Create .tiff image with both transparency channel and alpha channel

Question

By setting a .tiff file "tiff:alpha" tag to either unspecified or unassociated I can have photoshop display it's alpha channel as either a premultiplied transparency or a seperate alpha channel. However if I manually edit an image through photoshop, and I manually add an alpha channel, create transparency in the R/G/B channels and then check "export transparency" during the export process. I can have both the premultiplied channel and the alpha channel. I'm however not able to do this through any processing code using either GIMP or ImageMagick. How would I go about replicating this through script/code and is it even possible?

exec(`gimp -i -d -f -b '(let* ((img (car (file-png-load 1 \"example.png\" \"example.png\")))
     (drawable (car (gimp-image-active-drawable img))))
     (gimp-image-select-item img 0 drawable)
     (plug-in-sel2path 0 img drawable)
     (gimp-image-insert-channel img (car (gimp-channel-new-from-component img 5 \"Alpha 1\")) 0 5)
     (gimp-image-insert-channel img (car (gimp-channel-new-from-component img 5 \"Transparency\")) 0 6)
     (file-tiff-save 1 img drawable \"output.tiff\" \"output.tiff\" 1) (gimp-quit 0))'`, {cwd: `src/images/${r}`, env:process.env}, async (error, stdout, stderr) => {                        
        im.convert([`${appDir}/images/${r}/output.tiff`, "-define", 
        "tiff:alpha=unspecified", `${appDir}/images/${r}/output2.tiff`], () => {
            //script where I save the the output image
        })
})

The code above is run in a node.js environment.

A relevant similar question to what i want: How to append an alpha channel to a TIFF (or pdf) with Imagemagick (or imagick php)?

Could there possibly be another route outside of gimp/ImageMagick that might allow this? — ForsakenCreator, Dec 02 '22 at 07:48
Apparently one can do this with Imagemagick using ftxt:. See https://github.com/ImageMagick/ImageMagick/discussions/5840#discussioncomment-4291130 — fmw42, Dec 02 '22 at 16:17
Funnily enough that's my threat, currently trying to use the answer they gave me there. — ForsakenCreator, Dec 05 '22 at 14:52

score 1 · Answer 1 · answered Dec 01 '22 at 22:17

Not with Gimp

There are two API calls to save a TIFF in Gimp:

(file-tiff-save run-mode image drawable filename raw-filename compression)

and

(file-tiff-save2 run-mode image drawable filename raw-filename compression save-transp-pixels)

As you can see, the only difference between the two is the save-transp-pixels argument, documented thus:

Keep the color data masked by an alpha channel intact (do not store premultiplied components)

So it's either premultiplied or not premultiplied, but not both.

Create .tiff image with both transparency channel and alpha channel

1 Answers1

Not with Gimp