Opengl GLSL render to texture

Question

I'm trying to render to texture with OpenGL + GLSL shaders. For start I'm trying to fill every pixel of 30x30 texture with white color. I'm passing to vertex shader index from 0 to 899, representing each pixel of texture. Is this correct? Vertex shader:

flat in int index;

void main(void) {
gl_Position = vec4((index % 30) / 15 - 1, floor(index / 30) / 15 - 1, 0, 1); 
}

Fragment shader:

out vec4 color;


void main(void) {
color = vec4(1, 1, 1, 1);
}

derhass derhass · Accepted Answer · 2013-09-28T22:40:59

You are trying to render 900 vertices, with one vertex per pixel? Why are you doing that? What primitive type are you using. It would only make sense if you were using points, but then you would need some slight modification of the output coordinates to actually hit the fragment centers.

The usual way for this is to render just a quad (easily represented as a triangle strip with just 4 vertices) which is filling the whole framebuffer. To achieve this, you just need to setup the viewport to the full frambeuffer and render are quad from (-1,-1) to (1,1).

Note that in both approaches, you don't need vertex attributes. You could just use gl_VertexID (directly as replacement for index in your approach, or as aan index to a const array of 4 vertex coords for the quad).

Opengl GLSL render to texture

1 Answers