Extract image conversion from RGBA to BGRA premultiplied into common function.

2 files changed, 17 insertions, 0 deletions

diff --git a/src/XPM.cxx b/src/XPM.cxx
index f95580403..380ae14c4 100644
--- a/src/XPM.cxx
+++ b/src/XPM.cxx
@@ -264,6 +264,21 @@ void RGBAImage::SetPixel(int x, int y, ColourDesired colour, int alpha) {
 	pixel[3] = static_cast<unsigned char>(alpha);
 }
 
+// Transform a block of pixels from RGBA to BGRA with premultiplied alpha.
+// Used for DrawRGBAImage on some platforms.
+void RGBAImage::BGRAFromRGBA(unsigned char *pixelsBGRA, const unsigned char *pixelsRGBA, size_t count) noexcept {
+	for (size_t i = 0; i < count; i++) {
+		const unsigned char alpha = pixelsRGBA[3];
+		// Input is RGBA, output is BGRA with premultiplied alpha
+		pixelsBGRA[2] = pixelsRGBA[0] * alpha / 255;
+		pixelsBGRA[1] = pixelsRGBA[1] * alpha / 255;
+		pixelsBGRA[0] = pixelsRGBA[2] * alpha / 255;
+		pixelsBGRA[3] = alpha;
+		pixelsRGBA += bytesPerPixel;
+		pixelsBGRA += bytesPerPixel;
+	}
+}
+
 RGBAImageSet::RGBAImageSet() : height(-1), width(-1) {
 }
 
diff --git a/src/XPM.h b/src/XPM.h
index b7b895cdd..f8c908ecb 100644
--- a/src/XPM.h
+++ b/src/XPM.h
@@ -50,6 +50,7 @@ class RGBAImage {
 	float scale;
 	std::vector<unsigned char> pixelBytes;
 public:
+	static constexpr size_t bytesPerPixel = 4;
 	RGBAImage(int width_, int height_, float scale_, const unsigned char *pixels_);
 	explicit RGBAImage(const XPM &xpm);
 	RGBAImage(const RGBAImage &) = default;
@@ -65,6 +66,7 @@ public:
 	int CountBytes() const;
 	const unsigned char *Pixels() const;
 	void SetPixel(int x, int y, ColourDesired colour, int alpha);
+	static void BGRAFromRGBA(unsigned char *pixelsBGRA, const unsigned char *pixelsRGBA, size_t count) noexcept;
 };
 
 /**