source/blender/gpu/shaders/gpu_shader_edges_overlay_geom.glsl


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layout(triangles) in;
layout(triangle_strip, max_vertices = 3) out;

uniform float outlineWidth = 1.0;
uniform vec2 viewportSize;

in vec4 pos_xformed[];
in float widthModulator[];

noperspective out vec3 distanceToOutline;

// project to screen space
vec2 proj(int axis)
{
  vec4 pos = pos_xformed[axis];
  return (0.5 * (pos.xy / pos.w) + 0.5) * viewportSize;
}

float dist(vec2 pos[3], int v)
{
  // current vertex position
  vec2 vpos = pos[v];
  // endpoints of opposite edge
  vec2 e1 = pos[(v + 1) % 3];
  vec2 e2 = pos[(v + 2) % 3];

  float abs_det = length(cross(vec3(vpos - e1, 0), vec3(vpos - e2, 0)));  // could simplify
  return abs_det / distance(e2, e1);
}

vec3 distance[3];

void clearEdge(int v)
{
  float distant = 10 * outlineWidth;
  for (int i = 0; i < 3; ++i)
    distance[i][v] += distant;
}

void modulateEdge(int v)
{
  float offset = min(widthModulator[v], 1) * outlineWidth;
  for (int i = 0; i < 3; ++i)
    distance[i][v] -= offset;
}

void main()
{
  vec2 pos[3] = vec2[3](proj(0), proj(1), proj(2));

  for (int v = 0; v < 3; ++v)
    distance[v] = vec3(0);

  for (int v = 0; v < 3; ++v) {
    if (widthModulator[v] > 0) {
      distance[v][v] = dist(pos, v);
      modulateEdge(v);
    }
  }

  for (int v = 0; v < 3; ++v)
    if (widthModulator[v] <= 0)
      clearEdge(v);

  for (int v = 0; v < 3; ++v) {
    gl_Position = pos_xformed[v];
    distanceToOutline = distance[v];
    EmitVertex();
  }

  EndPrimitive();
}