3b1b-manim/manimlib/shaders/quadratic_bezier_stroke/frag.glsl

#version 330

#INSERT camera_uniform_declarations.glsl

in vec2 uv_coords;

in float uv_stroke_width;
in float uv_anti_alias_width;
in vec4 color;

in float bezier_degree;

out vec4 frag_color;

const float QUICK_DIST_WIDTH = 0.1;


float cube_root(float x){
    return sign(x) * pow(abs(x), 1.0 / 3.0);
}


// Distance from (x0, y0) to the curve y = x^2
float dist_to_curve(float x0, float y0){
    if(bezier_degree == 1.0){
        return y0;
    }
    if(false && uv_stroke_width < QUICK_DIST_WIDTH){
        // This is a quick approximation for computing
        // the distance to the curve.
        // Evaluate F(x, y) = y - x^2
        // divide by its gradient's magnitude
        return (y0 - x0 * x0) / sqrt(1 + 4 * x0 * x0);
    }
    // Otherwise, explicit solve for the minmal distance using the cubic formula
    //
    // The distance squared between (x0, y0) and a point (x, x^2) looks like
    //
    // (x0 - x)^2 + (y0 - x^2)^2 = x^4 + (1 - 2y0)x^2 - 2x0 * x + (x0^2 + y0^2)
    //
    // Setting the derivative equal to zero (and rescaling) looks like
    // 
    // x^3 + (0.5 - y0) * x - 0.5 * x0 = 0
    //
    // float p = 0.5 - y0;
    // float mhq = 0.25 * x0;  // negative half of q
    // float sqrt_disc = sqrt(mhq * mhq + p * p * p / 27.0);
    // float x = cube_root(mhq + sqrt_disc) + cube_root(mhq - sqrt_disc);
    // return distance(uv_coords, vec2(x, x * x));

    float x = x0;
    float p = (0.5 - y0);
    float q = -0.5 * x0;
    for(int i = 0; i < 2; i++){
        float fx = x * x * x + p * x + q;
        float dfx = 3 * x * x + p;
        x = x - fx / dfx;
    }
    return distance(uv_coords, vec2(x, x * x));
}


void main() {
    if (uv_stroke_width == 0) discard;

    float x0 = uv_coords.x;
    float y0 = uv_coords.y;
    // An sdf for the region around the curve we wish to color.
    float signed_dist = abs(dist_to_curve(x0, y0)) - 0.5 * uv_stroke_width;

    frag_color = color;
    // if(uv_stroke_width > QUICK_DIST_WIDTH) frag_color = vec4(1, 0, 0, 1);
    frag_color.a *= smoothstep(0.5, -0.5, signed_dist / uv_anti_alias_width);
}