GRAPHICS: Add ImageDecoder::isTransparent()

src/graphics/images/decoder.cpp

@@ -74,6 +74,49 @@ bool ImageDecoder::hasAlpha() const {
 	return hasAlpha(_format);
 }
 
+bool ImageDecoder::isTransparent() const {
+	if (_mipMaps.empty())
+		return false;
+
+	const MipMap &mipMap = *_mipMaps[0];
+
+	const int   pixels = mipMap.width * mipMap.height;
+	const byte *data   = mipMap.data;
+
+	switch (_format) {
+		case kPixelFormatR8G8B8A8:
+			for (int i = 0; i < pixels; i++, data += 4)
+				if (data[3] != 0xFF)
+					return true;
+			break;
+
+		case kPixelFormatB8G8R8A8:
+			for (int i = 0; i < pixels; i++, data += 4)
+				if (data[3] != 0xFF)
+					return true;
+			break;
+
+		case kPixelFormatA1R5G5B5:
+			for (int i = 0; i < pixels; i++, data += 2)
+				if (!(data[0] & 0x80))
+					return true;
+			break;
+
+		case kPixelFormatDXT3:
+			return isTransparentDXT3(mipMap.data, mipMap.size);
+			break;
+
+		case kPixelFormatDXT5:
+			return isTransparentDXT5(mipMap.data, mipMap.size);
+			break;
+
+		default:
+			break;
+	}
+
+	return false;
+}
+
 PixelFormat ImageDecoder::getFormat() const {
 	return _format;
 }

src/graphics/images/decoder.h

@@ -65,9 +65,12 @@ class ImageDecoder {
 	/** Is the image data compressed? */
 	bool isCompressed() const;
 
-	/** Does the image data have alpha? .*/
+	/** Does the image data have an alpha channel? */
 	bool hasAlpha() const;
 
+	/** Does the image have transparent alpha values? */
+	bool isTransparent() const;
+
 	/** Return the image data's format. */
 	PixelFormat getFormat() const;
 

src/graphics/images/s3tc.cpp

@@ -200,4 +200,34 @@ void decompressDXT5(byte *dest, Common::SeekableReadStream &src, uint32 width, u
 	}
 }
 
+bool isTransparentDXT3(const byte *data, uint32 size) {
+	for ( ; size > 15; size -= 16, data += 16)
+		if ((data[0] & data[1] & data[2] & data[3] & data[4] & data[5] & data[6] & data[7]) != 0xFF)
+			return true;
+
+	return false;
+}
+
+bool isTransparentDXT5(const byte *data, uint32 size) {
+	for ( ; size > 15; size -= 16, data += 16) {
+		const byte a0 = data[0];
+		const byte a1 = data[1];
+
+		uint64 abl = READ_LE_UINT32(data + 2) | ((uint64)READ_LE_UINT16(data + 6) << 32);
+
+		for (int i = 0; i < 16; i++, abl >>= 3) {
+			const uint8 a = abl & 7;
+
+			const bool o1 = ((a == 0) && (a0 == 0xFF)) || ((a == 1) && (a1 == 0xFF));
+			const bool o2 = (a0 == a1) && (a0 == 0xFF) && (a != 6);
+			const bool o3 = (a0 < a1) && (a == 7);
+
+			if (!(o1 || o2 || o3))
+				return true;
+		}
+	}
+
+	return false;
+}
+
 } // End of namespace Graphics

src/graphics/images/s3tc.h

@@ -42,6 +42,9 @@ void decompressDXT1(byte *dest, Common::SeekableReadStream &src, uint32 width, u
 void decompressDXT3(byte *dest, Common::SeekableReadStream &src, uint32 width, uint32 height, uint32 pitch);
 void decompressDXT5(byte *dest, Common::SeekableReadStream &src, uint32 width, uint32 height, uint32 pitch);
 
+bool isTransparentDXT3(const byte *data, uint32 size);
+bool isTransparentDXT5(const byte *data, uint32 size);
+
 } // End of namespace Graphics
 
 #endif // GRAPHICS_IMAGES_S3TC_H