Open indexed images in "greyscale" fails

[gernot66]

I'm using the IFF plugin 0.10.1 in GIMP 2.10.34 on a win11 x64 system

Do not open as greyscale if a greyscale can't be exportet to the file format
(spares the text below)

The issue that the plugin won't save greyscales is solved and it is proper since a standard OS3.x (i.e.) can't open greyscale .iff images but the plugin unfortunately loads indexed grey colors as greyscale. This would not be a big thing since it is easy to make an indexed image of the greyscale but the problem is i will lose any special sorting which i need in example for a workbench backdrop to keep the grey "aaaaaa" at index 0, respectively that it will be used as background color ("aaaaaa" is the standard OS3 grey). It is somewhat annyoing that each time i reopen such an indexed image i have to convert it to an indexed and sort the grey "aaaaaa" to index 0 (which isn't that easy imagine you have 128 indicies and all is grey in grey on grey, unfortunately GIMP doesn't shows the values when you sort the palette and i need for this a second program, which would be an issue of GIMP itself).

Any other filetype e.g PNG opens such an indexed pseudo greyscale proper as indexed, just to make clear this is not an issue of GIMP itself.

Until i helped myself with a workaround and use special indexed bitmaps to make my indexed greyscales for the backdrops in which at least one color is shifted by a single value to green, this isn't noticable and still grey but it's not a perfect solution. Further many files does not yet use such a special palette.

I could as well use "XnView" to convert to amiga IFF, but this viewer/convertor has the behave to fill a palette to the next bitplane value, i.e. 20 colors will end up as 5 bitplanes, strangewisely it fills a 1 bitplane image up to 8 which makes the file uselessly larger as needed, though this is even no good solution. Further to fill the palette up with black 000000 can be an issue for OS3.x backdrops because any color present in the palette must be used by the image and it can happen that black isn't used (for a photo i.e.) such images will fail to be displayed proper by the OS when converted to IFF using XnView. Thus images to use as backdrop are at best stripped to the colors used as GIMP usually does (further i guess the common for the IFF format is to fill with ffffff since this equals to "erased" for DPaint i.e., however the color must be used)

An example image:
3worlds.zip
it uses the maximum available colors for the workbench in 8 bpl depth (243 incl. 00000, aaaaaa, ffffff, almost the maximum but only when using FastIPrefs and FastWBPattern which uses the else locked index 3 to 7 another four are reserved for the pointer and four are quasi reserve, factual the image will have less as 243 colors - mostly, it depends on the source image quality and usually i don't use ffffff since this is to bright for a backdrop).
and one for 5 bpl:
ascending.zip
For 5 bpl i can use a maximum of 28 colors if i include the "standard eight" thus this image uses only the left 23 colors incl. 000000, aaaaaa. ffffff, the pointer colors are for 5 bpl out of range, indeed it is almost useless to have such a greyscale in 8 bpl depth since there is no visible difference to 5 bpl especially not on a CRT monitor).
Both images are from M. C. Escher and was hosted by "TWAIN" downloaded from archive.org converted to IFF using GIMP 2.10.34 and the named IFF plugin.

Btw, this is the only issue i encountered while converting a felt billion of images to amiga IFF,
thanks a lot for the effort without this plugin i would be almost helpless.
(that is recently i decided because of the somewhat smaller filesize to use PNG directly in OS3.1 using the proper datatype for PNG, but exactly for this i have to reopen them and will lose the sorting, but for anything i will create for OS1.3 i furtheron need your plugin just that this is no problem at all since the images i can use for "SimGen" or "WBPic" can have only 2 bpl and receive a special "blue on blue" palette to hide the images until i applied a random selected palette to them, different OS and a complete different task).

No matter if i use it or not i guess it would be fine to solve this import/export issue respectively to have a uniform behave not to convert eitherway to greyscale.

gernot66