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/* Shader for applying a bias and scale factor to a rectangle texture during drawing.
* Can be used to apply f(g)=(g + prescaleoffset)*scalefactor + postscaleoffset;
* After remapping, the gradient image of the rescaled image is computed by taking
* central derivatives in horizontal and vertical direction.
*
* Input: Intensity image in RED channel of texture.
* Output: RED channel = gradient dx.
* GREEN channel = gradient dy.
* BLUE channel = Remapped intensity image whose gradient was computed.
*
* (w)2006 by Mario Kleiner. Licensed under MIT license.
*/
#extension GL_ARB_texture_rectangle : enable
/* We currently only support rectangular textures: */
uniform sampler2DRect Image;
/* Input parameter constants to specify pre- post-scale bias and scaling factor:*/
uniform float prescaleoffset;
uniform float postscaleoffset;
uniform float scalefactor;
void main()
{
float v1, v2;
vec2 incolor;
/* Need to lookup two texture values for output position dx(x,y) == (s,t): */
v1 = texture2DRect(Image, gl_TexCoord[0].st + vec2(-1,0)).r;
v2 = texture2DRect(Image, gl_TexCoord[0].st + vec2(+1,0)).r;
/* Remap: */
v1 = ((v1 + prescaleoffset) * scalefactor) + postscaleoffset;
v2 = ((v2 + prescaleoffset) * scalefactor) + postscaleoffset;
/* Derivative dx into RED channel: */
gl_FragColor.r = 0.5 * (v2 - v1);
/* Need to lookup two texture values for output position dy(x,y) == (s,t): */
v1 = texture2DRect(Image, gl_TexCoord[0].st + vec2(0,-1)).r;
v2 = texture2DRect(Image, gl_TexCoord[0].st + vec2(0,+1)).r;
/* Remap: */
v1 = ((v1 + prescaleoffset) * scalefactor) + postscaleoffset;
v2 = ((v2 + prescaleoffset) * scalefactor) + postscaleoffset;
/* Derivative dy into GREEN channel: */
gl_FragColor.g = 0.5 * (v2 - v1);
/* Remapped input intensity into BLUE channel: */
incolor.rg = texture2DRect(Image, gl_TexCoord[0].st).ra;
gl_FragColor.b = ((incolor.r + prescaleoffset) * scalefactor) + postscaleoffset;
/* Just pass-through the alpha value, do not modify it: */
gl_FragColor.a = incolor.g;
/* Done. */
}
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