I'd like to be able to pass an arbitrary number of varying values per vertex from the vertex shader to the geometry shader. I know that OpenGL has no dynamic arrays, so the number should be specified at compile time. The whole thing should run on an Apple MacBook with a NVIDIA GeForce 9400M graphics card and a driver that only offers OpenGL 2.1, along with some extensions.
The problem here seems to be that the geometry shader takes its input in the form or an array with one element per vertex. As far as I can tell, there are no arrays of arrays available in my setup, and no arrays of interface blocks containing arrays either. So far, the best solution I could come up with is specifying a number of variables to pass this information, extracted from an array in the vertex shader and turned back into an array with a certain stride length in the geometry shader. That way, access to the values can still be performed using computed indices.
Is there a better, more elegant way?
User-defined varying variables can be declared as arrays in the vertex shader. This means that those, on input to the geometry shader, must be declared as two-dimensional arrays. See sections 4.3.6 and 7.6 of the OpenGL Shading Language Specification for more information.
For example, in the vertex shader, you may specify
varying vec2 value;
and in the geometry shader, this becomes a two-dimensional array, e.g. with triangles as input primitives
varying in vec2 value;
Note the counterintuitive order of array indices! Also beware that the array dimensions must be specified explicitly, using an integer constant. Using a non-constant integer variable or gl_VerticesIn yields a compiler error. Both remarks have been tested on the very MacBook Pro model mentioned in the question.