When we created the pipeline builder, we completely skipped over the blending logic, setting it as no blending. Blending is used for transparent objects and some graphical effects, so its important to have it. for that reason, we will make the rectangle we rendered last article transparent.

Blending in the pipeline

We cant really control blending from shaders, thats property of the pipeline. The gpu hardware itself does the blending mathematics for us, and has a bunch of options. We will be adding 2 new blending modes into the pipeline builder, one will be additive blending, where it just adds the colors, and the other alpha-blend, where it would mix the colors.

Lets add these 2 functions into the pipeline builder

void PipelineBuilder::enable_blending_additive()
{
    _colorBlendAttachment.colorWriteMask = VK_COLOR_COMPONENT_R_BIT | VK_COLOR_COMPONENT_G_BIT | VK_COLOR_COMPONENT_B_BIT | VK_COLOR_COMPONENT_A_BIT;
    _colorBlendAttachment.blendEnable = VK_TRUE;
    _colorBlendAttachment.srcColorBlendFactor = VK_BLEND_FACTOR_SRC_ALPHA;
    _colorBlendAttachment.dstColorBlendFactor = VK_BLEND_FACTOR_ONE;
    _colorBlendAttachment.colorBlendOp = VK_BLEND_OP_ADD;
    _colorBlendAttachment.srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE;
    _colorBlendAttachment.dstAlphaBlendFactor = VK_BLEND_FACTOR_ZERO;
    _colorBlendAttachment.alphaBlendOp = VK_BLEND_OP_ADD;
}

void PipelineBuilder::enable_blending_alphablend()
{
    _colorBlendAttachment.colorWriteMask = VK_COLOR_COMPONENT_R_BIT | VK_COLOR_COMPONENT_G_BIT | VK_COLOR_COMPONENT_B_BIT | VK_COLOR_COMPONENT_A_BIT;
    _colorBlendAttachment.blendEnable = VK_TRUE;
    _colorBlendAttachment.srcColorBlendFactor = VK_BLEND_FACTOR_SRC_ALPHA;
    _colorBlendAttachment.dstColorBlendFactor = VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
    _colorBlendAttachment.colorBlendOp = VK_BLEND_OP_ADD;
    _colorBlendAttachment.srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE;
    _colorBlendAttachment.dstAlphaBlendFactor = VK_BLEND_FACTOR_ZERO;
    _colorBlendAttachment.alphaBlendOp = VK_BLEND_OP_ADD;
}

When setting blending options in vulkan, we need to fill the formula on both color and alpha. The parameters work the same on both color and alpha. The formula works like this

outColor = srcColor * srcColorBlendFactor <op> dstColor * dstColorBlendFactor;

There are a few possible operators, but for the most part we will be doing VK_BLEND_OP_ADD, as its the most basic and guaranteed to work. There are many other much more advanced operators, but those come with extensions and we wont use them. Source (from srcColor and blend factor) refers to the color we are processing on our pipeline, and Destination (dstColor and blend factor) is the current value of the image we are rendering into.

With the formula above, lets explain what the addition blending does.

VK_BLEND_FACTOR_ONE sets the blend factor to just 1.0, so no multiplying. VK_BLEND_FACTOR_SRC_ALPHA on the other hand multiplies it by the alpha of the source. Our blending ends up as this formula

outColor = srcColor.rgb * srcColor.a + dstColor.rgb * 1.0

The alpha-blend one will look like this instead.

outColor = srcColor.rgb * srcColor.a + dstColor.rgb * (1.0 - srcColor.a)

Essentially making it into a lerp controlled by srcColor alpha, which will be from our shader.

Lets try to use it to see what it does. We dont have alpha set in our shaders, so lets just try the additive one. Change the blending on the init_mesh_pipeline() function.

//pipelineBuilder.disable_blending();
pipelineBuilder.enable_blending_additive();

You should now see the monkey mesh as semi-transparent, letting the color below it show.Play around with the blending modes to see what effects they result on.

Before we move to chapter 4, lets implement window resizing.

Next: Window Resizing