使用金属间接命令缓冲区时出错：“片段着色器不能与间接命令缓冲区一起使用”

Question

我正在开发一个基于 Metal、MTKView 的应用程序，该应用程序利用 A11 TBDR 架构在单个渲染通道中执行延迟着色。我使用了 Apple 的 Deferred Lighting sample code 作为参考，效果很好。

我想尝试使用 A11 硬件上 Metal 2 的间接命令缓冲区功能将几何缓冲区通道更改为 GPU 驱动。

我一直使用 Apple 的 Encoding Indirect Command Buffers on the GPU sample code 作为我的主要参考点。我可以在我的 iPhone XR 上运行这个示例（尽管可能跑题了，滚动不流畅，它会颤抖）。

当我尝试将几何缓冲区传递移动到间接命令缓冲区时，我遇到了困难。当我在几何缓冲区管道的MTLRenderPipelineDescriptor 上将supportIndirectCommandBuffers 设置为true 时，device.makeRenderPipelineState 失败并出现错误

AGXMetalA12 Code=3 "片段着色器不能与间接命令缓冲区一起使用"

我无法在文档中找到有关此错误的任何信息。我想知道，是否存在间接管道中不允许的某些类型的片段操作，或者我忽略了对 GPU 驱动的绘图的某种限制（可能是颜色附件的数量）？

SharedTypes.h

Metal 和 Swift 共享的标头

#ifndef SharedTypes_h
#define SharedTypes_h

#ifdef __METAL_VERSION__

#define NS_CLOSED_ENUM(_type, _name) enum _name : _type _name; enum _name : _type
#define NSInteger metal::int32_t

#else

#import <Foundation/Foundation.h>

#endif

#include <simd/simd.h>

typedef struct {
    uint32_t meshId;
    matrix_float3x3 normalViewMatrix;
    matrix_float4x4 modelMatrix;
    matrix_float4x4 shadowMVPTransformMatrix;
} InstanceData;

typedef struct {
    vector_float3 cameraPosition;
    float voxelScale;
    float blockScale;
    vector_float3 lightDirection;
    matrix_float4x4 viewMatrix;
    matrix_float4x4 projectionMatrix;
    matrix_float4x4 projectionMatrixInverse;
    matrix_float4x4 shadowViewProjectionMatrix;
} VoxelUniforms;

typedef NS_CLOSED_ENUM(NSInteger, BufferIndex)
{
    BufferIndexInstances  = 0,
    BufferIndexVertices = 1,
    BufferIndexIndices = 2,
    BufferIndexVoxelUniforms = 3,
};

typedef NS_CLOSED_ENUM(NSInteger, RenderTarget)
{
    RenderTargetLighting = 0,
    RenderTargetNormal_shadow = 1,
    RenderTargetVoxelIndex = 2,
    RenderTargetDepth = 3,
};

#endif /* SharedTypes_h */

GBuffer 着色器

#include <metal_stdlib>
using namespace metal;
#include "../SharedTypes.h"

struct VertexIn {
    packed_half3 position;
    packed_half3 texCoord3D;
    half ambientOcclusion;
    uchar normalIndex;
};

struct VertexInOut {
    float4 position [[ position ]];
    half3 worldPos;
    half3 eyeNormal;
    half3 localPosition;
    half3 localNormal;
    float eyeDepth;
    float3 shadowCoord;
    half3 texCoord3D;
};

vertex VertexInOut gBufferVertex(device InstanceData* instances [[ buffer( BufferIndexInstances ) ]],
                                 device VertexIn* vertices [[ buffer( BufferIndexVertices ) ]],
                                 constant VoxelUniforms &uniforms [[ buffer( BufferIndexVoxelUniforms ) ]],
                                 uint vid [[ vertex_id ]],
                                 ushort iid [[ instance_id ]])
{
    InstanceData instance = instances[iid];
    VertexIn vert = vertices[vid];
    VertexInOut out;
    float4 position = float4(float3(vert.position), 1);
    float4 worldPos = instance.modelMatrix * position;
    float4 eyePosition = uniforms.viewMatrix * worldPos;
    out.position = uniforms.projectionMatrix * eyePosition;
    out.worldPos = half3(worldPos.xyz);
    out.eyeDepth = eyePosition.z;

    half3 normal = normals[vert.normalIndex];
    out.eyeNormal = half3(instance.normalViewMatrix * float3(normal));
    out.shadowCoord = (instance.shadowMVPTransformMatrix * position).xyz;

    out.localPosition = half3(vert.position);
    out.localNormal = normal;
    out.texCoord3D = half3(vert.texCoord3D);
    return out;
}

fragment GBufferData gBufferFragment(VertexInOut in [[ stage_in ]],
                                     constant VoxelUniforms &uniforms [[ buffer( BufferIndexVoxelUniforms ) ]],
                                     texture3d<ushort, access::sample> voxelMap [[ texture(0) ]],
                                     depth2d<float> shadowMap [[ texture(1) ]],
                                     texture3d<half, access::sample> fogOfWarMap [[ texture(2) ]]
                                     ) {
    // voxel index
    half3 center = round(in.texCoord3D);
    uchar voxIndex = voxelMap.read(ushort3(center)).r - 1;

    // ambient occlusion
    half3 neighborPos = center + in.localNormal;
    half3 absNormal = abs(in.localNormal);
    half2 texCoord2D = tc2d(in.localPosition / uniforms.voxelScale, absNormal);
    half ao = getAO(voxelMap, neighborPos, absNormal.yzx, absNormal.zxy, texCoord2D);

    // shadow
    constexpr sampler shadowSampler(coord::normalized,
                                    filter::linear,
                                    mip_filter::none,
                                    address::clamp_to_edge,
                                    compare_func::less);

    float shadow_sample = ambientLightingLevel;
    for (short i = 0; i < shadowSampleCount; i++){
        shadow_sample += shadowMap.sample_compare(shadowSampler, in.shadowCoord.xy + poissonDisk[i] * 0.002, in.shadowCoord.z - 0.0018) * shadowContributionPerSample;
    }
    shadow_sample = min(1.0, shadow_sample);

    //fog-of-war
    half fogOfWarSample = fogOfWarMap.sample(fogOfWarSampler, (float3(in.worldPos) / uniforms.blockScale) + float3(0.5, 0.4, 0.5)).r;
    half notVisible = max(fogOfWarSample, 0.5h);

    // output
    GBufferData out;
    out.normal_shadow = half4(in.eyeNormal, ao * half(shadow_sample) * notVisible);
    out.voxelIndex = voxIndex;
    out.depth = in.eyeDepth;
    return out;
};

管道设置

extension RenderTarget {

    var pixelFormat: MTLPixelFormat {
        switch self {
        case .lighting: return .bgra8Unorm
        case .normal_shadow: return .rgba8Snorm
        case .voxelIndex: return .r8Uint
        case .depth: return .r32Float
        }
    }

    static var allCases: [RenderTarget] = [.lighting, .normal_shadow, .voxelIndex, .depth]
}

public final class GBufferRenderer {
    private let renderPipelineState: MTLRenderPipelineState
    weak var shadowMap: MTLTexture?

    public init(depthPixelFormat: MTLPixelFormat, colorPixelFormat: MTLPixelFormat, sampleCount: Int = 1) throws {
        let library = try LibraryMonad.getLibrary()
        let device = library.device
        let descriptor = MTLRenderPipelineDescriptor()
        descriptor.vertexFunction = library.makeFunction(name: "gBufferVertex")!
        descriptor.fragmentFunction = library.makeFunction(name: "gBufferFragment")!
        descriptor.depthAttachmentPixelFormat = depthPixelFormat
        descriptor.stencilAttachmentPixelFormat = depthPixelFormat
        descriptor.sampleCount = sampleCount
        for target in RenderTarget.allCases {
            descriptor.colorAttachments[target.rawValue].pixelFormat = target.pixelFormat
        }
        // uncomment below to trigger throw
        // descriptor.supportIndirectCommandBuffers = true
        renderPipelineState = try device.makeRenderPipelineState(descriptor: descriptor) // throws "Fragment shader cannot be used with indirect command buffers"
    }

    public convenience init(mtkView: MTKView) throws {
        try self.init(depthPixelFormat: mtkView.depthStencilPixelFormat, colorPixelFormat: mtkView.colorPixelFormat, sampleCount: mtkView.sampleCount)
    }
}

上述方法在以通常方式从 CPU 触发绘图时效果很好，但在设置 supportIndirectCommandBuffers 以准备 GPU 绘图时会引发错误。

我尝试剥离片段着色器以仅返回 GBuffer 的常量值，然后 makeRenderPipelineState 成功，但是当我重新添加纹理采样时，它又开始抱怨了。我似乎无法确定它到底不喜欢 frag 着色器的哪些方面。

Answer 1

查看代码以及 Metal 文档和 Metal Shading Language 规范，我想我知道为什么会出现此错误。

如果您查看 Metal 中 metal_command_buffer 标头中存在的 render_command 接口，您会发现要将参数传递给间接渲染命令，您只有这些函数：set_vertex_buffer 和 set_fragment_buffer，那里不是set_vertex_texture 或set_vertex_sampler，就像你在MTLRenderCommandEncoder 中那样。

但是，由于您的管道使用着色器，而着色器又使用纹理作为参数，并且您使用 supportIndirectCommandBuffers 表示您希望在间接命令中使用此管道，因此 Metal 没有选择，只能使管道创建失败。

如果您想将纹理或采样器传递给间接渲染命令，您应该使用参数缓冲区，您将传递给发出间接渲染命令的着色器，然后使用set_vertex_buffer 和set_fragment_buffer 绑定它们对于每个render_command。

规范：Metal Shading Language Specification（第 5.16 节）