julia_phase.glsl

uniform int iterations;
uniform vec2 scale, seed;

#define PI 3.14159265358979323846

void main(void) {
	vec2 z =(gl_TexCoord[0].xy - 0.5) * scale;
	int i = 1;
	for(; dot(z, z) <= 4.0 && i < iterations; i++) {
		z = vec2(z.x * z.x - z.y * z.y, (z.x + z.x) * z.y) + seed;
	}

	float gray = i > iterations ? 0.0 : abs(atan(z.x, z.y) / PI);

	gl_FragColor = vec4(gray, gray, gray, 0.0);
}