Remove unused ImagingConvertInPlace

[radarhere]

im.putalpha() is the only place where C's setmode() is called.

Pillow/src/PIL/Image.py

Lines 2050 to 2052 in 3220aca

	mode = getmodebase(self.mode) + "A"
	try:
	self.im.setmode(mode)

This is only trying to convert the image to LA, PA or RGBA.

However, the only operation setmode() can actually succeed at is converting RGB or RGBX to RGBA.

Pillow/src/_imaging.c

Lines 2079 to 2090 in 3220aca

	if (im->mode == mode) {
	; /* same mode; always succeeds */
	} else if (isRGB(im->mode) && isRGB(mode)) {
	/* color to color */
	im->mode = mode;
	im->bands = modelen;
	if (mode == IMAGING_MODE_RGBA) {
	(void)ImagingFillBand(im, 3, 255);
	}
	} else {
	/* trying doing an in-place conversion */
	if (!ImagingConvertInPlace(im, mode)) {

Pillow/src/libImaging/Convert.c

Lines 1739 to 1748 in 3220aca

	ImagingConvertInPlace(Imaging imIn, const ModeID mode) {
	ImagingSectionCookie cookie;
	ImagingShuffler convert;
	int y;
	/* limited support for inplace conversion */
	if (imIn->mode == IMAGING_MODE_L && mode == IMAGING_MODE_1) {
	convert = l2bit;
	} else if (imIn->mode == IMAGING_MODE_1 && mode == IMAGING_MODE_L) {
	convert = bit2l;

ImagingConvertInPlace() isn't able to convert anything to LA, PA or RGBA.

So this PR

• removes ImagingConvertInPlace(). setmode() is the only place where ImagingConvertInPlace() is called.
• simplifies setmode() to more clearly only convert RGB or RGBX images to RGBA. I've also renamed it to setalpha().

[akx]

I had been looking at removing ImagingConvertInPlace (in a bid to see if the shuffler functions could be implemented in a faster way) but didn't dare go through with it.

Some thoughts within though :)

[akx]

This comment is a little stale now?

[radarhere]

Is it? The method is still modifying the mode of an image in place. It's more specific now, is all.

[akx]

I suppose...

Should this function know how to do P-to-PA and L-to-LA too?

[radarhere]

The thought occurred to me. It is a bit different to RGB, since RGB already has the same pixelsize as RGBA.

Do you mind if that is a follow-up PR? I would rather tidy things up first.

[akx]

What's the deprecation policy for methods on the core image objects in general (is it documented)?

GitHub's Code Search isn't making it easy to find whether there are external users of im.im.setmode.

[radarhere]

There is no deprecation period.

#4532 (comment)

Pillow makes a commitment to stable public interfaces, which are defined at the Python layer. The C interfaces are explicitly internal, and no effort is made to keep them stable even between minor releases, and no support or warning is given when they change.

[akx]

That's an useful comment, thanks! Should maybe codify it into development documentation if it isn't already (for instance, I feel much better about touching the palette code even harder in #9829).

And I'll understand this as "the Python-facing interfaces defined in C extension module(s) in Pillow are also malleable" (since this is exactly that). 👍

[radarhere]

I've created #9879