Creating a uniform and using it in the shader

[dov]

In my quest of learning VSG I have extracted from the example source code that to create a uniform that I can change every frame I need to do the following four steps in C++:

1. Declare struct ToyUniform containing the uniform variables

struct ToyUniform {
    vsg::ivec2 iResolution;
}

2. Wrap the scruct in a using ToyUniformValue = vsg::Value<ToyUniform>
3. Allocate by create and assign set it up as follows:

  toyUniform = ToyUniformValue::create();
  toyUniform->properties.dataVariance = vsg::DataVariance::DYNAMIC_DATA;
  toyUniform->value().iResolution = {800,600};

4. Create a uniform descriptor:

  auto toyUniformDescriptor = vsg::DescriptorBuffer::create(toyUniform, 0, 0, VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER);

5. Add the uniform descriptor to the descriptor set:

  auto descriptorSet = vsg::DescriptorSet::create(descriptorSetLayout, vsg::Descriptors{toyUniformDescriptor});

The descriptorSet is then used as in vsgdraw.cpp.

On the shader side I declared it in a uniform and used it as follows in the vertex shader:

layout(location = 0) in vec2 inVertex;
layout(location = 1) in vec2 inTexture;

layout(set = 0, binding = 0) uniform UBO {
    ivec2 iResolution;
} ubo;
:
void main()
{
    gl_Position = ...; // This is ok since my fragment shader paints in the entire quad!
}

However, in the fragment shader, by writing the following shader I realized that the uniform is not initialized and the following shader outputs a constant red image.

#version 450

layout(set = 0, binding = 0) uniform UBO {
    ivec2 iResolution;
} ubo;

layout(location = 0) in vec2 fragCoord;
layout(location = 0) out vec4 fragColor;

void main()
{
  if (abs(ubo.iResolution.x) == 0)
      fragColor = vec4(1.0,0.0,0.0,1.0); // red
  else
      fragColor = vec4(0.0,1.0,0.0,1.0); // green
}

What am I missing?

Thanks in advance!

[robertosfield]

Broadly it looks OK, but we don't know what else is being doing by the rest of your application, perhaps it has another DescrptorSet that is being applied on set=0, binding=0.

[dov]

Thanks for confirming that I'm on the right track!

I uploaded the source code and the two shaders to the following gist:

https://gist.github.com/dov/f7d8e0adf755d6ced43ce685efeb1c43

If you have some more feedback on my code, I'd be happy to receive it!

[dov]

The mystery (in my mind at least!) deepens. I tried inspecting my application with renderdoc and it seems that the value of iResolution is correctly initialized in the fragment shader. So why does my if shader branch according to it being zero below and paint the output red?

#version 450

layout(set = 0, binding = 0) uniform UBO {
    ivec2 iResolution;
    vec2 iMouse;
    float iTime;
} ubo;

layout(location = 0) in vec2 fragCoord;
layout(location = 0) out vec4 fragColor;

void main()
{
  // Do a dummy test to see if iResolution has been initialized
  if (ubo.iResolution.x == 0)
      fragColor = vec4(1.0,0.0,0.0,1.0); // red
  else
      fragColor = vec4(0.0,1.0,0.0,1.0); // green
}

[dov]

I found it! My descriptorBindings was wrong! The correct contents should be:

  vsg::DescriptorSetLayoutBindings descriptorBindings{
    {0, VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER, 1, 
    VK_SHADER_STAGE_VERTEX_BIT|VK_SHADER_STAGE_FRAGMENT_BIT, nullptr} // { binding, descriptorType, descriptorCount, stageFlags, pImmutableSamplers}
  };

  auto descriptorSetLayout = vsg::DescriptorSetLayout::create(descriptorBindings);

with this in place my uniforms and shaders behave as expected. :-)

This should be added to my original uniform "recipe".

[robertosfield]

Great to hear you've spotted the problem.

Tracking down this type of issue is often helped by using the Vulkan Validation Layer. You can enable this by using the -d command line option in many of the examples like vsgviewer, or programatically via setting windowTraits::debugLayer = true;

[dov]

I finished my vsgshadertoy example, which allows rendering a shadertoy.com like shader in a vsg window.

I submitted the result as a pull request to vsgExamples. I'd be happy to receive feedback about how I can improve my VSG usage and understanding.