UI: Optimize the area border drawing

It is was not really a bottleneck but it was triggering my OCD when 1/3rd
of the drawcalls in a normal scene were basically only caused by this.

1 files changed, 42 insertions, 0 deletions

diff --git a/source/blender/gpu/shaders/gpu_shader_2D_area_borders_vert.glsl b/source/blender/gpu/shaders/gpu_shader_2D_area_borders_vert.glsl
new file mode 100644
index 00000000000..5326076e269
--- /dev/null
+++ b/source/blender/gpu/shaders/gpu_shader_2D_area_borders_vert.glsl
@@ -0,0 +1,42 @@
+
+uniform mat4 ModelViewProjectionMatrix;
+
+uniform vec4 rect;
+uniform int cornerLen;
+uniform float scale;
+
+in vec2 pos;
+
+const vec2 jitter_ofs[8] = vec2[8](
+	vec2( 0.468813, -0.481430), vec2(-0.155755, -0.352820),
+	vec2( 0.219306, -0.238501), vec2(-0.393286, -0.110949),
+	vec2(-0.024699,  0.013908), vec2( 0.343805,  0.147431),
+	vec2(-0.272855,  0.269918), vec2( 0.095909,  0.388710)
+);
+
+void main()
+{
+	int corner_id = (gl_VertexID / cornerLen) % 4;
+	int jitter_id = gl_VertexID / (cornerLen * 4) % 8;
+
+	vec2 final_pos = pos * scale;
+
+	if (corner_id == 0)
+		final_pos += rect.yw;  /* top right */
+	else if (corner_id == 1)
+		final_pos += rect.xw;  /* top left */
+	else if (corner_id == 2)
+		final_pos += rect.xz;  /* bottom left */
+	else
+		final_pos += rect.yz;  /* bottom right */
+
+	/* Only jitter verts inside the corner (not the one shared with the edges). */
+	if ((gl_VertexID % cornerLen) % (cornerLen - 2) != 0) {
+		/* Only jitter intern verts not boundaries. */
+		if ((gl_VertexID % 2) == 0) {
+			final_pos += jitter_ofs[jitter_id];
+		}
+	}
+
+	gl_Position = (ModelViewProjectionMatrix * vec4(final_pos, 0.0, 1.0));
+}