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varying vec2 pos;

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uniform sampler2D background;

uniform sampler2D foreground;

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uniform sampler2D noiseTexture;

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uniform float t; // transition value, between 0-1

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const vec4 colourHigh = vec4(0., 0., 0., 1.);

const vec4 colourLow = vec4(1., 0., 0., 1.);

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const float start = 0.05;

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void main() {

  vec2 newPos = pos;

  // flip y axis

  newPos.y = 1. - newPos.y;

  // read colour from images

  vec4 backgroundCol = texture2D(background, newPos);

  vec4 foregroundCol = texture2D(foreground, newPos);

  float noise = texture2D(noiseTexture, newPos/2.).r;

  float height = (noise * (1. - start) + start) - t;

  float useForeground = step(start, height); // 1 if height > burn cutoff

  float useBackground = 1. - step(0., height); // 1 if height < burn cutoff

  float colourT = smoothstep(0., start, height); // between 0 - 1 when between cuttoffs

  float useTransition = (1. - useForeground) * (1. - useBackground);

  gl_FragColor = useBackground * backgroundCol + useForeground * foregroundCol + useTransition * mix(colourLow, colourHigh, colourT);

}

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