRenderCommandEncoder objects. For details, see .)

Possible  values include:

• , which writes the same value to every pixel in the specified attachment descriptor. For more detail about this action, see .

• , which preserves the existing contents of the texture.

• , which allows each pixel in the attachment to take on any value at the start of the rendering pass.

If your application will render all pixels of the attachment for a given frame, use the default load action . The MTLLoadActionDontCare action allows the GPU to avoid loading the existing contents of the texture, ensuring the best performance. Otherwise, you can use the  action to clear the previous contents of the attachment, or the  action to preserve them. The MTLLoadActionClear action also avoids loading the existing texture contents, but it incurs the cost of filling the destination with a solid color.

Possible  values include:

• , which saves the final results of the rendering pass into the attachment.

• , which resolves the multisample data from the render target into single sample values, stores them in the texture specified by the attachment property resolveTexture, and leaves the contents of the attachment undefined. For details, see .

• , which leaves the attachment in an undefined state after the rendering pass is complete. This may improve performance as it enables the implementation to avoid any work necessary to preserve the rendering results.

For color attachments, the  action is the default store action, because applications almost always preserve the final color values in the attachment at the end of rendering pass. For depth and stencil attachments, is the default store action, because those attachments typically do not need to be preserved after the rendering pass is complete.

Specifying the Clear Load Action

If the  property of an attachment descriptor is set to , then a clearing value is written to every pixel in the specified attachment descriptor at the start of a rendering pass. The clearing value property depends upon the type of attachment.

• For ,  contains a  value that consists of four double-precision floating-point RGBA components and is used to clear the color attachment. The  function creates a clear color value from red, green, blue, and alpha components. The default clear color is (0.0, 0.0, 0.0, 1.0), or opaque black.

• For ,  contains one double-precision floating-point clearing value in the range [0.0, 1.0] that is used to clear the depth attachment. The default value is 1.0.

• For ,  contains one 32-bit unsigned integer that is used to clear the stencil attachment. The default value is 0.

Example: Creating a Render Pass Descriptor with Load and Store Actions

Listing 5-1 creates a simple render pass descriptor with color and depth attachments. First, two texture objects are created, one with a color-renderable pixel format and the other with a depth pixel format. Next the  convenience method of  creates a default render pass descriptor. Then the color and depth attachments are accessed through the properties of MTLRenderPassDescriptor. The textures and actions are set in colorAttachments[0], which represents the first color attachment (at index 0 in the array), and the depth attachment.

Listing 5-1  Creating a Render Pass Descriptor with Color and Depth Attachments

MTLTextureDescriptor *colorTexDesc = [MTLTextureDescriptor

texture2DDescriptorWithPixelFormat:MTLPixelFormatRGBA8Unorm

width:IMAGE_WIDTH height:IMAGE_HEIGHT mipmapped:NO];

id 
         
           colorTex = [device newTextureWithDescriptor:colorTexDesc];
         

MTLTextureDescriptor *depthTexDesc = [MTLTextureDescriptor

texture2DDescriptorWithPixelFormat:MTLPixelFormatDepth32Float

width:IMAGE_WIDTH height:IMAGE_HEIGHT mipmapped:NO];

id 
         
           depthTex = [device newTextureWithDescriptor:depthTexDesc];
         

MTLRenderPassDescriptor *renderPassDesc = [MTLRenderPassDescriptor renderPassDescriptor];

renderPassDesc.colorAttachments[0].texture = colorTex;

renderPassDesc.colorAttachments[0].loadAction = MTLLoadActionClear;

renderPassDesc.colorAttachments[0].storeAction = MTLStoreActionStore;

renderPassDesc.colorAttachments[0].clearColor = MTLClearColorMake(0.0,1.0,0.0,1.0);

renderPassDesc.depthAttachment.texture = depthTex;

renderPassDesc.depthAttachment.loadAction = MTLLoadActionClear;

renderPassDesc.depthAttachment.storeAction = MTLStoreActionStore;

renderPassDesc.depthAttachment.clearDepth = 1.0;

Example: Creating a Render Pass Descriptor for Multisampled Rendering

To use the  action, you must set the  property to a multisample-type texture, and the  property will contain the result of the multisample resolve operation. (If texture does not support multisampling, then the result of a multisample resolve action is undefined.) The , , and  properties may also be used for the multisample resolve operation to specify the mipmap level, cube slice, and depth plane of the multisample texture, respectively. In most cases, the default values for resolveLevel, resolveSlice, and resolveDepthPlane are usable. In Listing 5-2, an attachment is initially created and then its loadAction, storeAction,texture, and resolveTexture properties are set to support multisample resolve.

Listing 5-2  Setting Properties for an Attachment with Multisample Resolve

MTLTextureDescriptor *colorTexDesc = [MTLTextureDescriptor

texture2DDescriptorWithPixelFormat:MTLPixelFormatRGBA8Unorm

width:IMAGE_WIDTH height:IMAGE_HEIGHT mipmapped:NO];

id 
         
           colorTex = [device newTextureWithDescriptor:colorTexDesc];
         

MTLTextureDescriptor *msaaTexDesc = [MTLTextureDescriptor

texture2DDescriptorWithPixelFormat:MTLPixelFormatRGBA8Unorm

width:IMAGE_WIDTH height:IMAGE_HEIGHT mipmapped:NO];

msaaTexDesc.textureType = MTLTextureType2DMultisample;

msaaTexDesc.sampleCount = sampleCount;  //  must be > 1

id 
         
           msaaTex = [device newTextureWithDescriptor:msaaTexDesc];
         

MTLRenderPassDescriptor *renderPassDesc = [MTLRenderPassDescriptor renderPassDescriptor];

renderPassDesc.colorAttachments[0].texture = msaaTex;

renderPassDesc.colorAttachments[0].resolveTexture = colorTex;

renderPassDesc.colorAttachments[0].loadAction = MTLLoadActionClear;

renderPassDesc.colorAttachments[0].storeAction = MTLStoreActionMultisampleResolve;

renderPassDesc.colorAttachments[0].clearColor = MTLClearColorMake(0.0,1.0,0.0,1.0);

Using the Render Pass Descriptor to Create a Render Command Encoder

After you create a render pass descriptor and specify its properties, use the  method of a  object to create a render command encoder, as shown in Listing 5-3.

Listing 5-3  Creating a Render Command Encoder with the Render Pass Descriptor

id 
         
           renderCE = [commandBuffer
         

renderCommandEncoderWithDescriptor:renderPassDesc];

Displaying Rendered Content with Core Animation

Core Animation defines the  class, which is designed for the specialized behavior of a layer-backed view whose content is rendered using Metal. A CAMetalLayer object represents information about the geometry of the content (position and size), its visual attributes (background color, border, and shadow), and the resources used by Metal to present the content in a color attachment. It also encapsulates the timing of content presentation so that the content can be displayed as soon as it is available or at a specified time. For more information about Core Animation, see the .

Core Animation also defines the  protocol for objects that are displayable resources. The CAMetalDrawable protocol extends MTLDrawable and provides an object that conforms to the  protocol, so it can be used as a destination for rendering commands. To render into a CAMetalLayer object, you should get a new CAMetalDrawable object for each rendering pass, get the MTLTexture object that it provides, and use that texture to create the color attachment. Unlike color attachments, creation and destruction of a depth or stencil attachment are costly. If you need either depth or stencil attachments, create them once and then reuse them each time a frame is rendered.

Typically, you use the  method to designate CAMetalLayer as the backing layer type for your own custom UIView subclass, as shown in Listing 5-4. Otherwise, you can create a CAMetalLayer with its init method and include the layer in an existing view.

Listing 5-4  Using CAMetalLayer as the backing layer for a UIView subclass

+ (id) layerClass {

return [CAMetalLayer class];

}

To display content rendered by Metal in the layer, you must obtain a displayable resource (a  object) from the CAMetalLayer object and then render to the texture in this resource by attaching it to a  object. To do this, you first set properties of the CAMetalLayer object that describe the drawable resources it provides, then call its  method each time you begin rendering a new frame. If the CAMetalLayer properties are not set, thenextDrawable method call fails. The following CAMetalLayer properties describe the drawable object:

• The  property declares the  object that the resource is created from.

• The  property declares the pixel format of the texture. The supported values are  (the default) and .

• The  property declares the dimensions of the texture in device pixels. To ensure that your app renders content at the precise dimensions of the display (without requiring an additional sampling stage on some devices), take the target screen’s  or  property into account when calculating the desired size for your layer.

• The  property declares whether the texture can be used only as an attachment () or whether it can also be used for texture sampling and pixel read/write operations (). If , the layer object can optimize the texture for display. For most apps, the recommended value is .

• The  property declares whether changes to the layer's rendered resource are updated with standard Core Animation transaction mechanisms () or are updated asynchronously to normal layer updates (, the default value).

If the nextDrawable method succeeds, it returns a CAMetalDrawable object with the following read-only properties:

• The  property holds the texture object. You use this as an attachment when creating your rendering pipeline ( object).

• The  property points to the CAMetalLayer object that responsible for displaying the drawable.

To display the contents of a drawable object after rendering is complete, you must submit it to Core Animation by calling the drawable object’s  method. To synchronize presentation of a drawable with completion of the command buffer responsible for its rendering, you can call either the  or  convenience method on a  object. These methods use the scheduled handler (see ) to call the drawable’s present method, which covers most scenarios. The presentDrawable:atTime: method provides further control over when the drawable is presented.

Creating a Render Pipeline State

To use a  object to encode rendering commands, you must first specify a  object to define the graphics state for any draw calls. A render pipeline state object is a long-lived persistent object that can be created outside of a render command encoder, cached in advance, and reused across several render command encoders. When describing the same set of graphics state, reusing a previously created render pipeline state object may avoid expensive operations that re-evaluate and translate the specified state to GPU commands.

A render pipeline state is an immutable object. To create a render pipeline state, you first create and configure a mutable  object that describes the attributes of a render pipeline state. Then, you use the descriptor to create a  object.

Creating and Configuring a Render Pipeline Descriptor

To create a render pipeline state, first create a  object, which has properties that describe the graphics rendering pipeline state you want to use during the rendering pass, as depicted in Figure 5-2. The property of the new MTLRenderPipelineDescriptor object contains an array of  objects, and each descriptor represents a color attachment state that specifies the blend operations and factors for that attachment, as detailed in . The attachment descriptor also specifies the pixel format of the attachment, which must match the pixel format for the texture of the render pipeline descriptor with the corresponding attachment index, or an error occurs.

Set these properties for the  object:

• Set the  property to match the pixel format for the texture of  in .

• Set the  property to match the pixel format for the texture of  in .

• To specify the vertex or fragment shader in the render pipeline state, set the  or  property, respectively. Setting fragmentFunction to nil disables the rasterization of pixels into the specified color attachment, which is typically used for depth-only rendering or for outputting data into a buffer object from the vertex shader.

• If the vertex shader has an argument with per-vertex input attributes, set the  property to describe the organization of the vertex data in that argument, as described in .

• The default value of  for the  property is sufficient for most typical rendering tasks. To use only the vertex stage of the graphics pipeline (for example, to gather data transformed in a vertex shader), set this property to .

• If the attachment supports multisampling (that is, the attachment is a  type texture), then multiple samples can be created per pixel. To determine how fragments combine to provide pixel coverage, use the followingMTLRenderPipelineDescriptor properties.

  • The  property determines the number of samples for each pixel. When  is created, the  for the textures for all attachments must match this sampleCount property. If the attachment cannot support multisampling, then sampleCount is 1, which is also the default value.

  • If  is set to , then the alpha channel fragment output for colorAttachments[0] is read and used to determine a coverage mask.

  • If  is set to , then alpha channel fragment values for colorAttachments[0] are forced to 1.0, which is the largest representable value. (Other attachments are unaffected.)

Creating a Render Pipeline State from a Descriptor

After creating a render pipeline descriptor and specifying its properties, use it to create the  object. Because creating a render pipeline state can require an expensive evaluation of graphics state and a possible compilation of the specified graphics shaders, you can use either a blocking or an asynchronous method to schedule such work in a way that best fits the design of your app.

• To synchronously create the render pipeline state object, call either the  or  method of a  object. These methods block the current thread while Metal evaluates the descriptor’s graphics state information and compiles shader code to create the pipeline state object.

• To asynchronously create the render pipeline state object, call either the  or  method of a object. These methods return immediately—Metal asynchronously evaluates the descriptor’s graphics state information and compiles shader code to create the pipeline state object, then calls your completion handler to provide the new object.

When you create a  object you can also choose to create reflection data that reveals details of the pipeline’s shader function and its arguments. The and  methods provide this data. Avoid obtaining reflection data if it will not be used. For more information on how to analyze reflection data, see .

After you create a  object, call the  method of MTLRenderCommandEncoder to associate the render pipeline state with the command encoder for use in rendering.

Listing 5-5 demonstrates the creation of a render pipeline state object called pipeline.

Listing 5-5  Creating a Simple Pipeline State

MTLRenderPipelineDescriptor *renderPipelineDesc =

[[MTLRenderPipelineDescriptor alloc] init];

renderPipelineDesc.vertexFunction = vertFunc;

renderPipelineDesc.fragmentFunction = fragFunc;

renderPipelineDesc.colorAttachments[0].pixelFormat = MTLPixelFormatRGBA8Unorm;

// Create MTLRenderPipelineState from MTLRenderPipelineDescriptor

NSError *errors = nil;

id 
         
           pipeline = [device
         

newRenderPipelineStateWithDescriptor:renderPipelineDesc error:&errors];

assert(pipeline && !errors);

// Set the pipeline state for MTLRenderCommandEncoder

[renderCE setRenderPipelineState:pipeline];

The variables vertFunc and fragFunc are shader functions that are specified as properties of the render pipeline state descriptor called renderPipelineDesc. Calling the  method of the object synchronously uses the pipeline state descriptor to create the render pipeline state object. Calling the  method of MTLRenderCommandEncoder specifies the MTLRenderPipelineState object to use with the render command encoder.

Configuring Blending in a Render Pipeline Attachment Descriptor

Blending uses a highly configurable blend operation to mix the output returned by the fragment function (source) with pixel values in the attachment (destination). Blend operations determine how the source and destination values are combined with blend factors.

To configure blending for a color attachment, set the following  properties:

• To enable blending, set  to . Blending is disabled, by default.

•  identifies which color channels are blended. The default value MTLColorWriteMaskAll allows all color channels to be blended.

•  and  separately assign the blend operations for the RGB and Alpha fragment data with a MTLBlendOperation value. The default value for both properties is .

• , , , and  assign the source and destination blend factors.

Understanding Blending Factors and Operations

Four blend factors refer to a constant blend color value: MTLBlendFactorBlendColor, MTLBlendFactorOneMinusBlendColor, MTLBlendFactorBlendAlpha, and MTLBlendFactorOneMinusBlendAlpha. Call the method of MTLRenderCommandEncoder to specify the constant color and alpha values used with these blend factors, as described in .

Some blend operations combine the fragment values by multiplying the source values by a source  value (abbreviated SBF), multiplying the destination values by a destination blend factor (DBF), and combining the results using the arithmetic indicated by the  value. (If the blend operation is either MTLBlendOperationMin or MTLBlendOperationMax, the SBF and DBF blend factors are ignored.) For example, MTLBlendOperationAdd for both rgbBlendOperationand alphaBlendOperation properties defines the following additive blend operation for RGB and Alpha values:

• RGB = (Source.rgb * sourceRGBBlendFactor) + (Dest.rgb * destinationRGBBlendFactor)

• Alpha = (Source.a * sourceAlphaBlendFactor) + (Dest.a * destinationAlphaBlendFactor)

In the default blend behavior, the source completely overwrites the destination. This behavior is equivalent to setting both the sourceRGBBlendFactor and sourceAlphaBlendFactor to MTLBlendFactorOne, and the destinationRGBBlendFactor anddestinationAlphaBlendFactor to MTLBlendFactorZero. This behavior is expressed mathematically as:

• RGB = (Source.rgb * 1.0) + (Dest.rgb * 0.0)

• A = (Source.a * 1.0) + (Dest.a * 0.0)

Another commonly used blend operation, where the source alpha defines how much of the destination color remains, can be expressed mathematically as:

• RGB = (Source.rgb * 1.0) + (Dest.rgb * (1 - Source.a))

• A = (Source.a * 1.0) + (Dest.a * (1 - Source.a))

Using a Custom Blending Configuration

Listing 5-6 shows code for a custom blending configuration, using the blend operation MTLBlendOperationAdd, the source blend factor MTLBlendFactorOne, and the destination blend factor MTLBlendFactorOneMinusSourceAlpha.colorAttachments[0] is a  object with properties that specify the blending configuration.

Listing 5-6  Specifying a Custom Blending Configuration

MTLRenderPipelineDescriptor *renderPipelineDesc =                              [[MTLRenderPipelineDescriptor alloc] init];renderPipelineDesc.colorAttachments[0].blendingEnabled = YES; renderPipelineDesc.colorAttachments[0].rgbBlendOperation = MTLBlendOperationAdd;renderPipelineDesc.colorAttachments[0].alphaBlendOperation = MTLBlendOperationAdd;renderPipelineDesc.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactorOne;renderPipelineDesc.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactorOne;renderPipelineDesc.colorAttachments[0].destinationRGBBlendFactor =        MTLBlendFactorOneMinusSourceAlpha;renderPipelineDesc.colorAttachments[0].destinationAlphaBlendFactor =        MTLBlendFactorOneMinusSourceAlpha;NSError *errors = nil;id 
         
           pipeline = [device          newRenderPipelineStateWithDescriptor:renderPipelineDesc error:&errors];
         

Specifying Resources for a Render Command Encoder

The  methods discussed in this section specify resources that are used as arguments for the vertex and fragment shader functions, which are specified by the vertexFunction and fragmentFunction properties in a object. These methods assign a shader resource (buffers, textures, and samplers) to the corresponding argument table index (atIndex) in the render command encoder, as shown in Figure 5-3.

The following setVertex* methods assign one or more resources to corresponding arguments of a vertex shader function.

These setFragment* methods similarly assign one or more resources to corresponding arguments of a fragment shader function.

There are a maximum of 31 entries in the buffer argument table, 31 entries in the texture argument table, and 16 entries in the sampler state argument table.

The attribute qualifiers that specify resource locations in the Metal shading language source code must match the argument table indices in the Metal framework methods. In Listing 5-7, two buffers (posBuf and texCoordBuf) with indices 0 and 1, respectively, are defined for the vertex shader.

Listing 5-7  Metal Framework: Specifying Resources for a Vertex Function

[renderEnc setVertexBuffer:posBuf offset:0 atIndex:0];

[renderEnc setVertexBuffer:texCoordBuf offset:0 atIndex:1];

In Listing 5-8, the function signature has corresponding arguments with the attribute qualifiers buffer(0) and buffer(1).

Listing 5-8  Metal Shading Language: Vertex Function Arguments Match the Framework Argument Table Indices

vertex VertexOutput metal_vert(float4 *posData [[ buffer(0) ]],

float2 *texCoordData [[ buffer(1) ]])

Similarly, in Listing 5-9, a buffer, a texture, and a sampler (fragmentColorBuf, shadeTex, and sampler, respectively), all with index 0, are defined for the fragment shader.

Listing 5-9  Metal Framework: Specifying Resources for a Fragment Function

[renderEnc setFragmentBuffer:fragmentColorBuf offset:0 atIndex:0];

[renderEnc setFragmentTexture:shadeTex atIndex:0];

[renderEnc setFragmentSamplerState:sampler atIndex:0];

In Listing 5-10, the function signature has corresponding arguments with the attribute qualifiers buffer(0), texture(0), and sampler(0), respectively.

Listing 5-10  Metal Shading Language: Fragment Function Arguments Match the Framework Argument Table Indices

fragment float4 metal_frag(VertexOutput in [[stage_in]],                           float4 *fragColorData [[ buffer(0) ]],                           texture2d
         
           shadeTexValues [[ texture(0) ]],                           sampler samplerValues [[ sampler(0) ]] )
         

Vertex Descriptor for Data Organization

In Metal framework code, there can be one  for every pipeline state that describes the organization of data input to the vertex shader function and shares resource location information between the shading language and framework code.

In Metal shading language code, per-vertex inputs (such as scalars or vectors of integer or floating-point values) can be organized in one struct, which can be passed in one argument that is declared with the [[ stage_in ]] attribute qualifier, as seen in the VertexInput struct for the example vertex function vertexMath in Listing 5-11. Each field of the per-vertex input struct has the [[ attribute(index) ]] qualifier, which specifies the index in the vertex attribute argument table.

Listing 5-11  Metal Shading Language: Vertex Function Inputs with Attribute Indices

struct VertexInput {    float2    position [[ attribute(0) ]];    float4    color    [[ attribute(1) ]];    float2    uv1      [[ attribute(2) ]];    float2    uv2      [[ attribute(3) ]];};struct VertexOutput {    float4 pos [[ position ]];    float4 color;};vertex VertexOutput vertexMath(VertexInput in [[ stage_in ]]){  VertexOutput out;  out.pos = float4(in.position.x, in.position.y, 0.0, 1.0);  float sum1 = in.uv1.x + in.uv2.x;  float sum2 = in.uv1.y + in.uv2.y;  out.color = in.color + float4(sum1, sum2, 0.0f, 0.0f);  return out;}

To refer to the shader function input using the [[ stage_in ]] qualifier, describe a  object and then set it as the  property of . MTLVertexDescriptor has two properties: and .

The  property of MTLVertexDescriptor is a  object that defines how each vertex attribute is organized in a buffer that is mapped to a vertex function argument. The attributes property can support access to multiple attributes (such as vertex coordinates, surface normals, and texture coordinates) that are interleaved within the same buffer. The order of the members in the shading language code does not have to be preserved in the buffer in the framework code. Each vertex attribute descriptor in the array has the following properties that provide a vertex shader function information to locate and load the argument data:

• , which is an index to the buffer argument table that specifies which MTLBuffer is accessed. The buffer argument table is discussed in .

• , which specifies how the data should be interpreted in the framework code. If the data type is not an exact type match, it may be converted or expanded. For example, if the shading language type is half4 and the framework format is, then when the data is used as an argument to the vertex function, it may be converted from float to half and expanded from two to four elements (with 0.0, 1.0 in the last two elements).

• , which specifies where the data can be found from the start of a vertex.

Figure 5-4 illustrates a  in Metal framework code that implements an interleaved buffer that corresponds to the input to the vertex function vertexMath in the shading language code in .

Listing 5-12 shows the Metal framework code that corresponds to the interleaved buffer shown in Figure 5-4.

Listing 5-12  Metal Framework: Using a Vertex Descriptor to Access Interleaved Data

id 
         
           vertexFunc = [library newFunctionWithName:@"vertexMath"];            MTLRenderPipelineDescriptor* pipelineDesc =                                   [[MTLRenderPipelineDescriptor alloc] init];MTLVertexDescriptor* vertexDesc = [[MTLVertexDescriptor alloc] init];vertexDesc.attributes[0].format = MTLVertexFormatFloat2;vertexDesc.attributes[0].bufferIndex = 0;vertexDesc.attributes[0].offset = 0;vertexDesc.attributes[1].format = MTLVertexFormatFloat4;vertexDesc.attributes[1].bufferIndex = 0;vertexDesc.attributes[1].offset = 2 * sizeof(float);  // 8 bytesvertexDesc.attributes[2].format = MTLVertexFormatFloat2;vertexDesc.attributes[2].bufferIndex = 0;vertexDesc.attributes[2].offset = 8 * sizeof(float);  // 32 bytesvertexDesc.attributes[3].format = MTLVertexFormatFloat2;vertexDesc.attributes[3].bufferIndex = 0;vertexDesc.attributes[3].offset = 6 * sizeof(float);  // 24 bytesvertexDesc.layouts[0].stride = 10 * sizeof(float);    // 40 bytesvertexDesc.layouts[0].stepFunction = MTLVertexStepFunctionPerVertex;pipelineDesc.vertexDescriptor = vertexDesc;pipelineDesc.vertexFunction = vertFunc;
         

Each  object in the  array of the  object corresponds to the indexed struct member in VertexInput in the shader function. attributes[1].bufferIndex = 0 specifies the use of the buffer at index 0 in the argument table. (In this example, each  has the same , so each refers to the same vertex buffer at index 0 in the argument table.) The  values specify the location of data within the vertex, so attributes[1].offset = 2 * sizeof(float) locates the start of the corresponding data 8 bytes from the start of the buffer. The  values are chosen to match the data type in the shader function, so attributes[1].format = MTLVertexFormatFloat4 specifies the use of four floating-point values.

The  property of MTLVertexDescriptor is a . For each  in , the properties specify how vertex and attribute data are fetched from the corresponding in the argument table when Metal draws primitives. (For more on drawing primitives, see .) The  property of MTLVertexBufferLayoutDescriptor determines whether to fetch attribute data for every vertex, for some number of instances, or just once. If stepFunction is set to fetch attribute data for some number of instances, then the  property of MTLVertexBufferLayoutDescriptor determines how many instances. The  property specifies the distance between the data of two vertices, in bytes.

Figure 5-5 depicts the  that corresponds to the code in . layouts[0] specifies how vertex data is fetched from corresponding index 0 in the buffer argument table. layouts[0].stride specifies a distance of 40 bytes between the data of two vertices. The value of layouts[0].stepFunction, , specifies that attribute data is fetched for every vertex when drawing. If the value of stepFunction is, the stepRate property determines how often attribute data is fetched. For example, if stepRate is 1, data is fetched for every instance; if stepRate is 2, for every two instances, and so on.

Performing Fixed-Function Render Command Encoder Operations

Use these  methods to set fixed-function graphics state values:

•  specifies the region, in screen coordinates, which is the destination for the projection of the virtual 3D world. The viewport is 3D, so it includes depth values; for details, see .

•  determines whether to rasterize triangle and triangle strip primitives with lines () or as filled triangles (). The default value is MTLTriangleFillModeFill.

•  and  are used together to determine if and how culling is applied. You can use culling for hidden surface removal on some geometric models, such as an orientable sphere rendered with filled triangles. (A surface is orientable if its primitives are consistently drawn in either clockwise or counterclockwise order.)

  • The value of  indicates whether a front-facing primitive has its vertices drawn in clockwise () or counterclockwise () order. The default value isMTLWindingClockwise.

  • The value of  determines whether to perform culling (, if culling disabled) or which type of primitive to cull ( or ).

Use the following  methods to encode fixed-function state change commands:

•  specifies a 2D scissor rectangle. Fragments that lie outside the specified scissor rectangle are discarded.

•  sets the depth and stencil test state as described in .

•  specifies the stencil reference value.

•  specifies an adjustment for comparing shadow maps to the depth values output from fragment shaders.

•  determines whether to monitor if any samples pass the depth and stencil tests. If set to , then if any samples pass the depth and stencil tests, a non-zero value is written to a buffer specified by the  property of , as described in .

  You can use this mode to perform occlusion testing. If you draw a bounding box and no samples pass, then you may conclude that any objects within that bounding box are occluded and thus do not require rendering.

•  specifies the constant blend color and alpha values, as detailed in .

Working with Viewport and Pixel Coordinate Systems

Metal defines its Normalized Device Coordinate (NDC) system as a 2x2x1 cube with its center at (0, 0, 0.5). The left and bottom for x and y, respectively, of the NDC system are specified as -1. The right and top for x and y, respectively, of the NDC system are specified as +1.

The viewport specifies the transformation from NDC to the window coordinates. The Metal viewport is a 3D transformation specified by the  method of . The origin of the window coordinates is in the upper-left corner.

In Metal, pixel centers are offset by (0.5, 0.5). For example, the pixel at the origin has its center at (0.5, 0.5); the center of the adjacent pixel to its right is (1.5, 0.5). This is also true for textures.

Performing Depth and Stencil Operations

The depth and stencil operations are fragment operations that you specify as follows:

1. Specify a custom  object that contains settings for the depth/stencil state. Creating a custom MTLDepthStencilDescriptor object may require creating one or two  objects that are applicable to front-facing primitives and back-facing primitives.

2. Create a  object by calling the  method of MTLDevice with a depth/stencil state descriptor.

3. To set the depth/stencil state, call the  method of MTLRenderCommandEncoder with the .

4. If the stencil test is in use, call  to specify the stencil reference value.

If the depth test is enabled, the render pipeline state must include a depth attachment to support writing the depth value. To perform the stencil test, the render pipeline state must include a stencil attachment. To configure attachments, see .

If you will be changing the depth/stencil state regularly, then you may want to reuse the state descriptor object, modifying its property values as needed to create more state objects.

Use the properties of a  object as follows to set the depth and stencil state:

• To enable writing the depth value to the depth attachment, set  to .

•  specifies how the depth test is performed. If a fragment’s depth value fails the depth test, the fragment is discarded. For example, the commonly used MTLCompareFunctionLess function causes fragment values that are further away from the viewer than the (previously written) pixel depth value to fail the depth test; that is, the fragment is considered occluded by the earlier depth value.

• The  and  properties each specify a separate  object for front- and back-facing primitives. To use the same stencil state for both front- and back-facing primitives, you can assign the same to both  and  properties. To explicitly disable the stencil test for one or both faces, set the corresponding property to nil, the default value.

Explicit disabling of a stencil state is not necessary. Metal determines whether to enable a stencil test based on whether the stencil descriptor is configured for a valid stencil operation.

 shows an example of creation and use of a  object for the creation of a  object, which is then used with a render command encoder. In this example, the stencil state for the front-facing primitives is accessed from the  property of the depth/stencil state descriptor. The stencil test is explicitly disabled for the back-facing primitives.

Listing 5-13  Creating and Using a Depth/Stencil Descriptor

MTLDepthStencilDescriptor *dsDesc = [[MTLDepthStencilDescriptor alloc] init];

if (dsDesc == nil)

exit(1);   //  if the descriptor could not be allocated

dsDesc.depthCompareFunction = MTLCompareFunctionLess;

dsDesc.depthWriteEnabled = YES;

dsDesc.frontFaceStencil.stencilCompareFunction = MTLCompareFunctionEqual;

dsDesc.frontFaceStencil.stencilFailureOperation = MTLStencilOperationKeep;

dsDesc.frontFaceStencil.depthFailureOperation = MTLStencilOperationIncrementClamp;

dsDesc.frontFaceStencil.depthStencilPassOperation =

MTLStencilOperationIncrementClamp;

dsDesc.frontFaceStencil.readMask = 0x1;

dsDesc.frontFaceStencil.writeMask = 0x1;

dsDesc.backFaceStencil = nil;

id 
         
           dsState = [device
         

newDepthStencilStateWithDescriptor:dsDesc];

[renderEnc setDepthStencilState:dsState];

[renderEnc setStencilReferenceValue:0xFF];

The following properties define a stencil test in the :

•  is a bitmask; the GPU computes the bitwise AND of this mask with both the stencil reference value and the stored stencil value. The stencil test is a comparison between the resulting masked reference value and the masked stored value.

•  is a bitmask that restricts which stencil values are written to the stencil attachment by the stencil operations.

•  specifies how the stencil test is performed for fragments. In Listing 5-13, the stencil comparison function is , so the stencil test passes if the masked reference value is equal to masked stencil value already stored at the location of a fragment.

• , , and  specify what to do to a stencil value stored in the stencil attachment for three different test outcomes: if the stencil test fails, if the stencil test passes and the depth test fails, or if both stencil and depth tests succeed, respectively. In the preceding example, the stencil value is unchanged () if the stencil test fails, but it is incremented if the stencil test passes, unless the stencil value is already the maximum possible ().

Drawing Geometric Primitives

After you have established the pipeline state and fixed-function state, you can call the following  methods to draw the geometric primitives. These draw methods reference resources (such as buffers that contain vertex coordinates, texture coordinates, surface normals, and other data) to execute the pipeline with the shader functions and other state you have previously established with MTLRenderCommandEncoder.

•  renders a number of instances (instanceCount) of primitives using vertex data in contiguous array elements, starting with the first vertex at the array element at the indexvertexStart and ending at the array element at the index vertexStart + vertexCount - 1.

•  is the same as the previous method with an instanceCount of 1.

•  renders a number of instances (instanceCount) of primitives using an index list specified in the MTLBuffer object indexBuffer.indexCount determines the number of indices. The index list starts at the index that is indexBufferOffset byte offset within the data in indexBuffer. indexBufferOffset must be a multiple of the size of an index, which is determined byindexType.

•  is similar to the previous method with an instanceCount of 1.

For every primitive rendering method listed above, the first input value determines the primitive type with one of the MTLPrimitiveType values. The other input values determine which vertices are used to assemble the primitives. For all these methods, theinstanceStart input value determines the first instance to draw, and instanceCount input value determines how many instances to draw.

As previously discussed,  determines whether the triangles are rendered as filled or wireframe, and the  and  settings determine whether the GPU culls triangles during rendering. For more information, see ).

When rendering a point primitive, the shader language code for the vertex function must provide the [[ point_size ]] attribute, or the point size is undefined.

When rendering a triangle primitive with flat shading, the attributes of the first vertex (also known as the provoking vertex) are used for the whole triangle. The shader language code for the vertex function must provide the [[ flat ]] interpolation qualifier.

For details on all Metal shading language attributes and qualifiers, see .

Ending a Rendering Pass

To terminate a rendering pass, call  on the render command encoder. After ending the previous command encoder, you can create a new command encoder of any type to encode additional commands into the command buffer.

Code Example: Drawing a Triangle

The following steps, illustrated in , describe a basic procedure for rendering a triangle.

1. Create a  and use it to create a .

2. Create a  that specifies a collection of attachments that serve as the destination for encoded rendering commands in the command buffer.

   In this example, only the first color attachment is set up and used. (The variable currentTexture is assumed to contain a  that is used for a color attachment.) Then the  is used to create a new.

3. Create two  objects, posBuf and colBuf, and call  to copy vertex coordinate and vertex color data, posData and colData, respectively, into the buffer storage.

4. Call the  method of  twice to specify the coordinates and colors.

   The atIndex input value of the  method corresponds to the attribute buffer(atIndex) in the source code of the vertex function.

5. Create a  and establish the vertex and fragment functions in the pipeline descriptor:

   • Create a  with source code from progSrc, which is assumed to be a string that contains Metal shader source code.

   • Then call the  method of  to create the  vertFunc that represents the function called hello_vertex and to create the  fragFunc that represents the function calledhello_fragment.

   • Finally, set the  and  properties of the  with these MTLFunction objects.

6. Create a  from the  by calling  or a similar method of . Then the  method of uses the created pipeline state for rendering.

7. Call the  method of  to append commands to perform the rendering of a filled triangle (type ).

8. Call the  method to end encoding for this rendering pass. And call the  method of  to execute the commands on the device.

Listing 5-14  Metal Code for Drawing a Triangle

id 
         
           device = MTLCreateSystemDefaultDevice();
         

id 
         
           commandQueue = [device newCommandQueue];
         

id 
         
           commandBuffer = [commandQueue commandBuffer];
         

MTLRenderPassDescriptor *renderPassDesc

= [MTLRenderPassDescriptor renderPassDescriptor];

renderPassDesc.colorAttachments[0].texture = currentTexture;

renderPassDesc.colorAttachments[0].loadAction = MTLLoadActionClear;

renderPassDesc.colorAttachments[0].clearColor = MTLClearColorMake(0.0,1.0,1.0,1.0);

id 
         
           renderEncoder =
         

[commandBuffer renderCommandEncoderWithDescriptor:renderPassDesc];

static const float posData[] = {

0.0f, 0.33f, 0.0f, 1.f,

-0.33f, -0.33f, 0.0f, 1.f,

0.33f, -0.33f, 0.0f, 1.f,

};

static const float colData[] = {

1.f, 0.f, 0.f, 1.f,

0.f, 1.f, 0.f, 1.f,

0.f, 0.f, 1.f, 1.f,

};

id 
         
           posBuf = [device newBufferWithBytes:posData
         

length:sizeof(posData) options:nil];

id 
         
           colBuf = [device newBufferWithBytes:colorData
         

length:sizeof(colData) options:nil];

[renderEncoder setVertexBuffer:posBuf offset:0 atIndex:0];

[renderEncoder setVertexBuffer:colBuf offset:0 atIndex:1];

NSError *errors;

id 
         
           library = [device newLibraryWithSource:progSrc options:nil
         

error:&errors];

id 
         
           vertFunc = [library newFunctionWithName:@"hello_vertex"];
         

id 
         
           fragFunc = [library newFunctionWithName:@"hello_fragment"];
         

MTLRenderPipelineDescriptor *renderPipelineDesc

= [[MTLRenderPipelineDescriptor alloc] init];

renderPipelineDesc.vertexFunction = vertFunc;

renderPipelineDesc.fragmentFunction = fragFunc;

renderPipelineDesc.colorAttachments[0].pixelFormat = currentTexture.pixelFormat;

id 
         
           pipeline = [device
         

newRenderPipelineStateWithDescriptor:renderPipelineDesc error:&errors];

[renderEncoder setRenderPipelineState:pipeline];

[renderEncoder drawPrimitives:MTLPrimitiveTypeTriangle

vertexStart:0 vertexCount:3];

[renderEncoder endEncoding];

[commandBuffer commit];

In Listing 5-14, a  object represents the shader function called hello_vertex. The  method of  is used to specify the vertex resources (in this case, two buffer objects) that are passed as arguments into hello_vertex. The atIndex input value of the  method corresponds to the attribute buffer(atIndex) in the source code of the vertex function, as shown in Listing 5-15.

Listing 5-15  Corresponding Shader Function Declaration

vertex VertexOutput hello_vertex(

const global float4 *pos_data [[ buffer(0) ]],

const global float4 *color_data [[ buffer(1) ]])

{

...

}

Encoding a Single Rendering Pass Using Multiple Threads

In some cases, your app’s performance can be limited by the single-CPU workload of encoding commands for a single rendering pass. However, attempting to circumvent this bottleneck by separating the workload into multiple rendering passes encoded on multiple CPU threads can also adversely impact performance, because each rendering pass requires its own intermediate attachment store and load actions to preserve the render target contents.

Instead, use a  object, which manages multiple subordinate MTLRenderCommandEncoder objects that share the same command buffer and render pass descriptor. The parallel render command encoder ensures that the attachment load and store actions occur only at the start and end of the entire rendering pass, not at the start and end of each subordinate render command encoder’s set of commands. With this architecture, you can assign eachMTLRenderCommandEncoder object to its own thread in parallel in a safe and highly performant manner.

To create a parallel render command encoder, use the  method of a MTLCommandBuffer object. To create subordinate render command encoders, call the renderCommandEncoder method of theMTLParallelRenderCommandEncoder object once for each CPU thread from which you want to perform command encoding. All subordinate command encoders created from the same parallel render command encoder encode commands to the same command buffer. Commands are encoded to a command buffer in the order in which the render command encoders are created. To end encoding for a specific render command encoder, call the  method of MTLRenderCommandEncoder. After you have ended encoding on all render command encoders created by the parallel render command encoder, call the  method of MTLParallelRenderCommandEncoder to end the rendering pass.

Listing 5-16 shows the MTLParallelRenderCommandEncoder creating three MTLRenderCommandEncoder objects: rCE1, rCE2, and rCE3.

Listing 5-16  A Parallel Rendering Encoder with Three Render Command Encoders

MTLRenderPassDescriptor *renderPassDesc                      = [MTLRenderPassDescriptor renderPassDescriptor];renderPassDesc.colorAttachments[0].texture = currentTexture;renderPassDesc.colorAttachments[0].loadAction = MTLLoadActionClear;renderPassDesc.colorAttachments[0].clearColor = MTLClearColorMake(0.0,0.0,0.0,1.0);id 
         
           parallelRCE = [commandBuffer                      parallelRenderCommandEncoderWithDescriptor:renderPassDesc];id 
          
            rCE1 = [parallelRCE renderCommandEncoder];id 
           
             rCE2 = [parallelRCE renderCommandEncoder];id 
            
              rCE3 = [parallelRCE renderCommandEncoder];//  not shown: rCE1, rCE2, and rCE3 call methods to encode graphics commands////  rCE1 commands are processed first, because it was created first//  even though rCE2 and rCE3 end earlier than rCE1[rCE2 endEncoding];[rCE3 endEncoding];[rCE1 endEncoding];//  all MTLRenderCommandEncoders must end before MTLParallelRenderCommandEncoder[parallelRCE endEncoding];
            
           
          
         

The order in which the command encoders call  is not relevant to the order in which commands are encoded and appended to the . For , the  always contains commands in the order that the subordinate render command encoders were created, as seen in Figure 5-6.