如何在一个金属 API 场景中使用不同的片段着色器

Question

我最近一直在用Apple的Metal API做一些实验，现在我来到了一个主题问题 - 如何在一个Metal API场景中使用不同的片段着色器？可能吗？

背景：整个几何基元由一个简单的顶点碎片链渲染，里面定义/计算颜色（假设我们有一个立方体，它的所有面都用描述的方法渲染）。接下来，需要用纹理额外渲染基元的一部分（仅向其中一个面添加一些图片）。

我们是否需要使用不同的片段着色器来实现这一点？我想可以在第一步中使用一些默认纹理，这将提供一些解决方案。

你会推荐什么？

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编辑部分更进一步 ===

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正如Warren所建议的那样，我尝试使用MTLRenderPipelineState的两个不同对象和两对不同的渲染函数。有了以下代码，我没有得到想要的结果。每个状态在单独完成时都会按预期进行渲染，但一起执行只会给我们第一个被渲染的状态。

创作：

id <MTLFunction> fragmentProgram = [_defaultLibrary newFunctionWithName:@"color_fragment"];
// Load the vertex program into the library
id <MTLFunction> vertexProgram = [_defaultLibrary newFunctionWithName:@"lighting_vertex"];
// Create a vertex descriptor from the MTKMesh
MTLVertexDescriptor *vertexDescriptor = MTKMetalVertexDescriptorFromModelIO(_boxMesh.vertexDescriptor);
vertexDescriptor.layouts[0].stepRate = 1;
vertexDescriptor.layouts[0].stepFunction = MTLVertexStepFunctionPerVertex;
// Create a reusable pipeline state
MTLRenderPipelineDescriptor *pipelineStateDescriptor = [[MTLRenderPipelineDescriptor alloc] init];
pipelineStateDescriptor.label = @"MyPipeline";
pipelineStateDescriptor.sampleCount = _view.sampleCount;
pipelineStateDescriptor.vertexFunction = vertexProgram;
pipelineStateDescriptor.fragmentFunction = fragmentProgram;
pipelineStateDescriptor.vertexDescriptor = vertexDescriptor;
pipelineStateDescriptor.colorAttachments[0].pixelFormat = _view.colorPixelFormat;
pipelineStateDescriptor.depthAttachmentPixelFormat = _view.depthStencilPixelFormat;
pipelineStateDescriptor.stencilAttachmentPixelFormat = _view.depthStencilPixelFormat;
NSError *error = NULL;
_pipelineStateColor = [_device newRenderPipelineStateWithDescriptor:pipelineStateDescriptor error:&error];
if (!_pipelineStateColor) {
    NSLog(@"Failed to created pipeline state, error %@", error);
}
pipelineStateDescriptor.fragmentFunction = [_defaultLibrary newFunctionWithName:@"lighting_fragment"];
_pipelineStateTexture = [_device newRenderPipelineStateWithDescriptor:pipelineStateDescriptor error:&error];
if (!_pipelineStateTexture) {
    NSLog(@"Failed to created pipeline state, error %@", error);
}

渲染：

 - (void)renderInto:(id <MTLRenderCommandEncoder>)renderEncoder
 WithPipelineState:(id<MTLRenderPipelineState>)pipelineState
{
    [renderEncoder setRenderPipelineState:pipelineState];
    [renderEncoder setVertexBuffer:_boxMesh.vertexBuffers[0].buffer offset:_boxMesh.vertexBuffers[0].offset atIndex:0 ];
    [renderEncoder setVertexBuffer:_dynamicConstantBuffer offset:(sizeof(uniforms_t) * _constantDataBufferIndex) atIndex:1 ];
    [renderEncoder setVertexBuffer:_textureBuffer offset:0 atIndex:2];
    [renderEncoder setFragmentTexture:_textureData atIndex:0];
    MTKSubmesh* submesh = _boxMesh.submeshes[0];
    [renderEncoder drawIndexedPrimitives:submesh.primitiveType
                              indexCount:submesh.indexCount
                               indexType:submesh.indexType
                             indexBuffer:submesh.indexBuffer.buffer
                       indexBufferOffset:submesh.indexBuffer.offset];
}
- (void)_render
{
    dispatch_semaphore_wait(_inflight_semaphore, DISPATCH_TIME_FOREVER);
    [self _update];
    id <MTLCommandBuffer> commandBuffer = [_commandQueue commandBuffer];
    __block dispatch_semaphore_t block_sema = _inflight_semaphore;
    [commandBuffer addCompletedHandler:^(id<MTLCommandBuffer> buffer) {
        dispatch_semaphore_signal(block_sema);
    }];
    MTLRenderPassDescriptor* renderPassDescriptor = _view.currentRenderPassDescriptor;
    if(renderPassDescriptor != nil)
    {
        id <MTLRenderCommandEncoder> renderEncoder = [commandBuffer renderCommandEncoderWithDescriptor:renderPassDescriptor];
        renderEncoder.label = @"MyRenderEncoder";
        [renderEncoder setDepthStencilState:_depthState];
        [self renderInto:renderEncoder WithPipelineState:_pipelineStateColor];
        [self renderInto:renderEncoder WithPipelineState:_pipelineStateTexture];
        [renderEncoder endEncoding];
        [commandBuffer presentDrawable:_view.currentDrawable];
    }
    _constantDataBufferIndex = (_constantDataBufferIndex + 1) % kMaxInflightBuffers;
    [commandBuffer commit];
}

最后是片段着色器：

fragment float4 color_fragment(ColorInOut  in [[stage_in]])
{
    return float4(0.8f, 0.f, 0.1f, 0.5f);
}
fragment float4 texture_fragment(ColorInOut                       in [[stage_in]],
                                 texture2d<float, access::sample> texture [[texture(0)]])
{
    constexpr sampler s(coord::normalized,
                        address::clamp_to_zero,
                        filter::linear);
    return texture.sample(s, in.texture_coordinate);
}

Answer 1

通过创建多个渲染管线状态，可以在单个帧/通道中使用多个片段着色器。只需为每个顶点/片段函数对创建一个管道状态，并在发出绘制调用之前调用 渲染命令编码器上的 setRenderPipelineState: 以设置适当的管道状态。您需要编写单独的片段着色器函数来执行颜色直通和纹理采样。