feat(fits): add FITS image support

[cek]

No description provided.

[Tom94]

Heya, thanks a lot for the PR. Currently don't have all that much bandwidth, so might be a little bit until I get to review & merge.

[cek]

No worries at all -- no rush, and thanks! I'm just happy to be able to examine astrophoto images in my favorite viewer, rather than having to use one of the terrible alternatives. ;)

To be a more full-featured replacement for said alternatives, tev would also need a astrophoto-centric tone mapping operator and associated controls. However, such a specific/extreme tone mapper probably won't be of interest to most tev users, and thus would be a waste of UI space to render it all of the time. Once I have something working, I'm likely to have questions about how to integrate it such that it doesn't clutter the UI when not needed (e.g., a visibility toggle, defaulting to rendering it if a FITS image is loaded, for example...?) Or it may prove to be too specialized to add to tev proper. In any case, more to come.

[Tom94]

I was planning to add soft clipping (log-space sigmoid with configurable min/max brightness) to the HDR popup menu. Probably not the kind of tone mapping you are looking for 😅, but mentioning it just in case.

I think other specialized tone maps would also fit into the HDR popup for now… to be consolidated in an actual tone map selector when/if tev gets support for a bunch of other standard graphics ones (that’d share aforementioned min/max brightness controls and also consume image metadata like max FALL etc.)

[Tom94]

Thanks again for the PR! Looks mostly good to me -- just requesting a few minor changes.

EDIT: oh, and I almost forgot: could you add a fits entry to void ImageViewer::openImageDialog() as well?

[cek]

Thanks and sorry about that -- a little too quick on the PR trigger...

[Tom94]

Thanks a lot! I think I can take it from here. Just one more question to make sure: are uint8 fits images really linearly encoded?

If they were sRGB-encoded you could use toFloat32<true>(...) to do the decoding, but generic transfers like gamma curves require a bit of extra code.

And: is there no signed int8 format?

[cek]

The vast majority of FITS files hold either linear 12-to-16-bit raw sensor data as uint16, or linear processed (and possibly negative) data as fp32.

They can also hold processed data that is wildly nonlinear -- extreme "stretching" of shadow/lower-order bits where the interesting astro signals live. But FITS data is essentially never sRGB-encoded.

FITS has no real support for signed 8-bit data. I don't think I've ever seen an 8-bit FITS file in the wild, to be honest.

Thanks again!