OpenGL - Copy texture from screen smaller than that screen.

Question

I'm trying to capture the screen to a texture with a lower resolution than the screen itself (to render back onto the screen and create a blur/bloom effect), and it doesn't work quite well. I understand mipmaps can be used to do this, but I just can't get the correct sequence of commands to work.

My current code:

width=1024;
height=1024;

glGenTextures(1, &texture);

glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, texture);

glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glEnable (GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE,  GL_MODULATE);
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_FASTEST);
glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP );

glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 0, 0, width, height, 0);

// code for rendering the screen back on goes here

Answer 1

You can't capture and downfilter in one go. You have to capture the full screen to a larger texture first, then mipmaps should be created if auto-create mipmaps are enabled, then you can render one using that texture again, makig sure you adjust the mipmap level suitably.

However, that will look ugly, as the auto mipmapgeneration usually uses a box filter.

What I'd do is to set up some FBOs (Frame Buffer Objects) and GLSL shaders instead. That gives you finer control over all steps:

• create the original image in a texture
• apply some nice gaussian low-pass filtering
• blend the filtering with the original image to the frame buffer