luminance, episode 0.6: UBO, SSBO, Stackage.

Up to now, luminance has been lacking two cool features: UBO and SSBO. Both are buffer-backed uniform techniques. That is, a way to pass uniforms to shader stages through buffers.

The latest version of luminance has one of the two features. UBO were added and SSBO will follow for the next version, I guess.

What is UBO?

UBO stands for Uniform Bbuffer Object. Basically, it enables you to create uniform blocks in GLSL in feed them with buffers. Instead of passing values directly to the uniform interface, you just write whatever values you want to to buffers, and then pass the buffer as a source for the uniform block.

Such a technique has a lot of advantages. Among them, you can pass a lot of values. It’s also cool when you want to pass values instances of a structure (in the GLSL source code). You can also use them to share uniforms between several shader programs as well as quickly change all the uniforms to use.

In luminance, you need several things. First thing first, you need… a buffer! More specifically, you need a buffer Region to store values in. However, you cannot use any kind of region. You have to use a region that can hold values that will be fetched from shaders. This is done with a type called UB a. A buffer of UB a can be used as UBO.

Let’s say you want to store colors in a buffer, so that you can use them in your fragment shader. We’ll want three colors to shade a triangle. We need to create the buffer and get the region:

colorBuffer :: Region RW (UB (V3 Float)) <- createBuffer (newRegion 3)

The explicit type is there so that GHC can infer the correct types for the Region. As you can see, nothing fancy, except that we just don’t want a Region RW (V3 Float but Region RW (UB (V3 Float)). Why RW?

Then, we’ll want to store colors in the buffer. Easy peasy:

writeWhole colorBuffer (map UB colors)

colors :: [V3 Float]
colors = [V3 1 0 0,V3 0 1 0,V3 0 0 1] -- red, green, blue

At this point, colorBuffer represents a GPU buffer that holds three colors: red, green and blue. The next part is to get the uniform interface. That part is experimental in terms of exposed interface, but the core idea will remain the same. You’re given a function to build UBO uniforms as you also have a function to build simple and plain uniforms in createProgram:

createProgram shaderList $ \uni uniBlock -> {- … -}

Don’t spend too much time reading the signature of that function. You just have to know that uni is a function that takes Either String Natural – either a uniform’s name or its integral semantic – and gives you mapped U in return and that uniBlock does the same thing, but for uniform blocks instead.

Here’s our vertex shader:

in vec2 co;
out vec4 vertexColor;

// This is the uniform block, called "Colors" and storing three colors
// as an array of three vec3 (RGB).
uniform Colors {
  vec3 colors[3];
};

void main() {
  gl_Position = vec4(co, 0., 1.);
  vertexColor = vec4(colors[gl_VertexID], 1.);
}"

So we want to get a U a mapped to that "Colors" uniform block. Easy!

(program,colorsU) <- createProgram shaderStages $ \_ uniBlock -> uniBlock "Colors"

And that’s all! The type of colorsU is U (Region rw (UB (V3 Float))). You can then gather colorBuffer and colorsU in a uniform interface to send colorBuffer to colorsU!

You can find the complete sample here.

Finally, you can augment the type you can use UB with by implementing the UniformBlock typeclass. You can derive the Generic typeclass and then use a default instance:

data MyType = {- … -} deriving (Generic)

instance UniformBlock MyTpe -- we’re good to go with buffer of MyType!

luminance, luminance-samples and Stackage

I added luminance and luminance-samples into Stackage. You can then find them in the nightly snapshots and the future LTS ones.

What’s next?

I plan to add stencil support for the framebuffer, because it’s missing and people might like it included. I will of course add support for *SSBO** as soon as I can. I also need to work on cheddar but that project is complex and I’m still stuck with design decisions.

Thanks for reading my and for your feedback. Have you great week!