Optionally regenerate BC5 blue channel when decoding

BCnEnc.Net/Decoder/BcBlockDecoder.cs

@@ -127,16 +127,18 @@ internal class Bc5Decoder : BaseBcBlockDecoder<Bc5Block, RawBlock4X4Rgba32>
 	{
 		private readonly ColorComponent component1;
 		private readonly ColorComponent component2;
+		private readonly bool recalculateBlueChannel;
 
-		public Bc5Decoder(ColorComponent component1, ColorComponent component2)
+		public Bc5Decoder(ColorComponent component1, ColorComponent component2, bool recalculateBlueChannel)
 		{
 			this.component1 = component1;
 			this.component2 = component2;
+			this.recalculateBlueChannel = recalculateBlueChannel;
 		}
 
 		protected override RawBlock4X4Rgba32 DecodeBlock(Bc5Block block)
 		{
-			return block.Decode(component1, component2);
+			return block.Decode(component1, component2, recalculateBlueChannel);
 		}
 	}
 

BCnEnc.Net/Decoder/BcDecoder.cs

@@ -1598,7 +1598,7 @@ private IBcBlockDecoder<RawBlock4X4Rgba32> GetRgba32Decoder(CompressionFormat fo
 					return new Bc4Decoder(OutputOptions.Bc4Component);
 
 				case CompressionFormat.Bc5:
-					return new Bc5Decoder(OutputOptions.Bc5Component1, OutputOptions.Bc5Component2);
+					return new Bc5Decoder(OutputOptions.Bc5Component1, OutputOptions.Bc5Component2, OutputOptions.Bc5RecalculateBlueChannel);
 
 				case CompressionFormat.Bc7:
 					return new Bc7Decoder();
@@ -1773,7 +1773,7 @@ public int GetBlockSize(CompressionFormat format)
 
 				case CompressionFormat.Unknown:
 					return 0;
-				
+
 				default:
 					throw new ArgumentOutOfRangeException(nameof(format), format, null);
 			}

BCnEnc.Net/Decoder/Options/DecoderOutputOptions.cs

@@ -29,5 +29,10 @@ public class DecoderOutputOptions
 		/// The color channel to populate with the values of the second BC5 block.
 		/// </summary>
 		public ColorComponent Bc5Component2 { get; set; } = ColorComponent.G;
+
+		/// <summary>
+		/// BC5 option to recalculate the blue channel
+		/// </summary>
+		public bool Bc5RecalculateBlueChannel { get; set; } = false;
 	}
 }

BCnEnc.Net/Shared/EncodedBlocks.cs

@@ -251,18 +251,35 @@ public byte Green1
 
 		public void SetGreenIndex(int pixelIndex, byte greenIndex) => greenBlock.SetComponentIndex(pixelIndex, greenIndex);
 
-		public readonly RawBlock4X4Rgba32 Decode(ColorComponent component1 = ColorComponent.R, ColorComponent component2 = ColorComponent.G)
+		public readonly RawBlock4X4Rgba32 Decode(ColorComponent component1 = ColorComponent.R, ColorComponent component2 = ColorComponent.G, bool recalculateBlueChannel = false)
 		{
 			var output = new RawBlock4X4Rgba32();
 			var pixels = output.AsSpan;
 
 			var reds = redBlock.Decode();
 			var greens = greenBlock.Decode();
 
+			var blues = new byte[pixels.Length];
+			if (recalculateBlueChannel)
+			{
+				for (var i = 0; i < pixels.Length; i++)
+				{
+					var red = (float) reds[i] / 255;
+					var green = (float) greens[i] / 255;
+
+					var blue = 1 - red * red - green * green;
+					blue = blue < 0 ? 0f : (float) Math.Sqrt(blue);
+					blue = Math.Clamp((blue + 1) / 2f, 0f, 1f);
+
+					blues[i] = (byte) (blue * 255);
+				}
+			}
+
 			for (var i = 0; i < pixels.Length; i++)
 			{
 				pixels[i] = ComponentHelper.ComponentToColor(component1, reds[i]);
 				pixels[i] = ComponentHelper.ComponentToColor(pixels[i], component2, greens[i]);
+				pixels[i] = ComponentHelper.ComponentToColor(pixels[i], ColorComponent.B, blues[i]);
 			}
 
 			return output;