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?

Your code doesn't match the page you linked to. What's the definition of texture2DRect? — Ben Voigt
– Ben Voigt, Commented Jul 18, 2011 at 15:58
it's an opengl shader thing. texture2DRect is just a function that accesses the pixel information from an underlying texture, given the st coordinates. The page I linked to was actually a comment in the code describing the blurring algorithm and is not directly related to my question, which is "why does the blurring algorithm still work despite texture coordinates greater than 1?" — Alan
– Alan, Commented Jul 18, 2011 at 16:02
I understand what it's supposed to be, more or less, but it doesn't appear to be a GLSL built-in (at least my OpenGL 4.1 Reference Card doesn't include it). So I thought maybe it's a user-defined wrapper, and there could be some scaling inside the wrapper. — Ben Voigt
– Ben Voigt, Commented Jul 18, 2011 at 16:13
apparently, I discovered it's an extension of opengl stackoverflow.com/questions/6736531/… — Alan
– Alan, Commented Jul 19, 2011 at 15:33

Alan · Accepted Answer · 2011-07-19 16:20:33Z

8

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.

edited Jul 19, 2011 at 16:20

Alan

3,0574 gold badges26 silver badges28 bronze badges

answered Jul 18, 2011 at 18:13

Dr. Snoopy

56.5k7 gold badges148 silver badges161 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Alan Over a year ago

Thanks Matias, I also asked a related question that links to documentation on rectangular textures, for future reference stackoverflow.com/q/6736531/770572

user244795 · Accepted Answer · 2011-07-18 15:55:22Z

-1

Depends on the host code. If you saw something like

glTexParameteri (GL_TEXTURE_3D, GL_TEXTURE_WRAP_S, GL_CLAMP);

Then the out of bounds s dimension will be zeros, IIRC. Similar for t.

answered Jul 18, 2011 at 15:55

user244795

7286 silver badges14 bronze badges

1 Comment

Alan Over a year ago

As s dimension proceeds from 1 all the way to 6.4, the image just gets more and more blurry, which is not the behavior I would expect if it was just zero-ing out. Any ideas?

Collectives™ on Stack Overflow

Why does this OpenGL shader use texture coordinates beyond 1.0?

2 Answers 2

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related