Workbench: Xray: Make dithered depth dependant on alpha.

The range is 0.25 to 0.75 opacity when the Xray opacity is between 0.0 to 1.0.

This is to avoid loosing completely the sense of occlusion when having no
other solid drawing than the wireframe and loosing the transparency when
xray alpha is at 1.0.

Also replace Bayer (checkerboard) method by interlieved gradient noise to
minimize the chance to loose an occluded line completely.

Other noise function could be tested in the future.

source/blender/draw/engines/workbench/shaders/workbench_checkerboard_depth_frag.glsl

@@ -1,8 +1,35 @@
+
+/* 4x4 bayer matrix. */
+#define P(x) ((x + 0.5) * (1.0 / 16.0))
+const vec4 dither_mat[4] = vec4[4](
+	vec4( P(0.0),  P(8.0),  P(2.0), P(10.0)),
+	vec4(P(12.0),  P(4.0), P(14.0),  P(6.0)),
+	vec4( P(3.0), P(11.0),  P(1.0),  P(9.0)),
+	vec4(P(15.0),  P(7.0), P(13.0),  P(5.0))
+);
+
+uniform float threshold = 0.5;
+
+/* Noise dithering pattern
+ * 0 - Bayer matrix
+ * 1 - Interlieved gradient noise
+ */
+#define NOISE 1
+
 void main()
 {
-	vec2 p= vec2(floor(gl_FragCoord.x), floor(gl_FragCoord.y));
-	vec2 test = mod(p, 2.0);
-	if (mod(test.x + test.y, 2.0)==0.0) {
+#if NOISE == 0
+	ivec2 tx = ivec2(gl_FragCoord.xy) % 4;
+	float noise = dither_mat[tx.x][tx.y];
+#elif NOISE == 1
+	/* Interlieved gradient noise by Jorge Jimenez
+	 * http://www.iryoku.com/next-generation-post-processing-in-call-of-duty-advanced-warfare */
+	float noise = fract(52.9829189 * fract(0.06711056 * gl_FragCoord.x + 0.00583715 * gl_FragCoord.y));
+#else
+#error
+#endif
+
+	if (noise > threshold) {
 		discard;
 	} else {
 		gl_FragDepth = 1.0;

source/blender/draw/engines/workbench/workbench_forward.c

@@ -353,6 +353,7 @@ void workbench_forward_engine_init(WORKBENCH_Data *vedata)
 		psl->checker_depth_pass = DRW_pass_create("Checker Depth", state);
 		grp = DRW_shgroup_create(e_data.checker_depth_sh, psl->checker_depth_pass);
 		DRW_shgroup_call_add(grp, DRW_cache_fullscreen_quad_get(), NULL);
+		DRW_shgroup_uniform_float_copy(grp, "threshold", 0.75f - wpd->shading.xray_alpha * 0.5f);
 	}
 }