GLSL: unable to read texture from a FBO and render to another FBO with fragment shader

Question

I'm trying to "read" the texture attached to a first FBO (fboA), modifying it (with fragment shader) and render to a second FBO (fboB).

I'm not able to figure it out, all I got is a black or white texture.

Here is the code:

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboB);
glEnable(GL_TEXTURE_RECTANGLE_ARB);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, textureFromFboA);

GLuint t1Location = glGetUniformLocation(shaderProgram, "texture");
glUniform1i(t1Location, 0);

glUseProgram(shaderProgram);

glBegin(GL_QUADS);

glTexCoord2f(0.0f, 0.0f);
glVertex2f (0.0f, 0.0f);

glTexCoord2f(imageRect.size.width, 0.0f); 
glVertex2f (imageRect.size.width, 0.0f);

glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (imageRect.size.width, imageRect.size.height);

glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f,imageRect.size.height);

glEnd();
glDisable(GL_TEXTURE_RECTANGLE_ARB);
glFlush();

glUseProgram(0);

This is the fragment shader code:

#version 120

uniform sampler2D texture;

void main(void)
{
    gl_FragColor = texture2D(texture, gl_TexCoord[0].st) * 0.8;
}

I would expect to have a darker texture rendered in fboB but I only get an all black texture. This happens also if I write gl_FragColor = texture2D(texture, gl_TexCoord[0].st);.

On the contrary, if I write gl_FragColor = vec2(1.0, 0.0, 0.0, 1.0); I correctly have an all red texture as output.

If I comment out the glUseProgram() statement, the code works fine and texture in fboB is an exact copy of texture in fboA.

Why this happens? Am I missing something?

Answer 1

uniform sampler2D texture;

Rectangle textures are not the same texture type as 2D textures. Yes, they're two-dimensional, but they still maintain a distinct texture type. Therefore, they cannot be accessed via a sampler2D.

So change that to a samplerRect. You will also need to use proper texture coordinates, because rectangle textures take texel-space coordinates instead of normalized coordinates.

Alternatively, you can just use a 2D texture. NPOT textures have been around for well over half a decade; you don't have to use rectangle textures to have non-power-of-two render targets.