HLSL修改像素着色器中的深度

Question

我需要渲染一张从外部获取的带深度的图片。我可以构建两个纹理，然后将它们传递到一个着色器中（我可以验证像素着色器中的采样值是正确的）。

以下是我的HLSL代码：

// image texture
Texture2D m_TextureColor : register(t0);

// depth texture with values [0..1]
Texture2D<float> m_TextureDepth : register(t1);

// sampler to forbid linear filtering since we're dealing with pixels
SamplerState m_TextureSampler { Filter = MIN_MAG_MIP_POINT; };

struct VS_IN
{
    float4 position : POSITION;
    float2 texcoord : TEXCOORD;
};

struct VS_OUT
{
    float4 position : SV_POSITION;
    float2 texcoord : TEXCOORD0;
};

struct PS_OUT
{
    float4 color : COLOR0;
    float depth : DEPTH0;
};

VS_OUT VS(VS_IN input)
{
    VS_OUT output = (VS_OUT)0;

    output.position = input.position;
    output.texcoord = input.texcoord;

    return output;
}

PS_OUT PS(VS_OUT input) : SV_Target
{
    PS_OUT output = (PS_OUT)0;

    output.color = m_TextureColor.SampleLevel(m_TextureSampler, input.texcoord, 0);

    // I want to modify depth of the pixel,
    // but it looks like it has no effect on depth no matter what I set here
    output.depth = m_TextureDepth.SampleLevel(m_TextureSampler, input.texcoord, 0);

    return output;
}

我使用这些数据（具有PrimitiveTopology.TriangleStrip），构建了顶点缓冲区。其中第一个参数 Vector4 是位置，第二个参数 Vector2 是纹理坐标：

new[]
{
    new Vertex(new Vector4(-1, -1, 0.5f, 1), new Vector2(0, 1)),
    new Vertex(new Vector4(-1, 1, 0.5f, 1), new Vector2(0, 0)),
    new Vertex(new Vector4(1, -1, 0.5f, 1), new Vector2(1, 1)),
    new Vertex(new Vector4(1, 1, 0.5f, 1), new Vector2(1, 0)),
}

一切正常：我可以看到我的图像，可以从深度纹理中取样深度并构建出可视化内容（这是我验证正在取样的深度值是否正确的方法）。然而，我无法弄清楚如何修改像素的深度，以便在深度测试进行时能够正确呈现。因为目前一切都取决于我设置为顶点位置的z值是什么。

以下是我如何设置DirectX11（我使用SharpDX和C#）：

var swapChainDescription = new SwapChainDescription
{
    BufferCount = 1,
    ModeDescription = new ModeDescription(bufferSize.Width, bufferSize.Height, new Rational(60, 1), Format.R8G8B8A8_UNorm),
    IsWindowed = true,
    OutputHandle = HostHandle,
    SampleDescription = new SampleDescription(1, 0),
    SwapEffect = SwapEffect.Discard,
    Usage = Usage.RenderTargetOutput,
};
var swapChainFlags = DeviceCreationFlags.None | DeviceCreationFlags.BgraSupport;
SharpDX.Direct3D11.Device.CreateWithSwapChain(DriverType.Hardware, swapChainFlags, swapChainDescription, out var device, out var swapchain);

设置后备缓冲区和深度/模板缓冲区：

// color buffer
using (var textureColor = SwapChain.GetBackBuffer<Texture2D>(0))
{
    TextureColorResourceView = new RenderTargetView(Device, textureColor);
}

// depth buffer
using (var textureDepth = new Texture2D(Device, new Texture2DDescription
{
    Format = Format.D32_Float,
    ArraySize = 1,
    MipLevels = 1,
    Width = BufferSize.Width,
    Height = BufferSize.Height,
    SampleDescription = new SampleDescription(1, 0),
    Usage = ResourceUsage.Default,
    BindFlags = BindFlags.DepthStencil,
    CpuAccessFlags = CpuAccessFlags.None,
    OptionFlags = ResourceOptionFlags.None
}))
{
    TextureDepthResourceView = new DepthStencilView(Device, textureDepth);
}

DeviceContext.OutputMerger.SetTargets(TextureDepthResourceView, TextureColorResourceView);

准备深度模板状态：

var description = DepthStencilStateDescription.Default();
description.DepthComparison = Comparison.LessEqual;
description.IsDepthEnabled = true;
description.DepthWriteMask = DepthWriteMask.All;
DepthState = new DepthStencilState(Device, description);

并且使用它：

DeviceContext.OutputMerger.SetDepthStencilState(DepthState);

这是我构建发送到着色器的颜色/深度纹理的方式：

public static (ShaderResourceView resource, Texture2D texture) CreateTextureDynamic(this Device device, System.Drawing.Size size, Format format)
{
    var textureDesc = new Texture2DDescription
    {
        MipLevels = 1,
        Format = format,
        Width = size.Width,
        Height = size.Height,
        ArraySize = 1,
        BindFlags = BindFlags.ShaderResource,
        Usage = ResourceUsage.Dynamic,
        SampleDescription = new SampleDescription(1, 0),
        CpuAccessFlags = CpuAccessFlags.Write,
    };

    var texture = new Texture2D(device, textureDesc);
    return (new ShaderResourceView(device, texture), texture);
}

另外，由于我需要经常更新它们：

public static void UpdateResource(this Texture2D texture, int[] buffer, System.Drawing.Size size)
{
    var dataBox = texture.Device.ImmediateContext.MapSubresource(texture, 0, MapMode.WriteDiscard, MapFlags.None, out var dataStream);
    Parallel.For(0, size.Height, rowIndex => Marshal.Copy(buffer, size.Width * rowIndex, dataBox.DataPointer + dataBox.RowPitch * rowIndex, size.Width));
    dataStream.Dispose();
    texture.Device.ImmediateContext.UnmapSubresource(texture, 0);
}

public static void UpdateResource(this Texture2D texture, float[] buffer, System.Drawing.Size size)
{
    var dataBox = texture.Device.ImmediateContext.MapSubresource(texture, 0, MapMode.WriteDiscard, MapFlags.None, out var dataStream);
    Parallel.For(0, size.Height, rowIndex => Marshal.Copy(buffer, size.Width * rowIndex, dataBox.DataPointer + dataBox.RowPitch * rowIndex, size.Width));
    dataStream.Dispose();
    texture.Device.ImmediateContext.UnmapSubresource(texture, 0);
}

我也在谷歌上搜寻了很多关于这个问题的文章，找到了类似的帖子：https://www.gamedev.net/forums/topic/573961-how-to-set-depth-value-in-pixel-shader/ 然而我自己无法解决它。

提前感谢！

- chainerlt

Answer 1

要写入深度缓冲区，您需要针对 SV_Depth 系统值语义。因此，您的像素着色器输出结构将更像以下内容：

struct PS_OUT
{
    float4 color : SV_Target;
    float depth : SV_Depth;
};

在您的示例中，着色器不需要指定SV_Target（输出变量已在结构体内定义）。因此，它应该是这样的：

PS_OUT PS(VS_OUT input)
{
    PS_OUT output = (PS_OUT)0;

    output.color = m_TextureColor.SampleLevel(m_TextureSampler, input.texcoord, 0);

    // Now that output.depth is defined with SV_Depth, and you have depth-write enabled,
    // this should write to the depth buffer.
    output.depth = m_TextureDepth.SampleLevel(m_TextureSampler, input.texcoord, 0);

    return output;
}

请注意，如果显式写入深度（特别是在 AMD 硬件上），您可能会遇到一些性能损失，因为这会强制绕过其早期深度硬件优化。所有使用该深度缓冲区的未来绘图调用都将禁用早期-Z 优化，因此通常最好尽可能晚地执行深度写入操作。

- Varrak

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我非常清楚性能惩罚的问题，但是因为API将其流式传输给我，所以没有其他办法。非常感谢！ - chainerlt