Why does this OpenGL shader use texture coordinates beyond 1.0?

Question

I'm trying to get familiar with shaders in opengl. Here is some sample code that I found (working with openframeworks). The code simply blurs an image in two passes, first horizontally, then vertically. Here is the code from the horizontal shader. My only confusion is the texture coordinates. They exceed 1.

void main( void )
{
    vec2 st = gl_TexCoord[0].st;
    //horizontal blur
    //from http://www.gamerendering.com/2008/10/11/gaussian-blur-filter-shader/

    vec4 color;

    color += 1.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -4.0, 0));
    color += 2.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -3.0, 0));
    color += 3.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -2.0, 0));
    color += 4.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -1.0, 0));

    color += 5.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt , 0));

    color += 4.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 1.0, 0));
    color += 3.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 2.0, 0));
    color += 2.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 3.0, 0));
    color += 1.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 4.0, 0));

    color /= 5.0;
    gl_FragColor = color;
}

I can't make heads or tails out of this code. Texture coordinates are supposed to be between 0 and 1, and I've read a bit about what happens when they're greater than 1, but that's not the behavior I'm seeing (or I don't see the connection). blurAmnt varies between 0.0 and 6.4, so s can go from 0 to 25.6. The image just gets blurred more or less depending on the value, I don't see any repeating patterns.

My question boils down to this: what exactly is happening when the texture coordinate argument in the call to texture2DRect exceeds 1? And why does the blurring behavior still function perfectly despite this?

Answer 1

The [0, 1] texture coordinate range only applies to the GL_TEXTURE_2D texture target. Since that code uses texture2DRect (and a samplerRect), it's using the GL_TEXTURE_RECTANGLE_ARB texture target, that this target uses Unnormalized texture coordinates, in the range [0, width]x[0, height].

That's why you have "weird" texture coords. Don't worry, they work fine with this texture target.