Slowest Font Renderer in the West

fragment.glsl

uniform int n_lin_segments;
uniform vec2 lin_segments[256];
uniform int n_quad_segments;
uniform vec2 quad_segments[512];

varying vec3 pos;
varying vec2 uv;
void main() { 
  vec2 point = uv;
  // point = (point + vec2(0.3, 0)) * 0.1;
  
  int scan_count = 0;

  gl_FragColor = vec4(uv, n_lin_segments, 1);

  // find winding number for linear segments.
  for(int i=0; i < n_lin_segments * 2; i+=2) {
     vec2 pt0 = lin_segments[i];
     vec2 pt1 = lin_segments[i+1];
     float t = (point.y - pt0.y) / (pt1.y - pt0.y);
     if (t >= 0 && t < 1) { 
       float x_intersect = t * (pt1.x - pt0.x) + pt0.x - point.x;
       if (x_intersect <= 0) scan_count += 1;           
     }
  }

  // find winding number for quadratic segments
  for(int i=0; i < n_quad_segments * 3; i += 3) {
     vec2 pt0 = quad_segments[i];
     vec2 pt1 = quad_segments[i+1];
     vec2 pt2 = quad_segments[i+2];
     // Get the quadratic terms by solving the 
     // parametric equation A*t*t + B*t + C = 0 for T in [0, 1) in x and y
     vec2 p0 = pt0 - point, p1 = pt1 - point, p2 = pt2 - point;
     vec2 A =    p0 - 2 * p1 + p2, 
          B = -2*p0 + 2 * p1,
          C =    p0;

     // find the values of T at point.y
     float discriminant = B.y * B.y - 4 * A.y * C.y;
     if (-0.0001 < A.y && A.y < 0.0001) { 
        // The Y-quadratic is degenerate, we have to solve linearly.
        float t_1 = -C.y / B.y;
        float x_1 = A.x * t_1 * t_1 + B.x * t_1 + C.x;
        if (t_1 >= 0 && t_1 < 1 && x_1 < 0) { scan_count += 1; }
     } 
     else if (discriminant <=  0) { 
        // curve does not touch the Y-line or is tangent 
     }
     else { 
        // two T-intercept points that resultin Y = point.y
        float sqrt_disc = sqrt(discriminant);
        float t_1 = (-B.y - sqrt_disc) / (2.0 * A.y);
        float x_1 = A.x * t_1 * t_1 + B.x * t_1 + C.x;


        float t_2 = (-B.y + sqrt_disc) / (2.0 * A.y);
        float x_2 = A.x * t_2 * t_2 + B.x * t_2 + C.x;
        if (t_2 >= 0 && t_2 < 1 && x_2 < 0) { scan_count += 1; }

        if (t_1 >= 0 && t_1 < 1 && x_1 < 0) { scan_count += 1; }
     }
  }

  // shade the parts where we have an odd winding number
  if (scan_count % 2 == 1) { 
    gl_FragColor = vec4(1, 0, 0, 1);
  }
}