In this particular context, the issue is not so much the “abuse” of the module in itself, but the remark about not wanting the added area to be influenced by subsequent modules in this particular use. But next time, you may want to add a bit of background at one side and then you do want subsequentmodules to act on the added area…
In essence, the exemple (di- and triptychs) would require complicating the module for one particular use case. There are already very suitable tools available for that use case, and they offer many possibilities to do this.
I don’t immediately see any need for complicating the module to employ it for creating diptychs. From what I’ve learned so far, we can use it as it is, (although the modules would be more useful if the expansion limit of 100 percent was expanded somewhat).
To use any images developed in dt in a composite image created with another program we need to export them as .jpeg, .tiff or whatever, and then import the images into a compositing program where the images are put onto a common canvas and finally exported as one composite image file.
With the new modules available (really looking forward to v. 4.8 ), I foresee a similar workflow but within dt only:
Develop the first image, create a snapshot of the result, and export. Develop the second image – using the snapshot of the first image for guidance in scaling/positioning/grading – and export it. Add the two exported image files to dt library and open the first in darkroom. Expand the canvas and then overlay the second image file besides the first, add a frame by the Framing module, (if we cannot expand the canvas for this purpose also), and export the finished composite.
In such a use case there is no problem that the canvas color will be influenced by any subsequent editing, because we will do no kind of grading after the overlay.
If it is not possible to expand the canvas beyond 100 percent, but one will anyhow want some frame/spacing between the two images, this could be taken care of by creating a mask with a rectangular field positioned on the border between the two images with the Retouch module, and fill it with the color of the frame around.
This seems fairly straight forward to me, but you mentioned something about “probably also pushing the overlay module beyond its limits” that I’m not aware of what is referring to.
EDIT: If one also make a snapshot of the first edited image in a reverse orientation, it will be even easier to see where lines in the two images meet.
the benefit of this module is to be able to apply the same color / sharpness /… edits to a composite image. And this is also the weak point over pure image editing software: the same edits are applied to the whole composite if not limited manually by masks.
If you‘re aware of this, you can make use of this in darktable - if not you‘d better do this in a dedicated image editing tool like krita or gimp
), I foresee a similar workflow but within dt only: