Constructing Images fromarray/buffer in Greyscale vs. Palettized

[TheRealQuantam]

I'm noticing in Pillow 9.2.0 that fromarray et al with P mode create a palette and attach it to the image, which does not happen when the buffer is loaded as L. This makes P much slower than L to load even when the default palette is immediately overwritten by putpalette (and it's thus highly recommended to load as L and then use putpalette). If this hasn't already been changed since then I'd suggest either

1. Defer palette initialization until the palette is actually used with P
2. Allow a palette to be passed to fromarray et al
3. Note in the documentation that it's recommended to load the image as L if you're going to putpalette (at least I didn't see anything like that)

[radarhere]

Could we get a code example to demonstrate the speed difference?

I'm noticing in Pillow 9.2.0

Let us know if you mean that this speed difference just started in 9.2.0. I suspect you don't mean that.

[radarhere]

Cool. I was going to suggest that if you have wheel, pip and the dependencies installed, python3 -m pip install git+https://github.com/radarhere/Pillow.git@palette is simplest, but you figured something out.

Is that PR sufficiently fast then?

[TheRealQuantam]

Almost an order of magnitude speedup. I'd still mention that L is significantly faster than P (twice as fast) in the documentation, though.

[radarhere]

On your machine, with the PR, this is a 1ms speed difference when loading 139 images. On my machine, it is an 0.5ms speed difference. I think documenting it would make users think it is a better problem than that.

[radarhere]

#6567 has now been merged.