Saving all frames of a paletted PNG with few colors can fail

[Yay295]

>>> image.save('copy.png',save_all=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\...\.venv\Lib\site-packages\PIL\Image.py", line 2459, in save
    save_handler(self, fp, filename)
  File "C:\...\.venv\Lib\site-packages\PIL\PngImagePlugin.py", line 1230, in _save_all
    _save(im, fp, filename, save_all=True)
  File "C:\...\.venv\Lib\site-packages\PIL\PngImagePlugin.py", line 1408, in _save
    im = _write_multiple_frames(
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\...\.venv\Lib\site-packages\PIL\PngImagePlugin.py", line 1117, in _write_multiple_frames
    im_frame = im_frame.convert(rawmode)
               ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\...\.venv\Lib\site-packages\PIL\Image.py", line 1087, in convert
    im = self.im.convert(mode, dither)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: conversion not supported

The image isn't mine to share, but it's a P mode image with only 9 colors. This causes it to hit this bit of code:

Pillow/src/PIL/PngImagePlugin.py

Lines 1265 to 1285 in 8b14ed7

	if mode == "P":
	#
	# attempt to minimize storage requirements for palette images
	if "bits" in im.encoderinfo:
	# number of bits specified by user
	colors = min(1 << im.encoderinfo["bits"], 256)
	else:
	# check palette contents
	if im.palette:
	colors = max(min(len(im.palette.getdata()[1]) // 3, 256), 1)
	else:
	colors = 256
	if colors <= 16:
	if colors <= 2:
	bits = 1
	elif colors <= 4:
	bits = 2
	else:
	bits = 4
	mode = f"{mode};{bits}"

Which changes mode to P;4. Then when getting the output rawmode:

Pillow/src/PIL/PngImagePlugin.py

Line 1297 in 8b14ed7

rawmode, mode = _OUTMODES[mode]

Pillow/src/PIL/PngImagePlugin.py

Lines 1052 to 1069 in 8b14ed7

	_OUTMODES = {
	# supported PIL modes, and corresponding rawmodes/bits/color combinations
	"1": ("1", b"\x01\x00"),
	"L;1": ("L;1", b"\x01\x00"),
	"L;2": ("L;2", b"\x02\x00"),
	"L;4": ("L;4", b"\x04\x00"),
	"L": ("L", b"\x08\x00"),
	"LA": ("LA", b"\x08\x04"),
	"I": ("I;16B", b"\x10\x00"),
	"I;16": ("I;16B", b"\x10\x00"),
	"I;16B": ("I;16B", b"\x10\x00"),
	"P;1": ("P;1", b"\x01\x03"),
	"P;2": ("P;2", b"\x02\x03"),
	"P;4": ("P;4", b"\x04\x03"),
	"P": ("P", b"\x08\x03"),
	"RGB": ("RGB", b"\x08\x02"),
	"RGBA": ("RGBA", b"\x08\x06"),
	}

It gets a rawmode of P;4. This would all be fine, except I passed True for the save_all parameter, which causes a branch here:

Pillow/src/PIL/PngImagePlugin.py

Lines 1415 to 1420 in 8b14ed7

	if save_all:
	im = _write_multiple_frames(
	im, fp, chunk, rawmode, default_image, append_images
	)
	if im:
	ImageFile._save(im, _idat(fp, chunk), [("zip", (0, 0) + im.size, 0, rawmode)])

Inside _write_multiple_frames is this bit of code:

Pillow/src/PIL/PngImagePlugin.py

Lines 1122 to 1125 in 8b14ed7

	if im_frame.mode == rawmode:
	im_frame = im_frame.copy()
	else:
	im_frame = im_frame.convert(rawmode)

Now since the actual image mode is P, but the requested rawmode is P;4, it tries to convert from P to P;4, which fails, because Image.convert() takes modes, not rawmodes, and P;4 is not a valid mode.

This method also attempts to convert to a rawmode a bit further on as well:

Pillow/src/PIL/PngImagePlugin.py

Lines 1186 to 1188 in 8b14ed7

	if default_image:
	if im.mode != rawmode:
	im = im.convert(rawmode)

A basic search of the rest of Pillow's code only shows one other instance of ".convert(rawmode)" (in WebPImagePlugin), but it's okay in that case because the rawmode is set on the prior line to be either "RGB" or "RGBA".

[radarhere]

I've created #8087