// chessboard — generated from https://github.com/gl-transitions/gl-transitions/blob/master/transitions/chessboard.glsl
// Author: lql
// License: MIT
// Description: A chessboard pattern of squares reveals the incoming image

uniform shader _from;
uniform shader _to;
uniform float2 _resolution;
uniform float progress;
uniform float ratio;

vec4 getFromColor(vec2 uv) { return vec4(unpremul(_from.eval(vec2(uv.x, 1.0 - uv.y) * _resolution))); }
vec4 getToColor(vec2 uv) { return vec4(unpremul(_to.eval(vec2(uv.x, 1.0 - uv.y) * _resolution))); }

uniform float grid_num; 

vec4 transition(vec2 uv) {
    vec2 st = uv * grid_num;
    vec2 idx = floor(st);
    vec2 grid = fract(st);

    vec4 a = getFromColor(uv);
    vec4 b = getToColor(uv);

    float checker = mod(idx.x + idx.y, 2.0);
    float mixFactor;

    if (progress <= 0.5) {
        mixFactor = (checker > 0.5) ? step(grid.x, progress * 2.0) : 0.0;
    } else {
        mixFactor = (checker < 0.5) ? step(grid.x, (progress - 0.5) * 2.0) : 1.0;
    }

    return mix(a, b, mixFactor);
}

half4 main(float2 _coord) {
  vec2 _uv = _coord / _resolution;
  vec4 _c = clamp(transition(vec2(_uv.x, 1.0 - _uv.y)), 0.0, 1.0);
  return half4(_c.rgb * _c.a, _c.a);
}