Affine transformation fill parameter seems to do nothing

[Erotemic]

I would like to use PIL to perform an affine warp on an image, but I want to specify a custom fill value for undefined pixels. The transform function seems to have a fill=1 argument, but it is undocumented and looking further in the code it seems to be unused.

Is there any way to get a custom fill value in an affine transform?

[sharkovsky]

I am also interested in this!

[wiredfool]

Internally, in _imaging.c, the fill parameter is passed in, but subsequent calls into the core transform routines set it to 1. In those routines, the fill parameter is a flag for memset(0) on the non-affected locations. In theory, it should be possible to set this fill to some pixel value.

I've got a test implementation in https://github.com/wiredfool/Pillow/tree/issue_2837

It extends the interface rather than repurposing the fill parameter, in case someone is using the fill parameter on an ImagingTransformHandler.