Hoist common calculation into function.

Use logical and to stop conversion warning.

1 files changed, 11 insertions, 3 deletions

diff --git a/src/XPM.cxx b/src/XPM.cxx
index 31d87a06d..42c4dd96a 100644
--- a/src/XPM.cxx
+++ b/src/XPM.cxx
@@ -264,15 +264,23 @@ void RGBAImage::SetPixel(int x, int y, ColourRGBA colour) noexcept {
 	pixel[3] = colour.GetAlpha();
 }
 
+namespace {
+
+unsigned char AlphaMultiplied(unsigned char value, unsigned char alpha) {
+	return (value * alpha / UCHAR_MAX) & 0xffU;
+}
+
+}
+
 // 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 / UCHAR_MAX;
-		pixelsBGRA[1] = pixelsRGBA[1] * alpha / UCHAR_MAX;
-		pixelsBGRA[0] = pixelsRGBA[2] * alpha / UCHAR_MAX;
+		pixelsBGRA[2] = AlphaMultiplied(pixelsRGBA[0], alpha);
+		pixelsBGRA[1] = AlphaMultiplied(pixelsRGBA[1], alpha);
+		pixelsBGRA[0] = AlphaMultiplied(pixelsRGBA[2], alpha);
 		pixelsBGRA[3] = alpha;
 		pixelsRGBA += bytesPerPixel;
 		pixelsBGRA += bytesPerPixel;