The low level scanline interface was implemented, though access to palette indexes for paletted images wasn't.

The scanline interface exists but I'm not sure what you're expecting, it doesn't provide access to the file data, but to Imager's image data, wherever the image came from.

You can certainly, for example, make two calls to getscanline() to get the left and right ends of a scanline of an image.

If you can give me some idea of what you're trying to do maybe I can be more help.

That TODO page is seriously out of date, the Future Plans page is more up to date, though I admit I haven't done anything on the large sample support.

Unfortunately, JPEG doesn't compress by line but by 8 x 8 pixel blocks, and the output from that stage is then also compressed, at least as I understand it, so to get your output you're going to have to decompress everything anyway.

Imager doesn't provide any access to the file at this level, you might be able to convince libjpeg to give you some information about each block before it converts it to pixels, but it's not something likely to ever be supported by Imager.