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).