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// Based on http://www.reddit.com/r/programming/comments/losip/shader_toy/c2upn1e

uniform sampler2D source;
uniform lowp float qt_Opacity;
varying vec2 qt_TexCoord0;
uniform float radius;
uniform float diffractionIndex;
uniform float targetWidth;
uniform float targetHeight;
uniform float posX;
uniform float posY;

void main()
{
    float h = diffractionIndex * 0.5 * radius;
    vec2 targetSize = vec2(targetWidth, targetHeight);
    vec2 center = vec2(posX, posY);
    vec2 xy = gl_FragCoord.xy - center.xy;
    float r = sqrt(xy.x * xy.x + xy.y * xy.y);
    vec2 new_xy = r < radius ? xy * (radius - h) / sqrt(radius * radius - r * r) : xy;
    vec2 tc = (new_xy + center) / targetSize;
    tc.y = 1.0 - tc.y;
    gl_FragColor = qt_Opacity * texture2D(source, tc);
}