Export: proposal for image format options

[scurest]

I want to propose changes to the behaviour of the image format options in the exporter

Automatic (the default)

Basically I want to propose doing this

if export image corresponds to an existing image:
    if existing_image.file_format == 'JPEG':
        file_format = 'JPEG'
    else:
        file_format = 'PNG'
else:
    file_format = 'PNG'

IOW, this option would always favor PNGs except that (quoting from #203)

if the exporter already has access to a source PNG or JPG, it should be just copying the already-compressed data from there directly

This fixes two things:

• Currently this option does not make good use of an already-compressed JPEG. It will only encode an existing image as a JPEG if its name ends with .jpg/.jpeg. It would be better not to use the name, but to look at the image.file_format field.

  This can make a big difference in performance. For Sponza for instance

  • exporting with Automatic, all the JPEGs are re-encoded as PNGs
    • export time: 32s
    • resulting filesize: 116M
  • exporting with JPEG Format (.jpg), all JPEGs just copied over
    • export time: 22s
    • resulting filesize: 45M

• When an image needs to be packed from channels, the packed image can sometimes be a JPEG if one of the constituents is (note: only sometimes!). This is lossy and causes channel-bleed (AO texture influences roughtness #818). I think a better default is to prefer being lossless.

PNG format (.png)

Remove it. Since Automatic would favor PNGs, it's main difference would be that it re-encodes JPEGs as PNG. It would basically become a "make my file bigger" option.

JPEG format (.jpg)

This is okay, no change.

[emackey]

I like this idea. In particular I think there should never be a case where an existing JPG needlessly becomes a PNG without any channel reassignments.

[donmccurdy]

Agreed. 👍

[scurest]

Okay, cool. I put up a PR for it.

[emackey]

Fixed in #883.

[globglob3D]

When an image needs to be packed from channels, the packed image can sometimes be a JPEG if one of the constituents is (note: only sometimes!). This is lossy and causes channel-bleed (#818). I think a better default is to prefer being lossless.

If all constituents are JPEG I think the resulting packed image should be JPEG too. Yes this would cause some amount of bleed but
you decrease the size of the texture by a lot (up to 10 times?). If you choose to have AO/R/N as JPEG initially it means you care more about final glb size than quality. Or perhaps we should add an option regarding packed image format in the exporter?

#1318

[donmccurdy]

More recent discussion in: #1099