GND: Implement error correction for lightmap textures

The blending stage interpolates colors linearly when multiplying with the higher-precision inputs (same as for diffuse textures).

Core/FileFormats/RagnarokGND.lua

@@ -10,7 +10,7 @@ local ffi = require("ffi")
 local uv = require("uv")
 
 local assert = assert
-local math_floor = math.floor
+local floor = math.floor
 local format = string.format
 local table_insert = table.insert
 local tonumber = tonumber
@@ -800,9 +800,9 @@ function RagnarokGND:GenerateLightmapTextureImage(posterizationLevel)
 	local rgbaImageBytes = buffer.new(width * height * 4)
 	local bufferStartPointer, reservedlength = rgbaImageBytes:reserve(width * height * 4)
 	for pixelV = 0, height - 1, 1 do
-		local sliceV = math_floor(pixelV / 8)
+		local sliceV = floor(pixelV / 8)
 		for pixelU = 0, width - 1, 1 do
-			local sliceU = math_floor(pixelU / 8)
+			local sliceU = floor(pixelU / 8)
 			local sliceID = sliceV * numSlicesPerRow + sliceU
 			local offsetU = pixelU % 8
 			local offsetV = pixelV % 8
@@ -827,10 +827,14 @@ function RagnarokGND:GenerateLightmapTextureImage(posterizationLevel)
 				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)
 			else -- Slightly wasteful, but the determinism enables testing (and it's barely noticeable anyway)
 				bufferStartPointer[writableAreaStartIndex + 0] = 255
 				bufferStartPointer[writableAreaStartIndex + 1] = 0

Tests/FileFormats/RagnarokGND.spec.lua

@@ -599,7 +599,7 @@ describe("RagnarokGND", function()
 			assertEquals(lightmapTextureImage.height, 8)
 
 			local rawImageBytes = tostring(lightmapTextureImage.rgbaImageBytes)
-			local expectedTextureChecksum = 288753058
+			local expectedTextureChecksum = 3839217290
 			local generatedTextureChecksum = miniz.crc32(rawImageBytes)
 			assertEquals(generatedTextureChecksum, expectedTextureChecksum)
 		end)

Tests/Tools/RagnarokTools.spec.lua

@@ -49,7 +49,7 @@ describe("RagnarokTools", function()
 		it("should export the GND lightmap data in a human-readable format if a valid GND buffer is passed", function()
 			local expectedLightmapChecksum = "be735bab85771a54892d4129d7aba3126e0f7f41f2c9891a28aa8dcfc897d2fa"
 			local expectedShadowmapChecksum = "4a7a36bedbb8e73797b91b2f568b59b8d4600dc3975e2265114339a5142e9175"
-			local expectedTextureChecksum = "dfbff3a58a516c0990147567388431dee2d05dae12a01bd5fb4d30230bb79573"
+			local expectedTextureChecksum = "b8323aa6889476237edea7d1ce894c889e3bb7dbd464bbe1068568994999c659"
 
 			local gndFileContents = C_FileSystem.ReadFile(path.join("Tests", "Fixtures", "no-water-plane.gnd"))
 			RagnarokTools:ExportLightmapsFromGND(gndFileContents)