More accurate conversion from uint to ushort and byte

Author: ds5678

AssetRipper.TextureDecoder.ColorGenerator/NumericConversionGenerator.cs

@@ -229,6 +229,13 @@ private static void WriteConvertMethod(IndentedTextWriter writer, CSharpPrimitiv
 										writer.WriteLine("byte converted = unchecked((byte)x);");
 										writer.WriteLine("return Unsafe.As<byte, TTo>(ref converted);");
 									}
+									else if (from.Size < sizeof(double) && to.Size < sizeof(double))
+									{
+										writer.WriteLine($"double interpolated = (double)value / (double){from.MaxValue};");
+										writer.WriteLine($"double exact = interpolated * (double){to.MaxValue};");
+										writer.WriteLine($"{to.LangName} converted = ({to.LangName})double.Round(exact, MidpointRounding.AwayFromZero);");
+										writer.WriteLine($"return Unsafe.As<{to.LangName}, TTo>(ref converted);");
+									}
 									else
 									{
 										writer.WriteComment($"There are more accurate ways to map {from.TypeName} onto {to.TypeName}, but this is the simplest.");

AssetRipper.TextureDecoder/Rgb/NumericConversion.g.cs

@@ -383,12 +383,10 @@ private static TTo ConvertUInt32<TTo>(uint value) where TTo : unmanaged
 		else if (typeof(TTo) == typeof(byte))
 		{
 			// See https://github.com/AssetRipper/TextureDecoder/issues/19
-			// There are more accurate ways to map UInt32 onto Byte, but this is the simplest.
-			unchecked
-			{
-				byte converted = (byte)((uint)value >> 24);
-				return Unsafe.As<byte, TTo>(ref converted);
-			}
+			double interpolated = (double)value / (double)uint.MaxValue;
+			double exact = interpolated * (double)byte.MaxValue;
+			byte converted = (byte)double.Round(exact, MidpointRounding.AwayFromZero);
+			return Unsafe.As<byte, TTo>(ref converted);
 		}
 		else if (typeof(TTo) == typeof(short))
 		{
@@ -398,12 +396,10 @@ private static TTo ConvertUInt32<TTo>(uint value) where TTo : unmanaged
 		else if (typeof(TTo) == typeof(ushort))
 		{
 			// See https://github.com/AssetRipper/TextureDecoder/issues/19
-			// There are more accurate ways to map UInt32 onto UInt16, but this is the simplest.
-			unchecked
-			{
-				ushort converted = (ushort)((uint)value >> 16);
-				return Unsafe.As<ushort, TTo>(ref converted);
-			}
+			double interpolated = (double)value / (double)uint.MaxValue;
+			double exact = interpolated * (double)ushort.MaxValue;
+			ushort converted = (ushort)double.Round(exact, MidpointRounding.AwayFromZero);
+			return Unsafe.As<ushort, TTo>(ref converted);
 		}
 		else if (typeof(TTo) == typeof(int))
 		{