[FEATURE REQUEST] oiiotool thumbnail extraction

[lgritz]

Not sure how this oversight happened, but there currently not an oiiotool command that extracts a thumbnail from an image. That would be handy.

For that matter, oiiotool also doesn't make it easy to create or add a thumbnail and save the resulting image. That is trickier, though, requires a bit more design maybe.

[lgritz]

Some possible design ideas (not necessarily mutually exclusive):

1. A separate command to extract thumbnail from the top image that already has been loaded:
   oiiotool input.exr --thumbnail_get -o thumb.exr
   Questions: Should it push the thumbnail to the top of stack as a separate image? Or should it replace the top image with its thumbnail? What happens if there is no thumbnail -- should that be an error, or return the full-res image, or auto-generate a reduced-resolution copy, or return an essentially blank tiny image? Maybe optional modifiers to --thumbnail_get can select among these options.

2. Augment the -i read command with a command to get the thumbnail rather than the main image:
   oiiotool -i:get_thumbnail=1 input.exr -o thumb.exr

3. Generate a thumbnail of an arbitrary image (presumably prior to a write):
   oiiotool input.exr ...commands... --thumbnail_generate -o out_with_thumb.exr

[grdanny]

What is a thumbnail in this context?
some exrs have a smaller version of the image embedded as sub image?
can't you just do the same thing with --resize?

[lgritz]

Lots of image formats allow storing a low rez image in the header that can be used for quick preview without reading the full-res image and resizing it. The OIIO APIs actually allow you to retrieve that preview image (if it exists), but curiously, we never exposed that via oiiotool in a way that would let you extract it and treat it as a real image.