// RozK
// Fisheye removal for GoPro 11+, 8:7 ratio, without hypersmooth
// Adapted from https://github.com/duducosmos/defisheye
// itself based on http://www.fmwconcepts.com/imagemagick/defisheye/index.php

#extension GL_ARB_texture_rectangle: enable

// TODO: investigate
// precision highp float;

// uniforms
uniform sampler2DRect myTextureY;
uniform sampler2DRect myTextureU;
uniform sampler2DRect myTextureV;
uniform vec2 myResolution;
uniform float pts;

// parameters
const float input_fov = 156.0;
const float output_fov = 124.45;
const vec2 pixel_scale = vec2(0.652485, 1.0);
const int subsampling = 4;

// subsampling constants
const float substep = 1.0 / float(subsampling);
const float substart = substep * 0.5 - 0.5;
const float subscale = 1.0 / float(subsampling * subsampling);

// variables
vec2 center;
float diameter;
float input_len;
float inv_output_len;

vec4 unfish(const in vec2 coord) {
    float len = max(0.001, length(coord));
    vec2 y_coord = center + coord * ((input_len / len) * atan(len * inv_output_len));
    vec2 uv_coord = y_coord * 0.5;
    return vec4(
        texture2DRect(myTextureY, y_coord).r,
        texture2DRect(myTextureU, uv_coord).r,
        texture2DRect(myTextureV, uv_coord).r,
        1.0
    );
}

void main() {
    center = myResolution * 0.5;
    diameter = length(myResolution);
    input_len = diameter / radians(input_fov);
    inv_output_len = (2.0 * tan(radians(output_fov * 0.5))) / diameter;

    vec2 coord = gl_TexCoord[0].xy - center;
    vec4 pixel = vec4(0.0, 0.0, 0.0, 0.0);

    float x, y = substart;
    for (int column = 0; column < subsampling; column++, y += substep) {
        x = substart;
        for (int row = 0; row < subsampling; row++, x += substep) {
            pixel += unfish((coord + vec2(x, y)) * pixel_scale);
        }
    }

    gl_FragColor = pixel * subscale;
}