Is it possible to draw line thickness in a fragment shader?

Question

Is it possible for me to add line thickness in the fragment shader considering that I draw the line with GL_LINES? Most of the examples I saw seem to access only the texels within the primitive in the fragment shader and a line thickness shader would need to write to texels outside the line primitive to obtain the thickness. If it is possible however, a very small, basic, example, would be great.

Answer 1

Quite a lot is possible with fragment shaders. Just look what some guys are doing. I'm far away from that level myself but this code can give you an idea:

#define resolution vec2(500.0, 500.0)
#define Thickness 0.003

float drawLine(vec2 p1, vec2 p2) {
  vec2 uv = gl_FragCoord.xy / resolution.xy;

  float a = abs(distance(p1, uv));
  float b = abs(distance(p2, uv));
  float c = abs(distance(p1, p2));

  if ( a >= c || b >=  c ) return 0.0;

  float p = (a + b + c) * 0.5;

  // median to (p1, p2) vector
  float h = 2 / c * sqrt( p * ( p - a) * ( p - b) * ( p - c));

  return mix(1.0, 0.0, smoothstep(0.5 * Thickness, 1.5 * Thickness, h));
}

void main()
{
  gl_FragColor = vec4(
      max(
        max(
          drawLine(vec2(0.1, 0.1), vec2(0.1, 0.9)),
          drawLine(vec2(0.1, 0.9), vec2(0.7, 0.5))),
        drawLine(vec2(0.1, 0.1), vec2(0.7, 0.5))));
}

Another alternative is to check with texture2D for the color of nearby pixel - that way you can make you image glow or thicken (e.g. if any of the adjustment pixels are white - make current pixel white, if next to nearby pixel is white - make current pixel grey).

Answer 2

Here's my approach. Let p1 and p2 be the two points defining the line, and let point be the point whose distance to the line you wish to measure. Point is most likely gl_FragCoord.xy / resolution;

Here's the function.

float distanceToLine(vec2 p1, vec2 p2, vec2 point) {
    float a = p1.y-p2.y;
    float b = p2.x-p1.x;
    return abs(a*point.x+b*point.y+p1.x*p2.y-p2.x*p1.y) / sqrt(a*a+b*b);
}

Then use that in your mix and smoothstep functions.

Also check out this answer: https://stackoverflow.com/a/9246451/911207

Answer 3

No, it is not possible in the fragment shader. However, you can use a geometry shader to expand your line to a quad (or, actually two triangles) which can pose as a thick line.

Here's a nice discussion on that subject (with code samples).

Answer 4

A simple hack is to just add a jitter in the vertex shader: gl_Position += vec4(delta, delta, delta, 0.0); where delta is the pixelsize i.e. 1.0/viewsize

Do the line-draw pass twice using zero, and then the delta as jitter (passed in as a uniform).