Passing vertex attributes with layouts in geometry shader - opengl

Question

Let's say we have a GL program consisting of the following vertex, geometry and fragment shaders.
vertex:
#version 410
layout (location = 0) in vec2 pos;
layout (location = 1) in vec2 size;
layout (location = 2) in float rot;
layout (location = 3) in vec4 color;
layout (location = 0) out vec2 p;
layout (location = 1) out vec2 s;
layout (location = 2) out float r;
layout (location = 3) out vec4 c;
void main(){
gl_Position = vec4(pos,0.0,1.0);
p = pos;
s = size;
r = rot;
c = color;
}
geometry:
#version 410
layout (points) in;
layout (triangle_strip, max_vertices = 4) out;
layout (location = 0) in vec2 pos;
layout (location = 1) in vec2 size;
layout (location = 2) in float rot;
layout (location = 3) in vec4 color;
layout (location = 0) out vec2 p;
layout (location = 1) out vec2 s;
layout (location = 2) out float r;
layout (location = 3) out vec4 c;
void main()
{
gl_Position = gl_in[0].gl_Position + vec4(-0.2, -0.2, 0.0, 0.0); // 1:bottom-left
EmitVertex();
gl_Position = gl_in[0].gl_Position + vec4( 0.2, -0.2, 0.0, 0.0); // 2:bottom-right
EmitVertex();
gl_Position = gl_in[0].gl_Position + vec4(-0.2, 0.2, 0.0, 0.0); // 3:top-left
EmitVertex();
gl_Position = gl_in[0].gl_Position + vec4( 0.2, 0.2, 0.0, 0.0); // 4:top-right
EmitVertex();
EndPrimitive();
p = pos;
s = size;
r = rot;
c = color;
}
fragment:
#version 410
layout (location = 0) in vec2 pos;
layout (location = 1) in vec2 size;
layout (location = 2) in float rot;
layout (location = 3) in vec4 color;
out vec4 FragColor;
void main()
{
FragColor = color;
}
The program takes points and display them as quads.
What I'm trying to do here is to pass the 4 vertex attributes (pos, size, rot and color) to the fragment shader through the geometry shader.
If I remove the geometry shader from my program, it manages to pass the vertex attributes to the fragment shaders and it displays colored dots.
If I keep the geometry shader and remove the layouts (in and out), it displays black quads.
Am I missing something here?

Answer 1

There are a couple of issues in your code. I've renamed all of the variables so you can see what's going on (This probably wouldn't be a massive problem, since you are using layout qualifiers, but I'll stick with a naming that makes it obvious what's happening).
#version 410
// per-vertex attributes from the vertex buffers
layout (location = 0) in vec2 vs_pos;
layout (location = 1) in vec2 vs_size;
layout (location = 2) in float vs_rot;
layout (location = 3) in vec4 vs_color;
// per-vertex inputs to the geometry shader
layout (location = 0) out vec2 gs_pos;
layout (location = 1) out vec2 gs_size;
layout (location = 2) out float gs_rot;
layout (location = 3) out vec4 gs_color;
void main(){
gl_Position = vec4(vs_pos,0.0,1.0);
// pass vars from vertex shader, to geometry shader
gs_pos = vs_pos;
gs_size = vs_size;
gs_rot = vs_rot;
gs_color = vs_color;
}
#version 410
layout (points) in;
layout (triangle_strip, max_vertices = 4) out;
// when passed to the geometry shader, these variables are now arrays!
// be sure to declare them as such...
layout (location = 0) in vec2 gs_pos[];
layout (location = 1) in vec2 gs_size[];
layout (location = 2) in float gs_rot[];
layout (location = 3) in vec4 gs_color[];
// the outputs to the fragment shader are presented as
// single attributes though
layout (location = 0) out vec2 fs_pos;
layout (location = 1) out vec2 fs_size;
layout (location = 2) out float fs_rot;
layout (location = 3) out vec4 fs_color;
void main()
{
// pass vars for 1st vertex generated
// (you may be able to get away with setting these once prior
// to the first EmitVertex call, not sure, it's been a while!)
fs_pos = gs_pos[0];
fs_size = gs_size[0];
fs_rot = gs_rot[0];
fs_color = gs_color[0];
gl_Position = gl_in[0].gl_Position + vec4(-0.2, -0.2, 0.0, 0.0);
EmitVertex(); //< emits all params above!
// pass vars for 2nd vertex generated, etc...
fs_pos = gs_pos[0];
fs_size = gs_size[0];
fs_rot = gs_rot[0];
fs_color = gs_color[0];
gl_Position = gl_in[0].gl_Position + vec4( 0.2, -0.2, 0.0, 0.0);
EmitVertex(); //< emits all params above!
fs_pos = gs_pos[0];
fs_size = gs_size[0];
fs_rot = gs_rot[0];
fs_color = gs_color[0];
gl_Position = gl_in[0].gl_Position + vec4(-0.2, 0.2, 0.0, 0.0);
EmitVertex(); //< emits all params above!
fs_pos = gs_pos[0];
fs_size = gs_size[0];
fs_rot = gs_rot[0];
fs_color = gs_color[0];
gl_Position = gl_in[0].gl_Position + vec4( 0.2, 0.2, 0.0, 0.0);
EmitVertex(); //< emits all params above!
EndPrimitive();
}
And finally the geometry shader.
#version 410
// the inputs to the fragment shader would have been interpolated
// across the geometry primitive emitted by the GS.
layout (location = 0) in vec2 fs_pos;
layout (location = 1) in vec2 fs_size;
layout (location = 2) in float fs_rot;
layout (location = 3) in vec4 fs_color;
out vec4 FragColor;
void main()
{
FragColor = fs_color;
}