hsv_color_wheel.glsl

// can run on https://thebookofshaders.com/edit.php

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;

vec3 hsv2rgb(vec3 c) {
    vec4 K = vec4(1.0, 2.0/3.0, 1.0/3.0, 3.0);
    vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
    return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
}

void main() {
    vec2 coord = gl_FragCoord.xy/u_resolution.xy;
    coord = coord * 2.0 - 1.0;
    float r = length(coord);
    if (r > 1.0) {
        gl_FragColor = vec4(0.0);
        return;
    }
    
    float angle = atan(coord.y, coord.x);
    float hue = angle / (2.0 * 3.14) + 0.5;
    float saturation = r;
    float value = 1.0; // 0.0 to 1.0

    gl_FragColor = vec4(hsv2rgb(vec3(hue, saturation, value)), 1);
}