gmic filter preview

[ChiliDogMilieu]

This facility recently upgraded GIMP & GMIC to 3.04 & 3.7.4. (MXLin,Deb13,Trixie). GMIC default preview apparently no longer supports multiple selected layers. Please confirm, and if possible, advise if this capability can be compiled back in. Thank you.

[Prodigalmanboy]

I can confirm this on our three workstations.

This issue is unique to 3.7.4, and I can not believe that it is a purposeful choice by the dev, as it makes GMIC much less useful, to the point I have gone back to a 3.7.3. to avoid the below hoops.

The only workaround is to create a new layer, save the image, then close and reload the image in order for the changes to be reflected in GMIC. Obviously, this is a huge pain the butt.

[dtschump]

The code of G'MIC-Qt is not evolving anymore (just a few fix when needed), since the passing of his main developer (Sébastien Fourey). Latest commits shows that : c-koi/gmic-qt@master...GreycLab:gmic-qt:master
Last commit dates from Nov, 2024, whereas versions 3.7.3 and 3.7.4 were released on March 2026.

I admit I don't really understand your issue, when you say the preview does not support multiple layers.
Could you please describe steps to reproduce the problem, maybe with screenshots ?

Thanks for your help.

[dtschump]

Oups sorry, no.
The commit page is here : https://github.com/GreycLab/gmic-qt/commits/master/
and there are indeed a few commits in March.
Maybe one of them has broken something.

[dtschump]

Just checked the latest commits and it should have no effect on your issue.
So I don't understand what is happening for you. I've tested a few G'MIC-Qt filters that require multiple input layers (e.g. Drop Water), and it seems to work as expected.

[Prodigalmanboy]

Steps:

1. Open image
2. Use any GIMP built-filter (in attached screen-capture I use Emboss)
3. Open GMIC. GMIC does not show changes made by Emboss

I have since had my children try 3.7.4. on their machines. and the same issues exists.

gimp-3_yof8NAOuui.mp4

[dtschump]

Thanks for the description.
My first guess is that this issue is related to the new version of GIMP (3.2), which has introduced non-destructive filters.
With non-destructive filters, the processed layer is not explicitely modified, and the layer data given to G'MIC corresponds probably to the non-processed layer data. instead.
Hopefully, this may be fixed by some smart calls to the GIMP plug-in API, but as the plug-in developer has passed away, I'm afraid we have to wait someone (possibly a GIMP developer) looks at this specific issue and proposes a solution.
If you try the exact same version of the G'MIC-Qt plug-in on GIMP <3.2, it probably works as you expect it to work.

[ChiliDogMilieu]

Greetings. Perhaps there are two issues. Screenshots and additional info will be sent. Thank you.

[ChiliDogMilieu]

gimp210gmic297 - has 'preview mode', allowing multiple layer preview.
gimp300gmic374 - apparently has no 'preview mode' on the right. 'preview' dropdown on the left does not seem to
enable multiple layer preview.

---

[dtschump]

OK, thanks for the additional info.
What I can say is that these are two separate issues.

• @ChiliDogMilieu: Indeed, the feature for previewing multiple output layers seems to have disappeared from G'MIC-Qt, and I have to admit I'm not really sure when that happened. It's a feature I didn't use myself, and apparently Sébastien removed it without me recalling whether we had discussed it or not. These days, the plugin's code isn't really evolving anymore. I try as much as possible to keep it compatible with recent versions of GIMP, but that's about it. I don't have the resources right now to add new features to the plugin. So, unless someone submits a patch to add this feature, I consider it to be gone.
• @Prodigalmanboy : Your issue is different, and after investigation, it is indeed related to the fact that GIMP effects are now applied non-destructively to layers. And when G'MIC-Qt requests the contents of the layer's image buffer via the GIMP plugin API, it retrieves the unprocessed image. Technically speaking, I suppose this should be fairly “easy” to fix, if we can make the right calls to the GIMP API to retrieve the filtered image data. I've done a bit of searching online, and there isn't much documentation yet on the new API functions introduced in GIMP 3.2. Without documentation, it might be difficult. I’m going to try contacting one of the GIMP developers on the Pixls.Us forum to see if they can help me out. In my opinion, it should be fixable with just a few lines of code—but you still have to know what to write!

That's all I can do for now, sorry. There aren't really any active contributors working on the plugin's code at the moment, and as for me, I'm mainly focused on the core framework code and the command-line interface.

[ChiliDogMilieu]

Greetings. Thank you for the update. The multi-layer preview is not super-critical, and work-arounds are in place. If this part of Gmic-Qt is augmented locally, the code will be made available to the GMIC team.

[dtschump]

@Prodigalmanboy I thought a lot about how to handle GIMP non-destructive filters in G'MIC-Qt.
Currently, if a non-destructive filter is applied to a layer and G'MIC-Qt is called, the plug-in acts on the non-filtered layer input. But if you apply the G'MIC filter, the non-destructive filter is still there.
This is not completely wrong.
You can think of a GIMP non-destructive filter as something "on top" of a layer, that is applied as an additional processing layer.
If you really want that G'MIC-Qt process the filtered layer content, it's as easy as merging all the filters togeter (i.e. effectively apply filters on the layer pixel data), then call G'MIC-Qt on this merged layer (see screenshot).

I've tested and this allows G'MIC-Qt to act both on a "bottom" filter (before applying non-destructive filters, which is a nice option), or a "top" filter (after applying the non-destructive filters).
So I think the current behavior is nice as it is.

[Prodigalmanboy]

David, what you suggested above does not work for me.... and to be honest, being that one never had this problem before, it sounds like a bit of a "deflection" more than it sounds like a solution.

If I open an image, and run some non-destructive filters, there is no way to merge the layer as that menu entry is greyed-out. Possibly I am not understanding, so please give a step by step.

I have found that if I "flatten" the image after running Gimp non-destructive filters, and then call GMIC-Qt it reflects the changes. But one does not always want to flatten one's image...

Also, if GMIC-Qt is already open, and I run some Gimp non-destructive filter, GMIC-Qt can be made to reflect the changes by previewing a GMIC filter such as Frame [Cube] or Symmetrize - but this is not reliable.

All in all, being creative in GIMP using GMIC-Qt just got a LOT harder...

[dtschump]

@Prodigalmanboy the fact is : GIMP has completely changed the default behavior of filters since GIMP 3.2 : Most filters are now non-destructive and does not explicitely apply on the layer data.
This is not a minor change. This means that any plug-in that "process" the layer data does it before all the non-destructive filters associated to that layer.

Now, in GIMP, if you want a filter to be rendered explicitely, you have to "merge it", in order to make it "destructive" again (just as it was before). To do so :

• Check that the GIMP filter you applied is indeed non-destructive (all are not). If that is the case, there a small logo 'fx' that appears at the left of the layer thumbnail in the layer menu.
• Click on this 'fx' label, it opens a small context window showing the name of the filter(s) (multiple filters if any).
• Click on the 'Merge All Active Filters Down" icon to explictely merge your filter to your layer.
• The 'fx' label should disappear : the layer is now modified.
• Open the G'MIC-Qt plug-in : you can process your filtered layer.

This indeed adds some extra steps, but this is not due to the plug-in, this is due to how layer filters now work in GIMP 3.2.

[dtschump]

@Prodigalmanboy Just to expand on my answer and help you understand that the new non-destructive filters are very different from the previous filters in GIMP: Take a layer (color image) and apply a GIMP non-destructive directional blur filter to it. Then take a solid brush and draw lines on your layer: the lines will be blurred as well.
This is something that was not happening in previous versions of GIMP (the added strokes were not filtered).

The introduction of non-destructive filters thus radically changes the way one thinks about filters in GIMP; it’s a completely different philosophy (filters are now “overlays” on a layer, applied at the end of the rendering pipeline).
This means it's not related to G'MIC-Qt. In the example I just gave you, there is no call to G'MIC-Qt.

So, yes, your workflow has to be modified a little bit to take these new non-destructive filters into account.