P to PA conversion loses color

[Yay295]

What did you do?

using https://github.com/python-pillow/Pillow/blob/main/Tests/images/hopper.jpg

h = Image.open('hopper.jpg')
h.convert('P').show()
h.convert('PA').show()
h.convert('P').convert('PA').show()
h.convert('PA').convert('P').show()

What did you expect to happen?

I expected all of the images to look essentially the same.

What actually happened?

The image converted to P then to PA has no color.

What are your OS, Python, and Pillow versions?

• OS: Windows 10
• Python: 3.9.6
• Pillow: 9.2.0

[Yay295]

I think the issue is here:

Pillow/src/libImaging/Convert.c

Lines 1029 to 1040 in 7f39a21

	static void
	p2pa(UINT8 *out, const UINT8 *in, int xsize, ImagingPalette palette) {
	int x;
	int rgb = strcmp(palette->mode, "RGB");
	for (x = 0; x < xsize; x++, in++) {
	const UINT8 *rgba = &palette->palette[in[0] * 4];
	*out++ = in[0];
	*out++ = in[0];
	*out++ = in[0];
	*out++ = rgb == 0 ? 255 : rgba[3];
	}
	}

This function was previously changed two times:

1. 4f3b449#diff-504185fb56f577513c6b401b51e9d36abfabf172655309861f5f4f34892abf2f
2. https://github.com/python-pillow/Pillow/pull/6337/files#diff-504185fb56f577513c6b401b51e9d36abfabf172655309861f5f4f34892abf2f

I think the correct function would be:

static void
p2pa(UINT8 *out, const UINT8 *in, int xsize, ImagingPalette palette) {
    int x;
    int rgb = strcmp(palette->mode, "RGB");
    for (x = 0; x < xsize; x++, in++) {
        const UINT8 *rgba = &palette->palette[in[0]*4];
        *out++ = in[0];
        *out++ = in[1]; // <- change 0 to 1
        *out++ = in[2]; // <- change 0 to 2
        *out++ = rgb == 0 ? 255 : rgba[3];
    }
}

[radarhere]

The problem isn't in the pixels themselves. The PA mode is similar to the P mode - each pixel is just points to an index in the palette - except that PA also has an alpha channel. The problem is that the palette isn't correctly transferred.

from PIL import Image
h = Image.open('hopper.jpg')

im_p = h.convert('P')
print(im_p.getpalette()[:40])  # [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 51, 0, 0, 102, 0, 0, 153]

im_pa = h.convert('P').convert('PA')
print(im_pa.getpalette()[:40])  # [0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6, 7, 7, 7, 8, 8, 8, 9, 9, 9, 10, 10, 10, 11, 11, 11, 12, 12, 12, 13]

I've created PR #6497 to resolve this, by copying the palette into the new image.

[Yay295]

Oh yeah, that makes sense. Shouldn't PA only need two bytes for each pixel though? It looks like LA and La also use more space than they need.

Pillow/src/libImaging/Storage.c

Lines 77 to 99 in 95cff6e

	} else if (strcmp(mode, "PA") == 0) {
	/* 8-bit palette with alpha */
	im->bands = 2;
	im->pixelsize = 4; /* store in image32 memory */
	im->linesize = xsize * 4;
	im->palette = ImagingPaletteNew("RGB");
	} else if (strcmp(mode, "L") == 0) {
	/* 8-bit greyscale (luminance) images */
	im->bands = im->pixelsize = 1;
	im->linesize = xsize;
	} else if (strcmp(mode, "LA") == 0) {
	/* 8-bit greyscale (luminance) with alpha */
	im->bands = 2;
	im->pixelsize = 4; /* store in image32 memory */
	im->linesize = xsize * 4;
	} else if (strcmp(mode, "La") == 0) {
	/* 8-bit greyscale (luminance) with premultiplied alpha */
	im->bands = 2;
	im->pixelsize = 4; /* store in image32 memory */
	im->linesize = xsize * 4;

[radarhere]

I can't think of any reason why that should be, I'll only say that PA and LA have been stored in 4 bytes since PIL.

If you'd like to make that change, you could create a PR. Or if you'd like to just request it, you could create a new issue.

[radarhere]

The request was created as #6503