ios - 在 Metal 中如何清除深度缓冲区或模板缓冲区？

Question

在 Metal 中，当我已经有一个 RenderCommandEncoder 并且已经使用它完成了一些工作时，我如何清除 深度缓冲区 或 模板缓冲区 (但不是两个我都需要保留一个)？例如，在 OpenGL 中我们有 glClearDepthf/GL_DEPTH_BUFFER_BIT和 glClearStencil/GL_STENCIL_BUFFER_BIT但我没有在 Metal 中找到任何等价物。

Answer 1

虽然 Metal 确实没有提供在渲染过程中清除深度或模板缓冲区的机制，但可以创建一个近乎平凡的管道状态，让您可以根据需要选择性地执行此操作。

在将一些 OpenGL 代码移植到 Metal 的过程中，我发现自己需要清除与当前设置的视口(viewport)边界相对应的深度缓冲区部分。这是我的解决方案:

在我的设置代码中，我创建了一个专门的 MTLRenderPipelineState 和 MTLDepthStencilState ，它们仅用于清除深度缓冲区，并将它们与我的其他长期资源一起存储在我的 MTKView 子类中:

@property (nonatomic, retain) id<MTLRenderPipelineState> pipelineDepthClear;
@property (nonatomic, retain) id<MTLDepthStencilState> depthStencilClear;

[...]

// Special depth stencil state for clearing the depth buffer
MTLDepthStencilDescriptor *depthStencilDescriptor = [[MTLDepthStencilDescriptor alloc] init];

// Don't actually perform a depth test, just always write the buffer
depthStencilDescriptor.depthCompareFunction = MTLCompareFunctionAlways;
depthStencilDescriptor.depthWriteEnabled = YES;
depthStencilDescriptor.label = @"depthStencilClear";
depthStencilClear = [self.device newDepthStencilStateWithDescriptor:depthStencilDescriptor];

// Special pipeline state just for clearing the depth buffer.
MTLRenderPipelineDescriptor *renderPipelineDescriptor = [[MTLRenderPipelineDescriptor alloc] init];

// Omit the color attachment, since we don't want to write the color buffer for this case.

renderPipelineDescriptor.depthAttachmentPixelFormat = self.depthStencilPixelFormat;
renderPipelineDescriptor.rasterSampleCount = self.sampleCount;

renderPipelineDescriptor.vertexFunction = [self.library newFunctionWithName:@"vertex_depth_clear"];
renderPipelineDescriptor.vertexFunction.label = @"vertexDepthClear";

renderPipelineDescriptor.fragmentFunction = [self.library newFunctionWithName:@"fragment_depth_clear"];
renderPipelineDescriptor.fragmentFunction.label = @"fragmentDepthClear";

MTLVertexDescriptor *vertexDescriptor = [[MTLVertexDescriptor alloc] init];
vertexDescriptor.attributes[0].format = MTLVertexFormatFloat2;
vertexDescriptor.attributes[0].offset = 0;
vertexDescriptor.attributes[0].bufferIndex = 0;
vertexDescriptor.layouts[0].stepRate = 1;
vertexDescriptor.layouts[0].stepFunction = MTLVertexStepFunctionPerVertex;
vertexDescriptor.layouts[0].stride = 8;

renderPipelineDescriptor.vertexDescriptor = vertexDescriptor;

NSError* error = NULL;
renderPipelineDescriptor.label = @"pipelineDepthClear";
self.pipelineDepthClear = [self.device newRenderPipelineStateWithDescriptor:renderPipelineDescriptor error:&error];

并在我的 .metal 文件中设置匹配的顶点和片段函数:

struct DepthClearVertexIn
{
    float2 position [[ attribute(0) ]];
};

struct DepthClearVertexOut
{
    float4 position [[ position ]];
};

struct DepthClearFragmentOut
{
    float depth [[depth(any)]];
};

vertex DepthClearVertexOut 
vertex_depth_clear(     DepthClearVertexIn in [[ stage_in ]])
{
    DepthClearVertexOut out;
    // Just pass the position through. We're clearing in NDC space.
    out.position = float4(in.position, 0.5, 1.0);
    return out;
}

fragment DepthClearFragmentOut fragment_depth_clear()
{
    DepthClearFragmentOut out;
    out.depth = 1.0;
    return out;
}

最后，我的 clearDepthBuffer() 方法的主体如下所示:

// Set up the pipeline and depth/stencil state to write a clear value to only the depth buffer.
[view.commandEncoder setDepthStencilState:view.depthStencilClear];
[view.commandEncoder setRenderPipelineState:view.pipelineDepthClear];

// Normalized Device Coordinates of a tristrip we'll draw to clear the buffer
// (the vertex shader set in pipelineDepthClear ignores all transforms and just passes these through)
float clearCoords[8] = {
    -1, -1,
    1, -1,
    -1, 1,
    1, 1
};

[view.commandEncoder setVertexBytes:clearCoords length:sizeof(float) * 8 atIndex:0];
[view.commandEncoder drawPrimitives:MTLPrimitiveTypeTriangleStrip vertexStart:0 vertexCount:4];

// Be sure to reset the setDepthStencilState and setRenderPipelineState for further drawing

由于顶点着色器根本不转换坐标，我在 NDC 空间中指定输入几何，因此从 (-1, -1) 到 (1, 1) 的矩形覆盖整个视口(viewport)。如果您适本地设置几何和/或变换，则可以使用相同的技术来清除深度缓冲区的任何部分。

类似的技术应该适用于清除模板缓冲区，但我将把它作为练习留给读者。 ;)