/*Lanczos (6tap) scaling
  Copyright (C) 2010 Team XBMC
  http://www.xbmc.org
  Copyright (C) 2011 Stefanos A.
  http://www.opentk.com
  Copyright (C) 2011 BlackStar and DOLLS
  This Program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2, or (at your option)
  any later version.
  This Program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  GNU General Public License for more details.
*/

uniform sampler2D tex0;
uniform vec4 OGL2Size;
uniform vec4 OGL2InvSize;
uniform vec4 OGL2Param;

#define FIX(c) max(abs(c), 1e-5);

const float PI = 3.1415926535897932384626433832795;

vec3 weight3(float x)
{
    const float radius = 3.0;
    vec3 sample = FIX(2.0 * PI * vec3(x - 1.5, x - 0.5, x + 0.5));
    // Lanczos3. Note: we normalize outside this function, so no point in multiplying by radius.
    return /*radius **/ sin(sample) * sin(sample / radius) / (sample * sample);
}

vec3 pixel(float xpos, float ypos)
{
    return texture2D(tex0, vec2(xpos, ypos)).rgb;
}

vec3 line(float ypos, vec3 xpos1, vec3 xpos2, vec3 linetaps1, vec3 linetaps2)
{
    return
        pixel(xpos1.r, ypos) * linetaps1.r +
        pixel(xpos1.g, ypos) * linetaps2.r +
        pixel(xpos1.b, ypos) * linetaps1.g +
        pixel(xpos2.r, ypos) * linetaps2.g +
        pixel(xpos2.g, ypos) * linetaps1.b +
        pixel(xpos2.b, ypos) * linetaps2.b;
}

void main()
{
    vec2 stepxy = OGL2InvSize.xy;
    vec2 pos = gl_TexCoord[0].xy + stepxy * 0.5;
    vec2 f = fract(pos / stepxy);
    vec3 linetaps1   = weight3(0.5 - f.x * 0.5);
    vec3 linetaps2   = weight3(1.0 - f.x * 0.5);
    vec3 columntaps1 = weight3(0.5 - f.y * 0.5);
    vec3 columntaps2 = weight3(1.0 - f.y * 0.5);
    // make sure all taps added together is exactly 1.0, otherwise some
    // (very small) distortion can occur
    float suml = dot(linetaps1, vec3(1.0)) + dot(linetaps2, vec3(1.0));
    float sumc = dot(columntaps1, vec3(1.0)) + dot(columntaps2, vec3(1.0));
    linetaps1 /= suml;
    linetaps2 /= suml;
    columntaps1 /= sumc;
    columntaps2 /= sumc;
    vec2 xystart = (-2.5 - f) * stepxy + pos;
    vec3 xpos1 = vec3(xystart.x, xystart.x + stepxy.x, xystart.x + stepxy.x * 2.0);
    vec3 xpos2 = vec3(xystart.x + stepxy.x * 3.0, xystart.x + stepxy.x * 4.0, xystart.x + stepxy.x * 5.0);
    gl_FragColor = vec4(line(xystart.y                 , xpos1, xpos2, linetaps1, linetaps2) * columntaps1.r +
                        line(xystart.y + stepxy.y      , xpos1, xpos2, linetaps1, linetaps2) * columntaps2.r +
                        line(xystart.y + stepxy.y * 2.0, xpos1, xpos2, linetaps1, linetaps2) * columntaps1.g +
                        line(xystart.y + stepxy.y * 3.0, xpos1, xpos2, linetaps1, linetaps2) * columntaps2.g +
                        line(xystart.y + stepxy.y * 4.0, xpos1, xpos2, linetaps1, linetaps2) * columntaps1.b +
                        line(xystart.y + stepxy.y * 5.0, xpos1, xpos2, linetaps1, linetaps2) * columntaps2.b,1.0);
}
