When saving, allow alpha differences to indicate different frames

Tests/test_file_apng.py

@@ -374,6 +374,20 @@ def test_apng_save(tmp_path):
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
 
+def test_apng_save_alpha(tmp_path):
+    test_file = str(tmp_path / "temp.png")
+
+    im = Image.new("RGBA", (1, 1), (255, 0, 0, 255))
+    im2 = Image.new("RGBA", (1, 1), (255, 0, 0, 127))
+    im.save(test_file, save_all=True, append_images=[im2])
+
+    with Image.open(test_file) as reloaded:
+        assert reloaded.getpixel((0, 0)) == (255, 0, 0, 255)
+
+        reloaded.seek(1)
+        assert reloaded.getpixel((0, 0)) == (255, 0, 0, 127)
+
+
 def test_apng_save_split_fdat(tmp_path):
     # test to make sure we do not generate sequence errors when writing
     # frames with image data spanning multiple fdAT chunks (in this case

src/PIL/PngImagePlugin.py

@@ -1138,9 +1138,9 @@ def _write_multiple_frames(im, fp, chunk, rawmode, default_image, append_images)
                 else:
                     base_im = previous["im"]
                 delta = ImageChops.subtract_modulo(
-                    im_frame.convert("RGB"), base_im.convert("RGB")
+                    im_frame.convert("RGBA"), base_im.convert("RGBA")
                 )
-                bbox = delta.getbbox()
+                bbox = delta.im.getbbox(False)
                 if (
                     not bbox
                     and prev_disposal == encoderinfo.get("disposal")

src/_imaging.c

@@ -2160,9 +2160,15 @@ _isblock(ImagingObject *self) {
 }
 
 static PyObject *
-_getbbox(ImagingObject *self) {
+_getbbox(ImagingObject *self, PyObject *args) {
     int bbox[4];
-    if (!ImagingGetBBox(self->image, bbox)) {
+
+    int consider_alpha = 1;
+    if (!PyArg_ParseTuple(args, "|i", &consider_alpha)) {
+        return NULL;
+    }
+
+    if (!ImagingGetBBox(self->image, bbox, consider_alpha)) {
         Py_INCREF(Py_None);
         return Py_None;
     }
@@ -3574,7 +3580,7 @@ static struct PyMethodDef methods[] = {
 
     {"isblock", (PyCFunction)_isblock, METH_NOARGS},
 
-    {"getbbox", (PyCFunction)_getbbox, METH_NOARGS},
+    {"getbbox", (PyCFunction)_getbbox, METH_VARARGS},
     {"getcolors", (PyCFunction)_getcolors, METH_VARARGS},
     {"getextrema", (PyCFunction)_getextrema, METH_NOARGS},
     {"getprojection", (PyCFunction)_getprojection, METH_NOARGS},

src/libImaging/GetBBox.c

@@ -19,7 +19,7 @@
 #include "Imaging.h"
 
 int
-ImagingGetBBox(Imaging im, int bbox[4]) {
+ImagingGetBBox(Imaging im, int bbox[4], int consider_alpha) {
     /* Get the bounding box for any non-zero data in the image.*/
 
     int x, y;
@@ -58,10 +58,11 @@ ImagingGetBBox(Imaging im, int bbox[4]) {
         INT32 mask = 0xffffffff;
         if (im->bands == 3) {
             ((UINT8 *)&mask)[3] = 0;
-        } else if (
+        } else if (consider_alpha && (
             strcmp(im->mode, "RGBa") == 0 || strcmp(im->mode, "RGBA") == 0 ||
             strcmp(im->mode, "La") == 0 || strcmp(im->mode, "LA") == 0 ||
-            strcmp(im->mode, "PA") == 0) {
+            strcmp(im->mode, "PA") == 0
+        )) {
 #ifdef WORDS_BIGENDIAN
             mask = 0x000000ff;
 #else

src/libImaging/Imaging.h

@@ -317,7 +317,7 @@ ImagingMerge(const char *mode, Imaging bands[4]);
 extern int
 ImagingSplit(Imaging im, Imaging bands[4]);
 extern int
-ImagingGetBBox(Imaging im, int bbox[4]);
+ImagingGetBBox(Imaging im, int bbox[4], int consider_alpha);
 typedef struct {
     int x, y;
     INT32 count;