Color blend shader for triangles and text

[rickyjames35]

My goal in a nutshell is to create a basic shader that allows me to blend/modify the vertex colors using a descriptor. I'm really just trying to dynamically change the alpha value at runtime but blending the whole color gives me the same effect with a ability to adjust any channel. I would also like to be able to do the same with text. I have it working with basic shapes but not text. Here is my vert shader.

#version 450

layout(push_constant) uniform PushConstants
{
    mat4 proj;
    mat4 modelView;
} mvp;

layout(location = 0) in vec3 inPosition;
layout(location = 1) in vec4 inColor;

layout(location = 0) out vec4 fragColor;

layout(set = 0, binding = 0) uniform ColorBlendUniform
{
    vec4 colorBlend;
};

void main()
{
    gl_Position = (mvp.proj * mvp.modelView) * vec4(inPosition, 1.0);
    fragColor = inColor * colorBlend;
}

Here is my vsg::ShaderSet.

ColorBlend::ColorBlend(vsg::ref_ptr<const vsg::Options> options)
{
    auto vert{vsg::read_cast<vsg::ShaderStage>("shaders/color_blend.vert.vsgb", options)};
    auto frag{vsg::read_cast<vsg::ShaderStage>("shaders/basic.frag.vsgb", options)};
    stages = vsg::ShaderStages{vert, frag};

    addAttributeBinding("position", "", 0, VK_FORMAT_R32G32B32_SFLOAT, vsg::vec3Array::create(0));
    addAttributeBinding("color", "", 1, VK_FORMAT_R32G32B32A32_SFLOAT, vsg::vec4Array::create(0));

    addPushConstantRange("pc", "", VK_SHADER_STAGE_VERTEX_BIT, 0, 128);

    addDescriptorBinding("colorBlend", "", 0, 0, VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER, 1, VK_SHADER_STAGE_VERTEX_BIT, vsg::floatValue::create());
}

Here's me creating 3 triangles using the shader set along with a descriptor per triangle. The last triangle is setup to allow me to dynamically blend the color during runtime.

    auto shaderSet{shader_sets::ColorBlend::create(options)};
    auto graphicsPipelineConfig{vsg::GraphicsPipelineConfigurator::create(shaderSet)};
    graphicsPipelineConfig->enableArray("position", VK_VERTEX_INPUT_RATE_VERTEX, sizeof(vsg::vec3));
    graphicsPipelineConfig->enableArray("color", VK_VERTEX_INPUT_RATE_VERTEX, sizeof(vsg::vec4));
    graphicsPipelineConfig->init();

    auto stateGroup{vsg::StateGroup::create()};
    root->addChild(stateGroup);
    graphicsPipelineConfig->copyTo(stateGroup);

    constexpr float scale{0.5f};
    constexpr uint32_t set{0};
    constexpr uint32_t binding{0};

    // Triangle 1
    {
        auto descBuffer{vsg::DescriptorBuffer::create(vsg::vec4Value::create(vsg::vec4{1.0f, 1.0f, 0.0f, 1.0f}), binding)};
        auto bindDescriptorSet{vsg::BindDescriptorSet::create(VK_PIPELINE_BIND_POINT_GRAPHICS, graphicsPipelineConfig->layout, set, vsg::Descriptors{descBuffer})};

        auto transform{vsg::MatrixTransform::create(vsg::dmat4{vsg::translate(vsg::vec3{0.0f, -0.5f, 0.0f}) * vsg::scale(vsg::vec3{scale, scale, 1.0f})})};
        stateGroup->addChild(transform);
        transform->addChild(bindDescriptorSet);
        transform->addChild(geometry::Triangle::create());
    }

    // Triangle 2
    {
        auto descBuffer{vsg::DescriptorBuffer::create(vsg::vec4Value::create(vsg::vec4{0.0f, 1.0f, 1.0f, 1.0f}), binding)};
        auto bindDescriptorSet{vsg::BindDescriptorSet::create(VK_PIPELINE_BIND_POINT_GRAPHICS, graphicsPipelineConfig->layout, set, vsg::Descriptors{descBuffer})};

        auto transform{vsg::MatrixTransform::create(vsg::dmat4{vsg::translate(vsg::vec3{-0.5f, 0.5f, 0.0f}) * vsg::scale(vsg::vec3{scale, scale, 1.0f})})};
        stateGroup->addChild(transform);
        transform->addChild(bindDescriptorSet);
        transform->addChild(geometry::Triangle::create());
    }

    // Triangle 3
    {
        auto colorBlend{vsg::vec4Value::create(vsg::vec4{1.0f, 1.0f, 1.0f, 1.0f})};
        colorBlend->properties.dataVariance = vsg::DYNAMIC_DATA; // Needs to be dynamic to allow for update of color later
        auto descBuffer{vsg::DescriptorBuffer::create(colorBlend, binding)};
        descBuffer->setValue("name", "triangle3ColorBlendBuffer"); // Allows me to find the color blend buffer later using a visitor
        auto bindDescriptorSet{vsg::BindDescriptorSet::create(VK_PIPELINE_BIND_POINT_GRAPHICS, graphicsPipelineConfig->layout, set, vsg::Descriptors{descBuffer})};

        auto transform{vsg::MatrixTransform::create(vsg::dmat4{vsg::translate(vsg::vec3{0.5f, 0.5f, 0.0f}) * vsg::scale(vsg::vec3{scale, scale, 1.0f})})};
        stateGroup->addChild(transform);
        transform->addChild(bindDescriptorSet);
        transform->addChild(geometry::Triangle::create());
    }

Here is the result
Screencast from 05-06-2025 05:57:41 PM.webm

I'm new to VSG so I'm likely missing out on best practices or easier ways to go about doing this. For example is seems strange to me that I had to bypass using graphicsPipelineConfig->assignDescriptor() and instead manually create the vsg::BindDescriptorSet but from what I can tell that's the way to go if you need a different uniform value per vsg::VertexIndexDraw within the same vsg::StateGroup.

My next goal is to do the same thing with text. My plan is to create a new text shader based on the text.vert shader by adding the ColorBlendUniform the same way I did for handing triangles. However this seems a bit more complicated. In order to create the descriptor I need the pipeline layout which in my previous example I got from the vsg::GraphicsPipelineConfigurator. In this case the vsg::GraphicsPipelineConfigurator is created in the layout technique. So before I go down a rabbit hole I'm wondering if there is a better way to dynamically adjust the alpha of basic shapes and text?