shadow2DRect problem

uniform sampler2DRectShadow shadow;

uniform int offset;
void main(){
vec3 r0 =;
r0.x = r0.x - float(offset);
float tmp = shadow2DRect(shadow, r0).x;
if (tmp < 0.5)
gl_FragColor = gl_Color;

r0keeps the fragment position ,then r0 is used as the texcoord parameter of shadow2DRect. However r0’s x and y component value can be negative,can texcoord be negative value?

Out of interest, what is this shader supposed to do? It’s not performing a ‘standard’ shadowmap comparison.

However r0’s x and y component value can be negative

 vec3 r0 =

I don’t see where r0.y is -ve.

r0.x = r0.x - float(offset);

r0.x could be -ve if gl_FragCoord.x is 0 and offset > 0.
So, it’s odd, and not legal either according to ARB_TEXTURE_RECTANGLE spec.

“the NPOTS texture image lies in a [0…w]x[0…h] range.”

and issues sections 13,14:

"13) How does the handling of the R texture component differ from
the handling of S and T?

 The R texture coordinate for rectangular textures is handled
 as it would be for standard two dimensional textures.  Thus the coordinates range from [0..1] and the wrapping mode is unchanged from the default."

" 14) Does this extension work with OpenGL 1.4’s shadow mapping?
Yes. The one non-obvious allowance to support OpenGL 1.4’s shadow mapping is that the R texture coordinate wrap mode remains UNCHANGED for rectangular textures. Clamping of the R texture coordinate for rectangular textures uses the standard [0,1] interval rather than the [0,w_s] or [0,h_s] intervals as in the case of S and T. This is because R represents a depth value in the [0,1] range whether using a 2D or rectangular texture."

The other part that’s odd about this code is that the 3rd texture coordinate is crucial for shadowmap comparison, but here the 3rd coord is given by glFragCoord.z. I can not belive for a second that that’s valid for a proper shadow map.

The code is one sample called Depth Peeling in the NVIDIA OpenGL SDK. The function of this shader is to discard the fragment which fail the GL_COMPARE_R_TO_TEXTURE .it use the fragment.xy to be the texcoord , so i think fragment.x or.y can be negative value ,so can it sample the shadowmap texture correctly?

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