#ifdef GL_ES
precision mediump float;
precision mediump int;
#endif

uniform sampler2D texture;
varying vec4 vertTexCoord;

struct ColorStop {
  float percent;
  vec4 color;
};

ColorStop[] gradient = ColorStop[4] (
  ColorStop(0.0, vec4(0.5, 1.0, 0.0, 1.0)),
  ColorStop(0.25, vec4(1.0, 0.5, 0.0, 1.0)),
  ColorStop(0.5, vec4(1.0, 1.0, 0.0, 1.0)),
  ColorStop(1.0, vec4(0.0, 0.0, 1.0, 1.0))
);

vec4 eval(ColorStop[4] grd, float t) {
  /* Logic from Java Gradient class translated,
   with preferred easing function and color mode. */
}

void main(void) {
  vec4 pxl = texture2D(texture, vertTexCoord.xy);
  float bri = (pxl.x + pxl.y + pxl.z) / 3.0;
  gl_FragColor = eval(gradient, bri);
}