OpenGL Simple Blur effect

Question

I'm trying to put some blur around a sun, to give it a glow effect.

I'm using the method here:

http://nehe.gamedev.net/tutorial/radial_blur__rendering_to_a_texture/18004/

Basically, I draw a sphere, it takes the current buffer and grabs it as a texture and redraws it a number of times, stretching it each time.

This works great when the object is in the centre of the screen. However, I want to have it offset to say, the top right.

This is the main part I believe that needs adjusting:

glColor4f(1.0f, 1.0f, 1.0f, alpha);                 // Set The Alpha Value (Starts At 0.2)
            glTexCoord2f(0+spost,1-spost);                      // Texture Coordinate   ( 0, 1 )
            glVertex2f(0,0);                                    // First Vertex     (   0,   0 )

            glTexCoord2f(0+spost,0+spost);                      // Texture Coordinate   ( 0, 0 )
            glVertex2f(0,480);                                  // Second Vertex    (   0, 480 )

            glTexCoord2f(1-spost,0+spost);                      // Texture Coordinate   ( 1, 0 )
            glVertex2f(640,480);                                // Third Vertex     ( 640, 480 )

            glTexCoord2f(1-spost,1-spost);                      // Texture Coordinate   ( 1, 1 )
            glVertex2f(640,0);

For the life of me though, I can't work out how to offset it each time so that the blurred images are not offset to the right. I understand that the whole screen is being captured, but there must be a way to offset this when the texture is drawn.....

How?

Answer 1

This maybe isn't a direct answer, to your question, but you shouldn't focus too much on effects via the fixed pipeline. NeHE tutorials are good, but a bit outdated. I recommend you just skim through the basics and incorporate shaders in your code. They're much faster and will allow you for creating much more complex effects easier.

Answer 2

Generally, if you want to scale around a point (sx,sy), you need to translate your world so that (sx,sy) is at the origin, do your scaling, and translate in the reverse so that (sx, sy) is back where it was originally.