Fixing pre-multiplied textures when needed

Author: larsbrubaker

RenderGl/ImageAlphaConverter.cs

@@ -0,0 +1,76 @@
+/*
+Copyright (c) 2026, Lars Brubaker
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+The views and conclusions contained in the software and documentation are those
+of the authors and should not be interpreted as representing official policies,
+either expressed or implied, of the FreeBSD Project.
+*/
+
+using System;
+
+namespace MatterHackers.RenderGl
+{
+	public static class ImageAlphaConverter
+	{
+		public static byte[] ConvertPremultipliedBgraToStraightAlpha(byte[] premultipliedBgra)
+		{
+			if (premultipliedBgra == null)
+			{
+				throw new ArgumentNullException(nameof(premultipliedBgra));
+			}
+
+			if (premultipliedBgra.Length % 4 != 0)
+			{
+				throw new ArgumentException("Expected BGRA pixels.", nameof(premultipliedBgra));
+			}
+
+			var straightAlphaBgra = new byte[premultipliedBgra.Length];
+			for (int pixelOffset = 0; pixelOffset < premultipliedBgra.Length; pixelOffset += 4)
+			{
+				byte alpha = premultipliedBgra[pixelOffset + 3];
+				straightAlphaBgra[pixelOffset + 0] = UnpremultiplyChannel(premultipliedBgra[pixelOffset + 0], alpha);
+				straightAlphaBgra[pixelOffset + 1] = UnpremultiplyChannel(premultipliedBgra[pixelOffset + 1], alpha);
+				straightAlphaBgra[pixelOffset + 2] = UnpremultiplyChannel(premultipliedBgra[pixelOffset + 2], alpha);
+				straightAlphaBgra[pixelOffset + 3] = alpha;
+			}
+
+			return straightAlphaBgra;
+		}
+
+		private static byte UnpremultiplyChannel(byte channel, byte alpha)
+		{
+			if (alpha == 0)
+			{
+				return 0;
+			}
+
+			if (alpha == 255)
+			{
+				return channel;
+			}
+
+			return (byte)Math.Min(255, (channel * 255 + alpha / 2) / alpha);
+		}
+	}
+}

VorticeD3D/NativeSceneEffects.cs

@@ -182,6 +182,7 @@ public void Dispose()
 
 		private sealed class ImageTextureSource : IDisposable
 		{
+			public bool ConvertPremultipliedToStraightAlpha;
 			public MatterHackers.Agg.Image.ImageBuffer SourceImage;
 			public ID3D11ShaderResourceView ShaderResourceView;
 			public ID3D11Texture2D Texture;
@@ -593,7 +594,10 @@ private ColorTextureTarget EnsureColorTarget(ColorTextureTarget existingTarget,
 			return newTarget;
 		}
 
-		private ImageTextureSource EnsureImageTextureSource(ImageTextureSource existingSource, MatterHackers.Agg.Image.ImageBuffer sourceImage)
+		private ImageTextureSource EnsureImageTextureSource(
+			ImageTextureSource existingSource,
+			MatterHackers.Agg.Image.ImageBuffer sourceImage,
+			bool convertPremultipliedToStraightAlpha = false)
 		{
 			if (sourceImage == null)
 			{
@@ -602,6 +606,7 @@ private ImageTextureSource EnsureImageTextureSource(ImageTextureSource existingS
 			}
 
 			if (ReferenceEquals(existingSource?.SourceImage, sourceImage)
+				&& existingSource.ConvertPremultipliedToStraightAlpha == convertPremultipliedToStraightAlpha
 				&& existingSource.ShaderResourceView != null)
 			{
 				return existingSource;
@@ -610,6 +615,7 @@ private ImageTextureSource EnsureImageTextureSource(ImageTextureSource existingS
 			existingSource?.Dispose();
 			var textureSource = new ImageTextureSource
 			{
+				ConvertPremultipliedToStraightAlpha = convertPremultipliedToStraightAlpha,
 				SourceImage = sourceImage,
 			};
 
@@ -625,7 +631,9 @@ private ImageTextureSource EnsureImageTextureSource(ImageTextureSource existingS
 				BindFlags = BindFlags.ShaderResource,
 			};
 
-			var pixels = sourceImage.GetBuffer();
+			var pixels = convertPremultipliedToStraightAlpha
+				? ImageAlphaConverter.ConvertPremultipliedBgraToStraightAlpha(sourceImage.GetBuffer())
+				: sourceImage.GetBuffer();
 			var handle = GCHandle.Alloc(pixels, GCHandleType.Pinned);
 			try
 			{
@@ -674,7 +682,13 @@ private void RenderBedShadowTexture(BedRenderCommand bedCommand)
 			}
 
 			EnsureBedTargets(bedCommand.TopBaseTexture.Width, bedCommand.TopBaseTexture.Height);
-			bedBaseTexture = EnsureImageTextureSource(bedBaseTexture, bedCommand.TopBaseTexture);
+			// AGG stores this generated texture with premultiplied color channels.
+			// Convert it back to straight alpha for the D3D textured mesh pipeline so
+			// a translucent white bed stays visually white instead of turning gray.
+			bedBaseTexture = EnsureImageTextureSource(
+				bedBaseTexture,
+				bedCommand.TopBaseTexture,
+				convertPremultipliedToStraightAlpha: true);
 
 			var signature = ComputeBedShadowSignature(bedCommand);
 			if (signature == lastBedShadowSignature