blender-archive/source/blender/gpu/shaders/gpu_shader_gpencil_stroke_frag.glsl

void main()
{
  const vec2 center = vec2(0, 0.5);
  vec4 tColor = vec4(geometry_out.mColor);
  /* if alpha < 0, then encap */
  if (geometry_out.mColor.a < 0) {
    tColor.a = tColor.a * -1.0;
    float dist = length(geometry_out.mTexCoord - center);
    if (dist > 0.25) {
      discard;
    }
  }
  /* Solid */
  fragColor = tColor;
}