在不同的着色器阶段使用不同的推常数 [英] Using different push-constants in different shader stages

问题描述

我有一个顶点着色器，带有一个包含一个浮点的推常数块:

I have a vertex shader with a push-constant block containing one float:

layout(push_constant) uniform pushConstants {
    float test1;
} u_pushConstants;

还有一个片段着色器，它带有另一个推常数块，其浮点值不同:

And a fragment shader with another push-constant block with a different float value:

layout(push_constant) uniform pushConstants {
    float test2;
} u_pushConstants;

test1 和 test2 应该是不同的.

管道布局的push-constant范围定义如下:

The push-constant ranges for the pipeline layout are defined like this:

std::array<vk::PushConstantRange,2> ranges = {
    vk::PushConstantRange{
        vk::ShaderStageFlagBits::eVertex,
        0,
        sizeof(float)
    },
    vk::PushConstantRange{
        vk::ShaderStageFlagBits::eFragment,
        sizeof(float), // Push-constant range offset (Start after vertex push constants)
        sizeof(float)
    }
};

然后在渲染过程中按如下方式推送实际常量:

The actual constants are then pushed during rendering like this:

std::array<float,1> constants = {123.f};
commandBufferDraw.pushConstants(
    pipelineLayout,
    vk::ShaderStageFlagBits::eVertex,
    0,
    sizeof(float),
    constants.data()
);
std::array<float,1> constants = {456.f};
commandBufferDraw.pushConstants(
    pipelineLayout,
    vk::ShaderStageFlagBits::eFragment,
    sizeof(float), // Offset in bytes
    sizeof(float),
    constants.data()
);

但是，在检查着色器内部的值时，两个值都为123. 似乎偏移量被完全忽略了.我使用不正确吗?

However, when checking the values inside the shaders, both have the value 123. It seems that the offsets are completely ignored. Am I using them incorrectly?

推荐答案

在管道布局中，您声明了顶点着色器将访问推入常量范围中[0，4)个字节中的数据范围.您声明片段着色器将访问推入常量范围中的[4，8)中的数据范围.

In your pipeline layout, you stated that your vertex shader would access the range of data from [0, 4) bytes in the push constant range. You stated that your fragment shader would access the range of data from [4, 8) in the push constant range.

但是您的着色器却讲述了一个不同的故事.

But your shaders tell a different story.

layout(push_constant) uniform pushConstants {
    float test2;
} u_pushConstants;

此定义非常清楚地表明，推动常数范围开始使用[0，4).但是您告诉Vulkan它使用[4，8). Vulkan应该相信哪个:您的着色器或管道布局?

This definition very clearly says that the push constant range starts uses [0, 4). But you told Vulkan it uses [4, 8). Which should Vulkan believe: your shader, or your pipeline layout?

要记住的一般经验法则是:着色器表示其含义.赋予管道创建的参数不能更改代码的含义.

A general rule of thumb to remember is this: your shader means what it says it means. Parameters given to pipeline creation cannot change the meaning of your code.

如果您打算让片段着色器真正使用[4，8)，则片段着色器必须真正使用它:

If you intend to have the fragment shader really use [4, 8), then the fragment shader must really use it:

layout(push_constant) uniform fragmentPushConstants {
    layout(offset = 4) float test2;
} u_pushConstants;

由于它的定义与VS版本不同，因此它的块名称也应不同. offset布局指定相关变量的偏移量.这是GLSL的标准内容，并且为Vulkan编译不会改变这一点.

Since it has a different definition from the VS version, it should have a different block name too. The offset layout specifies the offset of the variable in question. That's standard stuff from GLSL, and compiling for Vulkan doesn't change that.

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