Update shader_design.md

doc/docs/shader_design.md

@@ -82,19 +82,17 @@ All splat maps are sampled, and of the 16-32 values, the 4 strongest are blended
 
 *Side note:* Storing blend values in 3-bits is possible, where each of the 8 numbers represents an index in a array of 0-1 values: `{ 0.0f, .125f, .25f, .334f, .5f, .667f, .8f, 1.0f }`. In the future, this may be baked at runtime. However, editing using a 3-bit array of fixed values was exceedingly difficult and unsuccessful.
 
-*Side note 2:* The bilinear blend can be skipped for distant terrain, allowing only 1/4 of the samples normally required.
-
-The position of the pixel within its grid square is used to bilinear interpolate the values of the 4 surrounding samples. We disable the default GPU interpolation on texture lookups and interpolate ourselves here. 
+The position of the pixel within its grid square is used to bilinear interpolate the values of the 4 surrounding samples. We disable the default GPU interpolation on texture lookups and interpolate ourselves here. At distances where the The bilinear blend would occur across only 1 pixel in sceen space, the blend is skipped, allowing only 1/4 of the samples normally required.
 
 **Comparing the two methods:**
 
 * **Texture lookups** - Considering only lookups for which ground texture to use and loading the texture data:
   * Splat maps use 12-16 lookups per pixel depending on 16 or 32 textures:
     * 4 for the 4 splat maps for 16 textures. 8 for 32 textures. This gets the texture for the closest vertex point
     * 8 for the strongest 4 albedo_height textures and the 4 normal_rough textures
-  * Terrain3D uses 12-20 lookups per pixel depending on if an area has an overlay texture:
-    * 4 for the surrounding 4 grid points on the control map
-    * 8-16 for the 2-4 albedo_height & normal_rough for the base and overlay textures, for each of the 4 grid points
+  * Terrain3D uses 5-20 lookups per pixel depending on terrain distance:
+    * 1-4 for the surrounding 4 grid points on the control map
+    * 4-16 for the 2-4 albedo_height & normal_rough for the base and overlay textures, for each of the 4 grid points
 
 * **VRAM consumed** - Splat maps store 16 texture strength values in 16 bytes per pixel (16 * 8 bits = 128 bits). We could store that in 16 bits. Splat maps with 32 textures would require 32 bytes per pixel. We store that in 18 bits (5 base, 5 overlay, 8 blend value). On a 4096 x 4096 terrain with 16M pixels, splat maps consume 256MB for 16 textures, 512MB for 32. We can specify 32 textures in only 36MB for a 93% reduction in VRAM. This calculation considers only the portion of the maps that define where to place the textures on the terrain. Tools use up a lot more VRAM for other things.