IMAGE: Add a paletted transparency mode flag to PNGDecoder

Previously, the PNGDecoder would always convert images that
have a palette with a transparent color(s) to a full RGBA
surface automatically. There needed to be a way to diable
this and keep the image paletted for the Glk engine, since
some Infocom V6 game Blorb files reuse the palettes from
previous images, so I couldn't have the decoder using the
dummy palette that comes with the image

2 files changed, 20 insertions, 3 deletions

diff --git a/image/png.cpp b/image/png.cpp
index d8351711d9..e854651da6 100644
--- a/image/png.cpp
+++ b/image/png.cpp
@@ -43,7 +43,9 @@ PNGDecoder::PNGDecoder() :
         _outputSurface(0),
         _palette(0),
         _paletteColorCount(0),
-        _skipSignature(false) {
+        _skipSignature(false),
+		_keepTransparencyPaletted(false),
+		_transparentColor(-1) {
 }
 
 PNGDecoder::~PNGDecoder() {
@@ -159,7 +161,7 @@ bool PNGDecoder::loadStream(Common::SeekableReadStream &stream) {
 
 	// Images of all color formats except PNG_COLOR_TYPE_PALETTE
 	// will be transformed into ARGB images
-	if (colorType == PNG_COLOR_TYPE_PALETTE && !png_get_valid(pngPtr, infoPtr, PNG_INFO_tRNS)) {
+	if (colorType == PNG_COLOR_TYPE_PALETTE && (_keepTransparencyPaletted || !png_get_valid(pngPtr, infoPtr, PNG_INFO_tRNS))) {
 		int numPalette = 0;
 		png_colorp palette = NULL;
 		uint32 success = png_get_PLTE(pngPtr, infoPtr, &palette, &numPalette);
@@ -175,6 +177,16 @@ bool PNGDecoder::loadStream(Common::SeekableReadStream &stream) {
 			_palette[(i * 3) + 2] = palette[i].blue;
 
 		}
+
+		if (png_get_valid(pngPtr, infoPtr, PNG_INFO_tRNS)) {
+			png_bytep trans;
+			int numTrans;
+			png_color_16p transColor;
+			png_get_tRNS(pngPtr, infoPtr, &trans, &numTrans, &transColor);
+			assert(numTrans == 1);
+			_transparentColor = *trans;
+		}
+
 		_outputSurface->create(width, height, Graphics::PixelFormat::createFormatCLUT8());
 		png_set_packing(pngPtr);
 	} else {
diff --git a/image/png.h b/image/png.h
index adbc6d7a68..f983da467e 100644
--- a/image/png.h
+++ b/image/png.h
@@ -57,8 +57,9 @@ public:
 	const Graphics::Surface *getSurface() const { return _outputSurface; }
 	const byte *getPalette() const { return _palette; }
 	uint16 getPaletteColorCount() const { return _paletteColorCount; }
+	int getTransparentColor() const { return _transparentColor; }
 	void setSkipSignature(bool skip) { _skipSignature = skip; }
-
+	void setKeepTransparencyPaletted(bool keep) { _keepTransparencyPaletted = keep; }
 private:
 	Graphics::PixelFormat getByteOrderRgbaPixelFormat() const;
 
@@ -68,6 +69,10 @@ private:
 	// flag to skip the png signature check for headless png files
 	bool _skipSignature;
 
+	// Flag to keep paletted images paletted, even when the image has transparency
+	bool _keepTransparencyPaletted;
+	int _transparentColor;
+
 	Graphics::Surface *_outputSurface;
 };