Improved conversion from RGB to RGBa, LA and La

Tests/test_image_convert.py

@@ -183,6 +183,14 @@ def test_trns_RGB(tmp_path: Path) -> None:
     assert im_l.info["transparency"] == im_l.getpixel((0, 0))  # undone
     im_l.save(f)
 
+    im_la = im.convert("LA")
+    assert "transparency" not in im_la.info
+    im_la.save(f)
+
+    im_la = im.convert("La")
+    assert "transparency" not in im_la.info
+    assert im_la.getpixel((0, 0)) == (0, 0)
+
     im_p = im.convert("P")
     assert "transparency" in im_p.info
     im_p.save(f)
@@ -191,6 +199,10 @@ def test_trns_RGB(tmp_path: Path) -> None:
     assert "transparency" not in im_rgba.info
     im_rgba.save(f)
 
+    im_rgba = im.convert("RGBa")
+    assert "transparency" not in im_rgba.info
+    assert im_rgba.getpixel((0, 0)) == (0, 0, 0, 0)
+
     im_p = pytest.warns(UserWarning, im.convert, "P", palette=Image.Palette.ADAPTIVE)
     assert "transparency" not in im_p.info
     im_p.save(f)

src/PIL/Image.py

@@ -979,7 +979,7 @@ def convert_transparency(m, v):
         # transparency handling
         if has_transparency:
             if (self.mode in ("1", "L", "I", "I;16") and mode in ("LA", "RGBA")) or (
-                self.mode == "RGB" and mode == "RGBA"
+                self.mode == "RGB" and mode in ("La", "LA", "RGBa", "RGBA")
             ):
                 # Use transparent conversion to promote from transparent
                 # color to an alpha channel.

src/libImaging/Convert.c

@@ -499,26 +499,27 @@ rgba2rgb_(UINT8 *out, const UINT8 *in, int xsize) {
 }
 
 /*
- * Conversion of RGB + single transparent color to RGBA,
- * where any pixel that matches the color will have the
- * alpha channel set to 0
+ * Conversion of RGB + single transparent color either to
+ * RGBA or LA, where any pixel matching the color will have the alpha channel set to 0, or
+ * RGBa or La, where any pixel matching the color will have all channels set to 0
  */
 
 static void
-rgbT2rgba(UINT8 *out, int xsize, int r, int g, int b) {
+rgbT2a(UINT8 *out, UINT8 *in, int xsize, int r, int g, int b, int premultiplied) {
 #ifdef WORDS_BIGENDIAN
     UINT32 trns = ((r & 0xff) << 24) | ((g & 0xff) << 16) | ((b & 0xff) << 8) | 0xff;
-    UINT32 repl = trns & 0xffffff00;
+    UINT32 repl = premultiplied ? 0 : (trns & 0xffffff00);
 #else
     UINT32 trns = (0xffU << 24) | ((b & 0xff) << 16) | ((g & 0xff) << 8) | (r & 0xff);
-    UINT32 repl = trns & 0x00ffffff;
+    UINT32 repl = premultiplied ? 0 : (trns & 0x00ffffff);
 #endif
 
     int i;
 
-    for (i = 0; i < xsize; i++, out += sizeof(trns)) {
+    UINT8 *ref = in != NULL ? in : out;
+    for (i = 0; i < xsize; i++, ref += sizeof(trns), out += sizeof(trns)) {
         UINT32 v;
-        memcpy(&v, out, sizeof(v));
+        memcpy(&v, ref, sizeof(v));
         if (v == trns) {
             memcpy(out, &repl, sizeof(repl));
         }
@@ -941,12 +942,14 @@ static struct {
     {"RGB", "1", rgb2bit},
     {"RGB", "L", rgb2l},
     {"RGB", "LA", rgb2la},
+    {"RGB", "La", rgb2la},
     {"RGB", "I", rgb2i},
     {"RGB", "F", rgb2f},
     {"RGB", "BGR;15", rgb2bgr15},
     {"RGB", "BGR;16", rgb2bgr16},
     {"RGB", "BGR;24", rgb2bgr24},
     {"RGB", "RGBA", rgb2rgba},
+    {"RGB", "RGBa", rgb2rgba},
     {"RGB", "RGBX", rgb2rgba},
     {"RGB", "CMYK", rgb2cmyk},
     {"RGB", "YCbCr", ImagingConvertRGB2YCbCr},
@@ -1681,14 +1684,27 @@ ImagingConvertTransparent(Imaging imIn, const char *mode, int r, int g, int b) {
     ImagingSectionCookie cookie;
     ImagingShuffler convert;
     Imaging imOut = NULL;
+    int premultiplied = 0;
+    // If the transparency matches pixels in the source image, not the converted image
+    UINT8 *source;
+    int source_transparency = 0;
     int y;
 
     if (!imIn) {
         return (Imaging)ImagingError_ModeError();
     }
 
-    if (strcmp(imIn->mode, "RGB") == 0 && strcmp(mode, "RGBA") == 0) {
+    if (strcmp(imIn->mode, "RGB") == 0 && (strcmp(mode, "RGBA") == 0 || strcmp(mode, "RGBa") == 0)) {
         convert = rgb2rgba;
+        if (strcmp(mode, "RGBa") == 0) {
+            premultiplied = 1;
+        }
+    } else if (strcmp(imIn->mode, "RGB") == 0 && (strcmp(mode, "LA") == 0 || strcmp(mode, "La") == 0)) {
+        convert = rgb2la;
+        source_transparency = 1;
+        if (strcmp(mode, "La") == 0) {
+            premultiplied = 1;
+        }
     } else if ((strcmp(imIn->mode, "1") == 0 ||
                 strcmp(imIn->mode, "I") == 0 ||
                 strcmp(imIn->mode, "I;16") == 0 ||
@@ -1726,7 +1742,9 @@ ImagingConvertTransparent(Imaging imIn, const char *mode, int r, int g, int b) {
     ImagingSectionEnter(&cookie);
     for (y = 0; y < imIn->ysize; y++) {
         (*convert)((UINT8 *)imOut->image[y], (UINT8 *)imIn->image[y], imIn->xsize);
-        rgbT2rgba((UINT8 *)imOut->image[y], imIn->xsize, r, g, b);
+
+        source = source_transparency ? (UINT8 *)imIn->image[y] : NULL;
+        rgbT2a((UINT8 *)imOut->image[y], source, imIn->xsize, r, g, b, premultiplied);
     }
     ImagingSectionLeave(&cookie);