Passthrough Geometry Shader - geometry

Question

I'm trying to build a simple passthrough geometry shader, but
I cant make it work with
glDrawElements(GL_TRIANGLES, fooSize, GL_UNSIGNED_INT, NULL);
but it does work with
glDrawArrays(GL_TRIANGLES, 0, foo_INDEX);.
the geometry shader is..
#version 400
#extension GL_EXT_geometry_shader4 : enable
layout(triangles) in;
layout(triangle_strip, max_vertices = 3) out;
void main() {
for(int i = 0; i < gl_VerticesIn; i++) {
gl_Position = gl_PositionIn[i];
EmitVertex();
}
EndPrimitive();
}
So, someone has any idea why this geometry shader works with drawArrays and doesnt work with drawElements? Please.

Answer 1

You have written a very confused and confusing geometry shader.
Your first line indicates that you're using GLSL version 4.00. This language has geometry shaders as a core feature; no extensions required. Your second line then enables the EXT_geometry_shader4 extension.
The problem comes from the fact that core geometry shaders and EXT_geometry_shader4 are very different things.
For example, there is no gl_VerticesIn in core geometry shaders. The number of vertices you get is defined by your layout() in; statement. You used triangles, so the answer is 3. You can also find it by using gl_in.length(), where gl_in is an interface block array containing the default vertex input values. Similarly, gl_PositionIn doesn't exist in core geometry shaders; you use gl_in[index].gl_Position.
So your geometry shaders code seems to be designed for EXT_geometry_shader4... except for these lines:
layout(triangles) in;
layout(triangle_strip, max_vertices = 3) out;
These are pure core geometry shaders; in EXT_geometry_shader4, these parameters are specified at link time in OpenGL code, not in the shader itself.
So you've thoroughly confused the compiler by having half of it use the EXT version and half use the core version.
A proper core version of what you're trying to do would be this:
#version 400
layout(triangles) in;
layout(triangle_strip, max_vertices = 3) out;
void main() {
for(int i = 0; i < 3; i++) { // You used triangles, so it's always 3
gl_Position = gl_in[i].gl_Position;
EmitVertex();
}
EndPrimitive();
}
I have no idea if this will fix your problem, but these are problems with your shader.