Image.show() on image with mode LA does not show transparency

[gerritholl]

When I use Image.show() on an image with mode LA, transparency is not shown. When I use .save(...), the resulting image has transparency included correctly. With mode RGBA, transparency is correctly displayed on .show() (and correctly written on .save(...)).

What did you do?

I used .show() on an image with mode LA.

What did you expect to happen?

I expected it to display an image with transparency:

What actually happened?

It showed an image without transparency:

What are your OS, Python and Pillow versions?

• OS: openSUSE 15.0
• Python: 3.7.3
• Pillow: 6.0.0

from numpy import linspace, block, concatenate, zeros, full
from PIL import Image

L = linspace(0, 255, 200*200, dtype="u1").reshape(200, 200, 1)
A = block([[zeros((100, 100), "u1"),             full((100, 100), 255, "u1")],
            [full((100, 100), 255, "u1"),
            zeros((100, 100), "u1")]]).reshape(200, 200, 1)
im1 = Image.fromarray((concatenate((L, A), 2)), mode="LA")
im2 = Image.fromarray((concatenate((L, L, L, A), 2)), mode="RGBA")
im1.show()
im2.show()

[radarhere]

This is not a bug as such. ImageShow is deliberately converting the image from LA to L, removing the transparency.

Pillow/src/PIL/ImageShow.py

Lines 73 to 75 in 3368500

	base = Image.getmodebase(image.mode)
	if base != image.mode and image.mode != "1" and image.mode != "RGBA":
	image = image.convert(base)

However, the exception for RGBA that you see was added in 74da587. So it seems feasible to do so for LA as well.

However, ImageShow uses the BMP format for Windows, and Pillow cannot save LA as BMP. Looking at https://en.wikipedia.org/wiki/BMP_file_format

The 32-bit per pixel (32bpp) format supports 4,294,967,296 distinct colors and stores 1 pixel per 4-byte DWORD. Each DWORD can define the alpha, red, green and blue samples of the pixel.

I could be wrong, but I think that BMP can't handle greyscale with alpha.

So I've created PR #3869 to allow ImageShow to use the LA mode, except for Windows.