Client: Improve the accuracy of downsampled texture colors

It seems that some error correction is applied by the fixed-function pipeline when operating on the downsampled (reduced-color) textures? Merely truncating the colors, as previously done, yields visible artifacts with an accumulating error consistent with the dropped precision (1 per 32 for bpp = 5). The base colors would have to be either uint8 or floats, which carry more information after all. The results are otherwise fairly exact, at least on the few maps I tested. This may also apply to lightmaps? (TBD)

Core/NativeClient/WebGPU/Texture.lua

@@ -7,7 +7,7 @@ local webgpu = require("wgpu")
 
 local cast = ffi.cast
 local new = ffi.new
-local math_floor = math.floor
+local floor = math.floor
 
 local Texture = {
 	DEFAULT_TEXTURE_FORMAT = ffi.C.WGPUTextureFormat_RGBA8Unorm,
@@ -192,7 +192,7 @@ function Texture:GenerateCheckeredGridImage(textureWidthInPixels, textureHeightI
 		for v = 0, textureHeightInPixels - 1 do
 			local index = 4 * (v * textureWidthInPixels + u)
 
-			local useAlternateColor = math_floor(u / GRID_CELL_SIZE) % 2 == math_floor(v / GRID_CELL_SIZE) % 2
+			local useAlternateColor = floor(u / GRID_CELL_SIZE) % 2 == floor(v / GRID_CELL_SIZE) % 2
 			local cellColor = useAlternateColor and firstColor or secondColor
 
 			pixels[index + RGBA_OFFSET_RED] = ffi.cast("uint8_t", cellColor.red * 255)
@@ -234,10 +234,14 @@ function Texture:CreateReducedColorImage(inputImageBytes, width, height, posteri
 			posterizedBlue = bit.lshift(posterizedBlue, posterizationLevel)
 			posterizedAlpha = bit.lshift(posterizedAlpha, posterizationLevel)
 
-			bufferStartPointer[writableAreaStartIndex + 0] = posterizedRed
-			bufferStartPointer[writableAreaStartIndex + 1] = posterizedGreen
-			bufferStartPointer[writableAreaStartIndex + 2] = posterizedBlue
-			bufferStartPointer[writableAreaStartIndex + 3] = posterizedAlpha
+			local remainingColorDepthInBitsPerPixel = math.max(1, 8 - posterizationLevel)
+			local numAvailableColorValues = math.pow(2, remainingColorDepthInBitsPerPixel) -- 256, 128, 64, 32, 16, 8
+			local errorCorrection = 1 / numAvailableColorValues
+
+			bufferStartPointer[writableAreaStartIndex + 0] = posterizedRed + floor(errorCorrection * red)
+			bufferStartPointer[writableAreaStartIndex + 1] = posterizedGreen + floor(errorCorrection * green)
+			bufferStartPointer[writableAreaStartIndex + 2] = posterizedBlue + floor(errorCorrection * blue)
+			bufferStartPointer[writableAreaStartIndex + 3] = posterizedAlpha + floor(errorCorrection * alpha)
 
 			numBytesWritten = numBytesWritten + 4
 		end

Tests/NativeClient/WebGPU/Texture.spec.lua

@@ -209,10 +209,18 @@ describe("Texture", function()
 		it("should reduce the color depth of the texture image", function()
 			local testImage = "\255\192\128\000" .. "\007\007\007\007"
 			local reducedColorImage, width, height = Texture:CreateReducedColorImage(testImage, 2, 1)
-			local expectedimageBytes = "\248\192\128\000\000\000\000\000"
+			local expectedImageBytes = buffer.new():put("\255\198\132\000\000\000\000\000")
 			assertEquals(width, 2)
 			assertEquals(height, 1)
-			assertEquals(tostring(reducedColorImage), expectedimageBytes)
+			assertEquals(reducedColorImage[0], expectedImageBytes[0])
+			assertEquals(reducedColorImage[1], expectedImageBytes[1])
+			assertEquals(reducedColorImage[2], expectedImageBytes[2])
+			assertEquals(reducedColorImage[3], expectedImageBytes[3])
+			assertEquals(reducedColorImage[4], expectedImageBytes[4])
+			assertEquals(reducedColorImage[5], expectedImageBytes[5])
+			assertEquals(reducedColorImage[6], expectedImageBytes[6])
+			assertEquals(reducedColorImage[7], expectedImageBytes[7])
+			assertEquals(tostring(reducedColorImage), tostring(expectedImageBytes))
 		end)
 	end)
 end)