Confusion about maximum output from Geometry Shaders

Question

The OpenGL-Wiki states on the output limitations of geometry shaders:

The first limit, defined by GL_MAX_GEOMETRY_OUTPUT_VERTICES, is the maximum number that can be provided to the max_vertices output layout qualifier.

[...]

The other limit, defined by GL_MAX_GEOMETRY_TOTAL_OUTPUT_COMPONENTS is [...] the total number of output values (a component, in GLSL terms, is a component of a vector. So a float is one component; a vec3 is 3 components).

That's what the declarative part of my geometry shader looks like:

layout( triangles ) in;
layout( triangle_strip, max_vertices = 300 ) out;
out vec4 var1;

My vertex format only consists of 4 floats for position. So I believe to have the 4 components from the varying var1 plus the 4 components from the position, i.e. 8 in total.

I have queried the following values for the constants mentioned above:

GL_MAX_GEOMETRY_OUTPUT_VERTICES = 36320
GL_MAX_GEOMETRY_TOTAL_OUTPUT_COMPONENTS = 36321

With max_vertices set to 300, a total of 8*300 = 2400 components would be written. It is needless to say that this value is far below 36321 as well as the 300 of max_vertices is far below 36320. So everything should be okay, right?

However, when I build the shader, the linking fails:

error C6033: Hardware limitation reached, can only emit 128 vertices of this size

Can somebody explain to me what is going on and why this doesn't work as I expected?

theV0ID theV0ID · Accepted Answer · 2015-03-21T17:02:31

I made a really dumb mistake. For the record, if somebody else is having the same issue: Querying the values for GL_MAX_GEOMETRY_OUTPUT_VERTICES and GL_MAX_GEOMETRY_TOTAL_OUTPUT_COMPONENTS must be done through glGetInteger and not by just evaluating those macros.

Confusion about maximum output from Geometry Shaders

1 Answers