在金属中如何清除深度缓冲区或模板缓冲区？

Question

在 metal 中，当我已经有了一个 RenderCommandEncoder 并且我已经用它做了一些工作时，我该如何清除 深度缓冲区 或 模板缓冲区（但不是两者我都需要保留一个）？例如，在 OpenGl 中我们有 glClearDepthf / GL_DEPTH_BUFFER_BIT 和 glClearStencil / GL_STENCIL_BUFFER_BIT 但我没有在金属中找到任何等价物。

Answer 1

虽然 Metal 确实没有提供在渲染通道中间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，让您尽可能有选择地这样做你喜欢

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边界对应的深度缓冲区的一部分。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变换坐标，我在NDC中指定了输入几何体space，所以一个从(-1, -1)到(1, 1)的矩形覆盖了整个视口。如果您适当地设置几何 and/or 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 reader。 ;)

在金属中如何清除深度缓冲区或模板缓冲区？

In metal how to clear the depth buffer or the stencil buffer?

objective-c

ios

metal

metalkit