Write positions to texture OpenGL/GLSL - c++

Question

I want to write the model-space vertex positions of a 3D mesh to a texture in OGL. Currently in order to write to a texture I set it to a fullscreen quad and write to it using a separate pass (based on tutorial seen here.)
The problem is that, from what I understand, I cannot pass more than one vertex list to the shader as the vertex shader can only be bound to one vertex list at a time, currently occupied by the screenspace quad.
Vertex Shader code:
layout(location = 0) in vec4 in_position;
out vec4 vs_position;
void main() {
vs_position = in_position;
gl_Position = vec4(in_position.xy, 0.0, 1.0);
}
Fragment Shader code:
in vec4 position; // coordinate in the screenspace quad
out vec4 outColor;
void main() {
vec2 uv = vec2(0.5, 0.5) * position.xy + vec2(0.5, 0.5);
outColor = ?? // Here I need my vertex position
}
Possible solution (?):
My idea was to introduce another shader pass before this to output the positions as r, g, b so that the position of the current texel can be retrieved from the texture (the only input format large enough to store many vertecies).
Although not 100% accurate, this image might give you an idea of what I want to do:
Model space coordinate map
Is there a way to encode the positions to the texture without using a fullscreen quad on the GPU?
Please let me know if you want to see more code.

Answer 1

Instead of generating the quad CPU side I would attach a geometry shader and create the quad there, that should free up the slot for your model-geometry to be passed in.
Geometry shader:
layout(points) in;
layout(triangle_strip, max_vertices = 4) out;
out vec2 texcoord;
void main()
{
gl_Position = vec4( 1.0, 1.0, 0.5, 1.0 );
texcoord = vec2( 1.0, 1.0 );
EmitVertex();
gl_Position = vec4(-1.0, 1.0, 0.5, 1.0 );
texcoord = vec2( 0.0, 1.0 );
EmitVertex();
gl_Position = vec4( 1.0,-1.0, 0.5, 1.0 );
texcoord = vec2( 1.0, 0.0 );
EmitVertex();
gl_Position = vec4(-1.0,-1.0, 0.5, 1.0 );
texcoord = vec2( 0.0, 0.0 );
EmitVertex();
EndPrimitive();
}