Thorne Brandt

Shaders really start getting interesting when you start implementing randomness. The Noise section of The Book of Shaders includes some vague challenges about "using noise with distance." I assumed that this meant using some overlapping to create the illusion of a rough edges.

Here's a simple demonstration of a reusable function. You multiply the larger circle and make the solid circle slight smaller than the radius input.

float rough_circle(in vec2 _st, float radius){
    float noiseScale= 90;
    vec2 noise_st = _st * noiseScale;
    float _noise = noise(noise_st);
    float circle1 = circle(_st, radius);
    float circle2 = circle(_st, radius - (0.01));
    float color = (circle1 * _noise) + circle2;
    return color;
}

This Noise Chapter also challenged you to animate this shape. I usually make a normalized time value between 0 and 1 with this:

float sin_time = sin(u_time) * 0.5 + 0.5; 

Then I have a continuous linear *u_time* and a oscillating *sin_time.* Here's the final result:

    
uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

// 2D Random
float random (in vec2 st) {
    return fract(sin(dot(st.xy,
                         vec2(12.9898,78.233)))
                 * 43758.5453123);
}


float circle(in vec2 _st, float radius){
    return 1.0 - 
        smoothstep(radius-0.2, radius, length(_st));
}

float time_value = sin(u_time) * 0.5 + 0.5; 

// 2D Noise based on Morgan McGuire @morgan3d
// https://www.shadertoy.com/view/4dS3Wd
float noise (in vec2 st) {
    vec2 i = floor(st);
    vec2 f = fract(st);

    // Four corners in 2D of a tile
    float a = random(i);
    float b = random(i + vec2(1.0, 0.0));
    float c = random(i + vec2(0.0, 1.0));
    float d = random(i + vec2(1.0, 1.0));

    // Smooth Interpolation

    // Cubic Hermine Curve.  Same as SmoothStep()
    vec2 u = f*f*(3.0-2.0*f);
    // u = smoothstep(0.,1.,f);

    // Mix 4 coorners percentages
    return mix(a, b, u.x) +
            (c - a)* u.y * (1.0 - u.x) +
            (d - b) * u.x * u.y;
}

float rough_circle(in vec2 _st, float radius){
    float noiseScale= 90.*(time_value + 0.5);
    float time_scale = 0.0001;
    vec2 moving_st = vec2(_st.x, _st.y + (u_time*time_scale*0.3)+1.0);
    vec2 noise_st = moving_st * noiseScale;
    float _noise = noise(noise_st);
    float circle1 = circle(_st, radius);
    float circle2 = circle(_st, radius - (0.1*time_value));
    float color = (circle1 * _noise) + circle2;
    return color;
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    st -= 0.5;
    float color = 1.0-step(rough_circle(st, 0.5), time_value);
    gl_FragColor = vec4(vec3(color), 1.0);
}