From 6a2f2ced95771b082bd00f32bff02825205a0835 Mon Sep 17 00:00:00 2001
From: Wladislav Artsimovich <60887273+FrostKiwi@users.noreply.github.com>
Date: Fri, 1 Mar 2024 11:21:25 +0900
Subject: [PATCH] correct LUT miscalculation

---
 ...a-how-to-simulate-a-3d-texture-in-webgl.md | 40 ++++++++++---------
 1 file changed, 21 insertions(+), 19 deletions(-)

diff --git a/webgl/lessons/webgl-qna-how-to-simulate-a-3d-texture-in-webgl.md b/webgl/lessons/webgl-qna-how-to-simulate-a-3d-texture-in-webgl.md
index 73224ca6c..d3c3b04e3 100644
--- a/webgl/lessons/webgl-qna-how-to-simulate-a-3d-texture-in-webgl.md
+++ b/webgl/lessons/webgl-qna-how-to-simulate-a-3d-texture-in-webgl.md
@@ -61,22 +61,25 @@ Thanks in advance!
 You can simulate a 3d texture by storing each plane of the 3d texture in a 2d texture
 
 Then a function like this will let you use it as a 3d texture
-
-    vec4 sampleAs3DTexture(sampler2D tex, vec3 texCoord, float size) {
-        float sliceSize = 1.0 / size;                         // space of 1 slice
-        float slicePixelSize = sliceSize / size;              // space of 1 pixel
-        float sliceInnerSize = slicePixelSize * (size - 1.0); // space of size pixels
-        float zSlice0 = min(floor(texCoord.z * size), size - 1.0);
-        float zSlice1 = min(zSlice0 + 1.0, size - 1.0);
-        float xOffset = slicePixelSize * 0.5 + texCoord.x * sliceInnerSize;
-        float s0 = xOffset + (zSlice0 * sliceSize);
-        float s1 = xOffset + (zSlice1 * sliceSize);
-        vec4 slice0Color = texture2D(tex, vec2(s0, texCoord.y));
-        vec4 slice1Color = texture2D(tex, vec2(s1, texCoord.y));
-        float zOffset = mod(texCoord.z * size, 1.0);
-        return mix(slice0Color, slice1Color, zOffset);
-     }
-
+    
+    vec4 sampleAs3DTexture(sampler2D tex, vec3 texCoord, float size)
+    {
+    	float sliceSize = 1.0 / size;			 // space of 1 slice
+    	float slicePixelSize = sliceSize / size; // space of 1 pixel
+    	float width = size - 1.0;
+    	float sliceInnerSize = slicePixelSize * width; // space of size pixels
+    	float zSlice0 = floor(texCoord.z * width);
+    	float zSlice1 = min(zSlice0 + 1.0, width);
+    	float xOffset = slicePixelSize * 0.5 + texCoord.x * sliceInnerSize;
+    	float yRange = (texCoord.y * width + 0.5) / size;
+    	float s0 = xOffset + (zSlice0 * sliceSize);
+    	float s1 = xOffset + (zSlice1 * sliceSize);
+    	vec4 slice0Color = texture2D(tex, vec2(s0, yRange));
+    	vec4 slice1Color = texture2D(tex, vec2(s1, yRange));
+    	float zOffset = mod(texCoord.z * width, 1.0);
+    	return mix(slice0Color, slice1Color, zOffset);
+    }
+     
 If your 3d texture was 8x8x8 then you'd make a 2d texture that is 64x8 and put each plane of the 3d texture in your 2d texture. Then, knowing that was originally 8x8x8 you'd pass in `8.0` for the size to `sampleAs3DTexture`
 
     precision mediump float;
@@ -91,10 +94,9 @@ If your 3d texture was 8x8x8 then you'd make a 2d texture that is 64x8 and put e
       gl_FragColor = sampleAs3DTexture(u_my3DTexture, v_texCoord, CUBE_SIZE);
     }
 
-Note: the function above assumes you want bilinear filtering between the planes. If you don't you can simplify the function.
-
-There's [a video explanation of this code here][1] which is from [this sample][2].
+Note: the function above assumes you want bilinear filtering between the planes. If you don't you can simplify the function, by returning `return texture2D(tex, vec2( s0, yRange));` immediately after calculating s0. 
 
+There's [a video explanation of this code here][1] which is from [this sample][2]. The video explanation and the final code in the sample differ slightly. This is due to the code miscalculating LUT size and shifting all colors blue, which was corrected by the author in 2019.
 
   [1]: http://www.youtube.com/watch?v=rfQ8rKGTVlg#t=26m00s
   [2]: http://webglsamples.googlecode.com/hg/color-adjust/color-adjust.html