Question

In my program it is necessary for me to do off-screen rendering. For that purpose I use a FBO. In order to see if the image I draw is the correct for testing purposes I copy it from the FBO to a texture then render the texture to a quad. The problem is that when I copy from the FBO into the texture and render it the image appears dark/get a black color but the shapes are correct. I have tried using a texture as attachment in the FBO and rendering it directly (without copying it into another texture) and the colors are correct.
Below is the code for texture creation

//initial texture which works when rendered to a quad  
glGenTexturesEXT(3, &textureID[0]);
glBindTextureEXT(GL_TEXTURE_2D, textureID[0]);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); 
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 600, 512, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
//second one which should be a copy of the above but has tha dark color mentioned
glBindTextureEXT(GL_TEXTURE_2D, textureID[1]);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); // automatic mipmap 
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 600, 512, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glBindTextureEXT(GL_TEXTURE_2D, 0);
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_2D,textureID[0],0);
  //Attach depth buffer to FBO
glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, depth_rb);
st1=glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);

Now in the rendering function

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fb); //biding FBO
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
 //render code
unsigned char *pixels= new unsigned char [600*512*4];
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,0);  //unbiding
glBindTextureEXT(GL_TEXTURE_2D,textureID[1]); //if I change to textureID[0] result is fine
glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA8,600,512,0,GL_RGBA,GL_UNSIGNED_BYTE,pixels);
glBindTextureEXT(GL_TEXTURE_2D,0);
glViewport(0, 0, 600, 512);
glClearColor(1.0,1.0,1.0,1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glBindTextureEXT(GL_TEXTURE_2D, textureID[1]);
  //setting the correct matrixes + render a quad
 //before rendering a quad I set the color with
glColor3f(1.0, 1.0, 1.0);
delete[] pixels;
glutSwapBuffers();

I’m using glut for the setup. I have tried other functions such as glGetTexImage2D after unbiding the FBO and biding the texture as an alternative to glReadPixels(…) but with no success.

Answer 1

I don’t understand this call to glTexImage2D in your second code snippet. pixels will contain just garbage. What do you expect it to do?

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Textures have been core OpenGL for a very, very long time. It’s just glGenTextures and glBindTexture not ...EXT. Also I recommend to either use ...ARB versions of the FBO functionality, or just using OpenGL core framebuffer support (of later OpenGL versions).