在金属中如何清除深度缓冲区或模板缓冲区？

Question

在金属中，当我已经有了RenderCommandEncoder并且已经对其进行了某些工作时，如何清除depth buffer或stencil buffer（但我俩都不需要保留一个） ？例如，在OpenGl中，我们有glClearDepthf / GL_DEPTH_BUFFER_BIT和glClearStencil / GL_STENCIL_BUFFER_BIT，但我没有找到任何等效的金属。

Answer 1

虽然的确Metal并没有提供在渲染过程中间清除深度或模板缓冲区的机制，但有可能创建近乎平凡的管道状态，使您可以根据需要选择性地进行操作。

在将一些OpenGL代码移植到Metal的过程中，我发现自己需要清除深度缓冲区的一部分，该部分与当前设置的视口的边界相对应。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，并将它们与其他长期存在的资源一起存放在我的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（）方法的主体看起来像这样：

// 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变换坐标，因此我在NDC空间中指定了输入几何，因此从（-1，-1）到（1，1）的矩形覆盖了整个视口。如果适当地设置几何和/或变换，则可以使用相同的技术清除深度缓冲区的任何部分。

类似的技术应该可以清除模板缓冲区，但我将其留给读者练习。 ;）