From 4e5f6f01a1b8dfd2dc07f760e79a0ced6d4836a5 Mon Sep 17 00:00:00 2001
From: Neil <nyamatongwe@gmail.com>
Date: Sat, 15 Feb 2020 12:47:29 +1100
Subject: Backport: Extract image conversion from RGBA to BGRA premultiplied
 into common function.

Backport of changeset 7987:beeac488af4d.
---
 gtk/PlatGTK.cxx   | 15 +++++----------
 src/XPM.cxx       | 15 +++++++++++++++
 src/XPM.h         |  2 ++
 win32/PlatWin.cxx | 27 ++++++---------------------
 4 files changed, 28 insertions(+), 31 deletions(-)

diff --git a/gtk/PlatGTK.cxx b/gtk/PlatGTK.cxx
index 97009d4ff..f884ac565 100755
--- a/gtk/PlatGTK.cxx
+++ b/gtk/PlatGTK.cxx
@@ -597,18 +597,13 @@ void SurfaceImpl::DrawRGBAImage(PRectangle rc, int width, int height, const unsi
 		rc.top += (rc.Height() - height) / 2;
 	rc.bottom = rc.top + height;
 
-	int stride = cairo_format_stride_for_width(CAIRO_FORMAT_ARGB32, width);
+	const int stride = cairo_format_stride_for_width(CAIRO_FORMAT_ARGB32, width);
 	const int ucs = stride * height;
 	std::vector<unsigned char> image(ucs);
-	for (int iy=0; iy<height; iy++) {
-		for (int ix=0; ix<width; ix++) {
-			unsigned char *pixel = &image[0] + iy*stride + ix * 4;
-			const unsigned char alpha = pixelsImage[3];
-			pixel[2] = (*pixelsImage++) * alpha / 255;
-			pixel[1] = (*pixelsImage++) * alpha / 255;
-			pixel[0] = (*pixelsImage++) * alpha / 255;
-			pixel[3] = *pixelsImage++;
-		}
+	for (ptrdiff_t iy=0; iy<height; iy++) {
+		unsigned char *pixel = &image[0] + iy*stride;
+		RGBAImage::BGRAFromRGBA(pixel, pixelsImage, width);
+		pixelsImage += RGBAImage::bytesPerPixel * width;
 	}
 
 	cairo_surface_t *psurfImage = cairo_image_surface_create_for_data(&image[0], CAIRO_FORMAT_ARGB32, width, height, stride);
diff --git a/src/XPM.cxx b/src/XPM.cxx
index f2565bf83..9ce82a6f6 100644
--- a/src/XPM.cxx
+++ b/src/XPM.cxx
@@ -263,6 +263,21 @@ 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 9ff612d64..36e541a59 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;
 };
 
 /**
diff --git a/win32/PlatWin.cxx b/win32/PlatWin.cxx
index 3c5a5adf3..27d40fe22 100644
--- a/win32/PlatWin.cxx
+++ b/win32/PlatWin.cxx
@@ -784,15 +784,10 @@ void SurfaceGDI::DrawRGBAImage(PRectangle rc, int width, int height, const unsig
 			HBITMAP hbmOld = SelectBitmap(hMemDC, hbmMem);
 
 			for (int y=height-1; y>=0; y--) {
-				for (int x=0; x<width; x++) {
-					unsigned char *pixel = static_cast<unsigned char *>(image) + (y*width+x) * 4;
-					const unsigned char alpha = pixelsImage[3];
-					// Input is RGBA, output is BGRA with premultiplied alpha
-					pixel[2] = static_cast<unsigned char>((*pixelsImage++) * alpha / 255);
-					pixel[1] = static_cast<unsigned char>((*pixelsImage++) * alpha / 255);
-					pixel[0] = static_cast<unsigned char>((*pixelsImage++) * alpha / 255);
-					pixel[3] = *pixelsImage++;
-				}
+				// Bits flipped vertically
+				unsigned char *pixel = static_cast<unsigned char *>(image) + RGBAImage::bytesPerPixel * y * width;
+				RGBAImage::BGRAFromRGBA(pixel, pixelsImage, width);
+				pixelsImage += RGBAImage::bytesPerPixel * width;
 			}
 
 			const BLENDFUNCTION merge = { AC_SRC_OVER, 0, 255, AC_SRC_ALPHA };
@@ -1436,18 +1431,8 @@ void SurfaceD2D::DrawRGBAImage(PRectangle rc, int width, int height, const unsig
 			rc.top += std::floor((rc.Height() - height) / 2);
 		rc.bottom = rc.top + height;
 
-		std::vector<unsigned char> image(height * width * 4);
-		for (int yPixel=0; yPixel<height; yPixel++) {
-			for (int xPixel = 0; xPixel<width; xPixel++) {
-				unsigned char *pixel = &image[0] + (yPixel*width + xPixel) * 4;
-				const unsigned char alpha = pixelsImage[3];
-				// Input is RGBA, output is BGRA with premultiplied alpha
-				pixel[2] = (*pixelsImage++) * alpha / 255;
-				pixel[1] = (*pixelsImage++) * alpha / 255;
-				pixel[0] = (*pixelsImage++) * alpha / 255;
-				pixel[3] = *pixelsImage++;
-			}
-		}
+		std::vector<unsigned char> image(RGBAImage::bytesPerPixel * height * width);
+		RGBAImage::BGRAFromRGBA(image.data(), pixelsImage, static_cast<ptrdiff_t>(height) * width);
 
 		ID2D1Bitmap *bitmap = nullptr;
 		const D2D1_SIZE_U size = D2D1::SizeU(width, height);
-- 
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