Passing data through shaders

Question

How it possible to pass data through shaders, when using vertex, tess. control, tess. evaluation, geometry and fragment shaders. I've tried using interface block this way.

//vertex shaders
out VS_OUT { ... } vs_out;

Than I wrote this code in tessellation control shader:

in VS_OUT { ... } tc_in; 
out TC_OUT { ... } tc_out;

So, tesselation control shader called once for every vertex. Does it mean that tc_in must be not array but single variable. I'm not really sure about that because of sneaky gl_InvocationID.

Then things become tough. Tessellation evaluation shader. Something tells me that evaluation shader should take interface block as array.

in TC_OUT { ... } te_in[];
out TE_OUT { ...  } te_out[];

Moving to geometry shader. Geometry shader takes multiple vertices, so, definitely array interface block.

in TE_OUT  { ... } gs_in[];
out vec3 random_variable;
...
random_variable = gs_in[whatever_index];

Seems legit for me, but data didn't come to fragment shader.

I would appreciate any advice.

Answer 1

Tessellation control shaders take the vertices of a patch as modify them in some way, so their input and output is

in VS_OUT { ... } tc_in[]; 
out TC_OUT { ... } tc_out[];

The control shader gets called for every patch vertex (see which one by using gl_InvocationID), so you usually don't need any loops to implement it.

Tessellation evaluation shaders take these modified vertices and output a single vertex per invocation, thus we have

in TC_OUT { ... } te_in[];
out TE_OUT { ...  } te_out;

Geometry shaders take multiple vertices and may output a different number of vertices, but these are constructed explicitly using EmitVertex and EmitPrimitive, so there is only one output element that has to be filled before each call of EmitVertex:

in TE_OUT { ... } gs_in[];
out GS_OUT { ... } gs_out;

But don't forget to set glPosition in your geometry shader, otherwise OpenGL won't know where to place your vertices.

The interpolated values of gs_out are then passed to the fragment shader.