Enable writing I;16B images as PNG

[FirefoxMetzger]

While investigating an ImageIO issue concerning behavior on big-endian architectures I've noticed that pillow's PNG plugin currently doesn't allow writing 16-bit big-endian images to PNG. I;16 is already supported, so I was wondering if it would be possible to add I;16B as well.

From what I can see, the only change needed to support this would be to add the line "I;16B": ("I;16B", b"\x10\x00"), to this dict:

Pillow/src/PIL/PngImagePlugin.py

Lines 1035 to 1051 in 3c5324b

	_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"),
	"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"),
	}

With this change we would be able to do 16 bit round-trips on big-endian, i.e., we can decode the pixel data directly into a 16 bit big-endian buffer and then use that same buffer when encoding as PNG. This would improve performance and streamline working with native encoding on big-endian machines.

[radarhere]

I've created PR #7302 to resolve this.

[homm]

This would improve performance and streamline working with native encoding on big-endian machines.

I;16 is already native order for any machines, isn't it?

[FirefoxMetzger]

I;16 is already native order for any machines, isn't it?

Yes, I always thought so as well. However, if you check the unpack function for I;16 in Unpack.c it looks like I;16 is treated as little-endian:

Pillow/src/libImaging/Unpack.c

Lines 1152 to 1160 in 3c5324b

	unpackI16B_I16(UINT8 *out, const UINT8 *in, int pixels) {
	int i;
	for (i = 0; i < pixels; i++) {
	out[0] = in[1];
	out[1] = in[0];
	in += 2;
	out += 2;
	}
	}

So I am not sure if I;16 is supposed to be native (and there is a macro missing in the function), or if it is supposed to be little-endian.