Make Palette handle greyscale

[hbldh]

Currently grayscale and BW images does not work very well att all with the palette object. Needs to be implemented.

[vitorio]

I noticed this! As a (temporary?) workaround, I'm currently converting my grayscale image to RGB before dithering it.

[hbldh]

Ah, of course. That should work; feels somewhat dirty though. A grayscale palette might be easy to create though, since a grayscale value x maps to RGB tuple (x, x, x) and then handling the ugly conversions behind the scenes...

[llamasoft]

I was working on a unrelated project. Honestly, I think converting to RGB (or telling the user to do so) is probably the only way to go.
PIL will load images of any "mode" and modes return pixels in different forms. For example:

• 1 and L return raw 8-bit ints.
• LA returns a 2-tuple of 8-bit ints.
• RGB, HSV, LAB, and YCbCr return a 3-tuple of 8-byte ints, but the values all represent something different.
• RGBA and CMYK returns a 4-tuple of 8-bit ints, again with different meanings.
• F returns a 32-bit floating point value.

I don't think it's the code's responsibility to handle the conversion for the user, but if the user passes an Image type not in RGB mode or sequences of non-3 length, an exception should probably be raised.

[tatarize]

if image.mode !=' RGB':
    image.convert('RGB')

It not very harsh with regards to responsibility.