Geometry Shader Shenanigans

When I gave that talk a few weeks back I said, rather naively in hindsight, that geometry & tessellation shaders should work.. man was I wrong there. It turned out to be rather fiddly to find a balance that felt lispy, worked with my current analogies and worked across all GLSL version (or at least failed gracefully).

Lets start at the beginning.

Passing values between GLSL stages

To pass values from stage to stage in GLSL starts simply, you declare something as out in one stage and in in the next e.g:

out vec4 foo; // in the vertex shader
in vec4 foo; // in the fragment shader

Nice and simple. It also works for arrays of values.

Next we add a geometry (or tessellation) shader. Now we are working with primtives (or patches) so the outs from the last stage become an array of ins in the geometry stage.

out vec4 foo; // in the vertex shader
in vec4[2] foo; // in the geometry shader

The length of the array is dictate by the size of the primitive, so lines are length 2, triangles are length 3, etc.

But how about if the out was an array?

out vec4[10] foo; // in the vertex shader
in vec4[2][10] foo; // in the geometry shader

Simple right? I thought so and I updated my compiler to work with this. I kept having this nagging feeling though, something about interface blocks. Turns out I should have listened to the feeling sooner. Support for arrays of arrays only arrive in GLSL in v4.3 and before that you needed to use an interface block:

out VertexOuts
	vec4[10] foo;
out VertexOuts
	vec4[10] foo;
} gs_in[2];

Which seems ok, but it has subtleties to it that make this hard to abstract for all GLSL versions. Lets have a look at the lisp.

First a vertex shader:

(defun-g test-vert ((position :vec4) (uv :vec2))
  (values position normal))

the outputs from the above are a vec4 which is used as the gl-position and a vec2 which is passed to the next stage. Here is a valid fragment shader to go with this:

(defun-g test-frag ((uv :vec2) &uniform (tex :sampler-2d))
  (texture tex uv))

So far, so simple. Next let’s look at a geometry shader that would match that vertex shader’s outputs. We will assume we are rendering triangles.

(defun-g test-geom ((uv (:vec2 3)))

Makes sense right? We just array the inputs. However we know that our interface block is going to cause problems. In this case uv isn’t really vec2[3] it’s in blockName { vec2 }gs_in[3]. We have a mismatch between the abstraction and reality. However it’s a useful lie, it takes the GLSL behavior and makes things more consistent and thus easy to understand, so if we can keep it I’d prefer to.

The answer (at least for now) is to make an ‘ephemeral’ type, this is a type that can’t exists in GLSL for real, but one that our compiler can handle.

So this:

(defun-g test-geom ((uv (:vec2 3)))
  (let ((a uv)
        (index 1))
    (aref uv index)

becomes something like:

   vec2 uv;
} gs_in[3]

void main()
    int index = 1;

Notice that int index = 1 ended up in the GLSL but there is no a = <something>, that is because of that ephemeral type. The reason that it has to be ephemeral is that until we use aref we can’t access the uv slot inside the block. Also you cant write IN_BLOCK tmp = gs_in[1] or IN_BLOCK[3] tmp = gs_in as whilst interface blocks may look like structs, but they dont’ behave like them and those two example as simply illegal in GLSL. It’s a damn shame really as otherwise this would be much easier!

One other option we could have gone for instead of this ephemeral array business we could invent a ‘primitive’ type. So the geom shader could be:

(defun-g test-geom ((uv triangle))
  (let ((a uv)
        (index 1))
    (aref uv index)

But then test-geom is no longer a stand alone function, we would need to wait until test-geom was used in a pipeline to know what slots triangle contained. One of the beauties of CEPL is that all gpu functions are simply functions until they are used in a pipeline, they can be used as stages or just be called from other gpu functions. If we use triangle then we can’t compile this gpu function ahead of time, which would suck as that feature currently lets you find and fix bugs faster. On top of this triangle would also have to be ephemeral as there is no triangle type in GLSL

The same also goes for requiring the user to define the interface between vertex & geom shaders as structs. You still need the ephemeral & you require the user to repeat themselves.

This is one of those cases where every option is kinda sucky but the first one described feels the least sucky, and that is the one I have gone for.

Implement it

The above took a few days of experiments and pain to get ironed out, and then it needed to be implemented. As usual working in areas of the compiler that haven’t been touched for a while uncovered bugs and general weaknesses that needed fixes.

Another thing that was playing a lot on my mind was that one of the features CEPL has is support for is defining a stage as raw GLSL and using it in a pipeline, so whatever I made needed to not break that. Having users is awesome and I really want to make sure I don’t fuck up their workflow if I can help it. For example one person is already using the GLSL stage support in CEPL to use Tessellation shaders, which is something I’ve never used! The fact that they can do this makes me happy and eases the pain whilst I slowly find nice lispy ways of doing the same.

Oh, I almost forgot..


As we are in the world of Geometry & Tessellation we need to talk about primitives. One of the things I love about Varjo is that it can pass information from stage to stage so the user doesn’t have to write the same stuff multiple times, we should be able to do the same with primitives. I wanted to be able to take the OpenGL draw-mode and pass it through the compilation. This allows Varjo to not only write some of the GLSL automatically (e.g. layout(triangles) in; in the Geometry stage) but also to know the length of the array’d interface block going into the geometry stage. Having little details like this checked is worth my time as then Varjo can give much nicer and more targeted errors that GL can.

I’ll skip the details on this for now as this post is already getting long.

Where are we now?

We are here


I finally got these damn things working. Here we are using a geometry shader to draw the normals of the sphere.

There is still a bunch of stuff to do and there are some aspects that really pushed the limits of how ‘lispy’ stuff could be made. I’ll save that for another post. I’ll simply say thanks for reading and leave you with the pipeline for drawing the lines in the above.


This lisp code:

(defun-g normals-vert ((vert g-pnt) &uniform (model->clip :mat4))
  (values (* model->clip (v! (pos vert) 1))
          (s~ (* model->clip (v! (norm vert) 0)) :xyz)))

(defun-g normals-geom ((normals (:vec3 3)))
  (declare (varjo:output-primitive :kind :line-strip :max-vertices 6))
  (labels ((gen-line ((index :int))
             (let ((magnitude 0.2))
               (setf gl-position (gl-position (aref gl-in index)))
               (setf gl-position
                     (+ (gl-position (aref gl-in index))
                        (* (v! (aref normals index) 0f0)
    (gen-line 0)
    (gen-line 1)
    (gen-line 2)

(defun-g normals-frag ()
  (v! 1 1 0 1))

(def-g-> draw-normals ()
  :vertex (normals-vert g-pnt)
  :geometry (normals-geom (:vec3 3))
  :fragment (normals-frag))

makes this glsl:

("#version 450

in vec3 fk_vert_position;
in vec3 fk_vert_normal;
in vec2 fk_vert_texture;

    out vec3 _VERTEX_OUT_1;

uniform mat4 MODEL_62CLIP;

void main() {
    vec3 return1;
    vec4 g_G1550 = (MODEL_62CLIP * vec4(fk_vert_position,float(1)));
    return1 = (MODEL_62CLIP * vec4(fk_vert_normal,float(0))).xyz;
    gl_Position = g_G1550;
    vec3 g_G1552 = return1;
    _VERTEX_OUT_1 = g_G1552;
#version 450

layout (triangles) in;

    in vec3 _VERTEX_OUT_1;
} inputs[3];

layout (line_strip, max_vertices = 6) out;

void GEN_LINE(int INDEX);

void GEN_LINE(int INDEX) {
    float MAGNITUDE = 0.2f;
    gl_Position = gl_in[INDEX].gl_Position;
    gl_Position = (gl_in[INDEX].gl_Position + (vec4(inputs[INDEX]._VERTEX_OUT_1,0.0f) * MAGNITUDE));

void main() {
#version 450

layout(location = 0) out vec4 _FRAGMENT_OUT_0;

void main() {
    vec4 g_G1553 = vec4(float(1),float(1),float(0),float(1));
    _FRAGMENT_OUT_0 = g_G1553;

Published: April 18 2017

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