Add packing from RGBA to BGRa

Tests/test_mode_bgra.py

@@ -0,0 +1,23 @@
+from helper import unittest, PillowTestCase
+
+from PIL import Image
+
+
+class TestBGRa(PillowTestCase):
+
+    def test_bgra(self):
+        RGBA_RED_50 = b'\xff\x00\x00\x80'  # 50% red RGBA
+        BGRa_RED_50 = b'\x00\x00\x80\x80'  # 50% red BGRa
+        RGBa_RED_50 = b'\x80\x00\x00\x80'  # 50% red RGBa
+
+        im = Image.frombuffer("RGBA", (1, 1), BGRa_RED_50, "raw", "BGRa", 4, 1)
+        self.assertEqual(im.tobytes(), RGBA_RED_50)
+        self.assertEqual(im.tobytes('raw', 'BGRa'), BGRa_RED_50)
+
+        im = Image.frombuffer("RGBa", (1, 1), BGRa_RED_50, "raw", "BGRa", 4, 1)
+        self.assertEqual(im.tobytes(), RGBa_RED_50)
+        self.assertEqual(im.tobytes('raw', 'BGRa'), BGRa_RED_50)
+
+
+if __name__ == '__main__':
+    unittest.main()

libImaging/Pack.c

@@ -72,6 +72,10 @@
 #define C64L C64N
 #endif
 
+/* like (a * b + 127) / 255), but much faster on most platforms */
+#define MULDIV255(a, b, tmp)\
+        (tmp = (a) * (b) + 128, ((((tmp) >> 8) + (tmp)) >> 8))
+
 static void
 pack1(UINT8* out, const UINT8* in, int pixels)
 {
@@ -315,6 +319,21 @@ ImagingPackABGR(UINT8* out, const UINT8* in, int pixels)
     }
 }
 
+void
+ImagingPackBGRa(UINT8* out, const UINT8* in, int pixels)
+{
+    int i;
+    /* BGRa, reversed bytes with premultiplied alplha */
+    for (i = 0; i < pixels; i++) {
+	int alpha = out[3] = in[A];
+	int tmp;
+	out[0] = MULDIV255(in[B], alpha, tmp);
+	out[1] = MULDIV255(in[G], alpha, tmp);
+	out[2] = MULDIV255(in[R], alpha, tmp);
+	out += 4; in += 4;
+    }
+}
+
 static void
 packRGBL(UINT8* out, const UINT8* in, int pixels)
 {
@@ -525,6 +544,7 @@ static struct {
     {"RGBA",	"BGR",		24,	ImagingPackBGR},
     {"RGBA",	"BGRA",		32,	ImagingPackBGRA},
     {"RGBA",	"ABGR",		32,	ImagingPackABGR},
+    {"RGBA",	"BGRa",		32,	ImagingPackBGRa},
     {"RGBA",   	"R",            8,      band0},
     {"RGBA",   	"G",            8,      band1},
     {"RGBA",   	"B",            8,      band2},