gtk2/gsk/resources/glsl/border.glsl

// VERTEX_SHADER:
uniform vec4 u_color;
uniform vec4 u_widths;
uniform vec4[3] u_outline_rect;

_OUT_ vec4 final_color;
_OUT_ _ROUNDED_RECT_UNIFORM_ transformed_outside_outline;
_OUT_ _ROUNDED_RECT_UNIFORM_ transformed_inside_outline;

void main() {
  gl_Position = u_projection * u_modelview * vec4(aPosition, 0.0, 1.0);

  final_color = premultiply(u_color) * u_alpha;

  RoundedRect outside = create_rect(u_outline_rect);
  RoundedRect inside = rounded_rect_shrink (outside, u_widths);

  rounded_rect_transform(outside, u_modelview);
  rounded_rect_transform(inside, u_modelview);

  rounded_rect_encode(outside, transformed_outside_outline);
  rounded_rect_encode(inside, transformed_inside_outline);
}

// FRAGMENT_SHADER:
uniform vec4[3] u_outline_rect;

_IN_ vec4 final_color;
_IN_ _ROUNDED_RECT_UNIFORM_ transformed_outside_outline;
_IN_ _ROUNDED_RECT_UNIFORM_ transformed_inside_outline;

void main() {
  vec2 frag = get_frag_coord();

  float alpha = clamp(rounded_rect_coverage(decode_rect(transformed_outside_outline), frag) -
                      rounded_rect_coverage(decode_rect(transformed_inside_outline), frag),
                      0.0, 1.0);

  setOutputColor(final_color * alpha);
}