From b40f5be01f8d0aa50af46ac7a79cfe88efdcbe18 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20Foucault?= <foucault.clem@gmail.com>
Date: Tue, 17 Apr 2018 22:25:53 +0200
Subject: [PATCH] Eevee: Use textureGather for minmaxZbuffer downsampling.

I haven't noticed any performance improvement but it could be more important
for other hardware. At least it's not slower!
---
 .../eevee/shaders/effect_minmaxz_frag.glsl    | 61 +++++++++++++------
 1 file changed, 41 insertions(+), 20 deletions(-)

diff --git a/source/blender/draw/engines/eevee/shaders/effect_minmaxz_frag.glsl b/source/blender/draw/engines/eevee/shaders/effect_minmaxz_frag.glsl
index 05fef73b159..c11dbdd59ad 100644
--- a/source/blender/draw/engines/eevee/shaders/effect_minmaxz_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/effect_minmaxz_frag.glsl
@@ -4,6 +4,8 @@
  * Adapted from http://rastergrid.com/blog/2010/10/hierarchical-z-map-based-occlusion-culling/
  **/
 
+#extension GL_ARB_texture_gather : enable
+
 #ifdef LAYERED
 uniform sampler2DArray depthBuffer;
 uniform int depthLayer;
@@ -12,15 +14,21 @@ uniform sampler2D depthBuffer;
 #endif
 
 #ifdef LAYERED
-#define sampleLowerMip(t) texelFetch(depthBuffer, ivec3(t, depthLayer), 0).r
+#  define sampleLowerMip(t) texelFetch(depthBuffer, ivec3(t, depthLayer), 0).r
+#  define gatherLowerMip(t) textureGather(depthBuffer, vec3(t, depthLayer))
 #else
-#define sampleLowerMip(t) texelFetch(depthBuffer, t, 0).r
+#  define sampleLowerMip(t) texelFetch(depthBuffer, t, 0).r
+#  define gatherLowerMip(t) textureGather(depthBuffer, t)
 #endif
 
 #ifdef MIN_PASS
-#define minmax(a, b) min(a, b)
+#define minmax2(a, b) min(a, b)
+#define minmax3(a, b, c) min(min(a, b), c)
+#define minmax4(a, b, c, d) min(min(min(a, b), c), d)
 #else /* MAX_PASS */
-#define minmax(a, b) max(a, b)
+#define minmax2(a, b) max(a, b)
+#define minmax3(a, b, c) max(max(a, b), c)
+#define minmax4(a, b, c, d) max(max(max(a, b), c), d)
 #endif
 
 /* On some AMD card / driver conbination, it is needed otherwise,
@@ -38,32 +46,45 @@ void main()
 	texelPos *= 2;
 #endif
 
+#ifdef COPY_DEPTH
 	float val = sampleLowerMip(texelPos);
-#ifndef COPY_DEPTH
-	float val2 = sampleLowerMip(texelPos + ivec2(1, 0));
-	float val3 = sampleLowerMip(texelPos + ivec2(1, 1));
-	float val4 = sampleLowerMip(texelPos + ivec2(0, 1));
-	val = minmax(val, val2);
-	val = minmax(val, val3);
-	val = minmax(val, val4);
+#else
+	vec4 samp;
+#  ifdef GL_ARB_texture_gather
+	samp = gatherLowerMip(vec2(texelPos) / vec2(mipsize));
+#  else
+	samp.x = sampleLowerMip(texelPos);
+	samp.y = sampleLowerMip(texelPos + ivec2(1, 0));
+	samp.z = sampleLowerMip(texelPos + ivec2(1, 1));
+	samp.w = sampleLowerMip(texelPos + ivec2(0, 1));
+#  endif
+
+	float val = minmax4(samp.x, samp.y, samp.z, samp.w);
 
 	/* if we are reducing an odd-width texture then fetch the edge texels */
 	if (((mipsize.x & 1) != 0) && (texelPos.x == mipsize.x - 3)) {
 		/* if both edges are odd, fetch the top-left corner texel */
 		if (((mipsize.y & 1) != 0) && (texelPos.y == mipsize.y - 3)) {
-			val = minmax(val, sampleLowerMip(texelPos + ivec2(2, 2)));
+			samp.x = sampleLowerMip(texelPos + ivec2(2, 2));
+			val = minmax2(val, samp.x);
 		}
-		float val2 = sampleLowerMip(texelPos + ivec2(2, 0));
-		float val3 = sampleLowerMip(texelPos + ivec2(2, 1));
-		val = minmax(val, val2);
-		val = minmax(val, val3);
+#  ifdef GL_ARB_texture_gather
+		samp = gatherLowerMip((vec2(texelPos) + vec2(1.0, 0.0)) / vec2(mipsize));
+#  else
+		samp.y = sampleLowerMip(texelPos + ivec2(2, 0));
+		samp.z = sampleLowerMip(texelPos + ivec2(2, 1));
+#  endif
+		val = minmax3(val, samp.y, samp.z);
 	}
 	/* if we are reducing an odd-height texture then fetch the edge texels */
 	if (((mipsize.y & 1) != 0) && (texelPos.y == mipsize.y - 3)) {
-		float val2 = sampleLowerMip(texelPos + ivec2(0, 2));
-		float val3 = sampleLowerMip(texelPos + ivec2(1, 2));
-		val = minmax(val, val2);
-		val = minmax(val, val3);
+#  ifdef GL_ARB_texture_gather
+		samp = gatherLowerMip((vec2(texelPos) + vec2(0.0, 1.0)) / vec2(mipsize));
+#  else
+		samp.x = sampleLowerMip(texelPos + ivec2(0, 2));
+		samp.y = sampleLowerMip(texelPos + ivec2(1, 2));
+#  endif
+		val = minmax3(val, samp.x, samp.y);
 	}
 #endif