Add support for premultiplied alpha. Patch by Charles Nicholson.

src/nvtt/Compressor.cpp

@@ -478,6 +478,11 @@ bool Compressor::Private::initMipmap(Mipmap & mipmap, const InputOptions::Privat
 	// Convert linear float image to fixed image ready for compression.
 	mipmap.toFixedImage(inputOptions);
 
+	if (inputOptions.premultiplyAlpha)
+	{
+		premultiplyAlphaMipmap(mipmap, inputOptions);
+	}
+
 	return true;
 }
 
@@ -582,6 +587,29 @@ void Compressor::Private::scaleMipmap(Mipmap & mipmap, const InputOptions::Priva
 }
 
 
+void Compressor::Private::premultiplyAlphaMipmap(Mipmap & mipmap, const InputOptions::Private & inputOptions) const
+{
+	nvDebugCheck(mipmap.asFixedImage() != NULL);
+
+	Image * image = mipmap.asMutableFixedImage();
+
+	const uint w = image->width();
+	const uint h = image->height();
+
+	const uint count = w * h;
+
+	for (uint i = 0; i < count; ++i)
+	{
+		Color32 c = image->pixel(i);
+
+		c.r = (uint(c.r) * uint(c.a)) >> 8;
+		c.g = (uint(c.g) * uint(c.a)) >> 8;
+		c.b = (uint(c.b) * uint(c.a)) >> 8;
+
+		image->pixel(i) = c;
+	}
+}
+
 // Process an input image: Convert to normal map, normalize, or convert to linear space.
 void Compressor::Private::processInputImage(Mipmap & mipmap, const InputOptions::Private & inputOptions) const
 {

src/nvtt/Compressor.h

@@ -58,6 +58,7 @@ namespace nvtt
 
 		void downsampleMipmap(Mipmap & mipmap, const InputOptions::Private & inputOptions) const;
 		void scaleMipmap(Mipmap & mipmap, const InputOptions::Private & inputOptions, uint w, uint h, uint d) const;
+		void premultiplyAlphaMipmap(Mipmap & mipmap, const InputOptions::Private & inputOptions) const;
 		void processInputImage(Mipmap & mipmap, const InputOptions::Private & inputOptions) const;
 		void quantizeMipmap(Mipmap & mipmap, const CompressionOptions::Private & compressionOptions) const;
 		bool compressMipmap(const Mipmap & mipmap, const CompressionOptions::Private & compressionOptions, const OutputOptions::Private & outputOptions) const;

src/nvtt/InputOptions.cpp

@@ -118,6 +118,8 @@ void InputOptions::reset()
 	
 	m.maxExtent = 0;
 	m.roundMode = RoundMode_None;
+
+	m.premultiplyAlpha = false;
 }
 
 
@@ -329,6 +331,10 @@ void InputOptions::setRoundMode(RoundMode mode)
 	m.roundMode = mode;
 }
 
+void InputOptions::setPremultiplyAlpha(bool b)
+{
+	m.premultiplyAlpha = b;
+}
 
 void InputOptions::Private::computeTargetExtents() const
 {

src/nvtt/InputOptions.h

@@ -78,6 +78,8 @@ namespace nvtt
 		uint maxExtent;
 		RoundMode roundMode;
 		
+		bool premultiplyAlpha;
+
 		// @@ These are computed in nvtt::compress, so they should be mutable or stored elsewhere...
 		mutable uint targetWidth;
 		mutable uint targetHeight;

src/nvtt/nvtt.h

@@ -227,6 +227,9 @@ namespace nvtt
 		// Set resizing options.
 		NVTT_API void setMaxExtents(int d);
 		NVTT_API void setRoundMode(RoundMode mode);
+
+		// Set whether or not to premultiply color by alpha
+		NVTT_API void setPremultiplyAlpha(bool b);
 	};
 	
 	

src/nvtt/tools/compress.cpp

@@ -140,6 +140,7 @@ int main(int argc, char *argv[])
 	bool silent = false;
 	bool bc1n = false;
 	nvtt::Format format = nvtt::Format_BC1;
+	bool premultiplyAlpha = false;
 
 	const char * externalCompressor = NULL;
 
@@ -173,6 +174,10 @@ int main(int argc, char *argv[])
 		{
 			noMipmaps = true;
 		}
+		else if (strcmp("-premula", argv[i]) == 0)
+		{
+			premultiplyAlpha = true;
+		}
 
 		// Compression options.
 		else if (strcmp("-fast", argv[i]) == 0)
@@ -266,7 +271,8 @@ int main(int argc, char *argv[])
 		printf("  -tonormal\tConvert input to normal map.\n");
 		printf("  -clamp   \tClamp wrapping mode (default).\n");
 		printf("  -repeat  \tRepeat wrapping mode.\n");
-		printf("  -nomips  \tDisable mipmap generation.\n\n");
+		printf("  -nomips  \tDisable mipmap generation.\n");
+		printf("  -premula \tPremultiply alpha into color channel.\n\n");
 
 		printf("Compression options:\n");
 		printf("  -fast    \tFast compression.\n");
@@ -373,6 +379,11 @@ int main(int argc, char *argv[])
 		inputOptions.setMipmapGeneration(false);
 	}
 
+	if (premultiplyAlpha)
+	{
+		inputOptions.setPremultiplyAlpha(true);
+		inputOptions.setAlphaMode(nvtt::AlphaMode_Premultiplied);
+	}
 
 	nvtt::CompressionOptions compressionOptions;
 	compressionOptions.setFormat(format);