Sometimes G’MIC plugins show pieces of the image that I have deleted or that were deleted by someone else.
This morning I deleted the egg behind some flowers. Selecting the layer without the egg, I used G’MIC and several plugins showed the egg too in their previews.
Now I’m using an image of a group of roses which was made transparent and posted as a CC0.1 image.
Upon using G’MIC some plugins like Angoisse Anguish and Aurora are showing things in the background. It must be stuff the other person cut out of the image. But why are these things showing up in the background?
How can I prevent them from doing so?
This morning I worked on a transparent image of two columbine flowers. I duped the layer, cut out one of the flowers, but again in G’MIC there was a plugin that included the second flower in the preview. Very annoying, especially in the Array (faded) plugin.
And I just tested the Array plugin, and yes, it shows those things in the background if I select that plugin to run.
Any suggestions?
What software are you using G’MIC with?
It sounds like this program may be deleting items by setting the transparency to zero while leaving the color values unchanged.
Then the G’MIC effects are reading those RGB values from the image and ignoring the image transparency.
You may be able to fill the deleted areas with black as a workaround.
Thank you for taking the time to respond, 
I am using GIMP 2.10.32
I understand the third paragraph and it sounds like what is happening,
Could you explain the second one? How is the transparency set to zero?
How do I change the color values?
It is in sRGB mode.
To add black or even white would just undo all the hard work to get the image to be isolated on a transparent background.
Are there some settings I can change?
How is the transparency set to zero?
That is done by GIMP’s eraser tool to hide the erased image data.
I am not sure.
The GIMP eraser tool documentation confirms that it keeps the RGB values the same so that its anti-erase feature can restore the original image data, but I did not see anything on how to change that behavior.
I didn’t think to look at the tool options for an eraser. Wow!
That anti-erase stuff is interesting. I tried it on one of the images and I could see what the other person had erased.
But say we erase a person, and then share the image with someone else, they could use Gimp and put the person back in, That’s a bit scary.
Thank you for the link.
And for the explanation.
I’ll have to find out a bit more, because it’s just changed what I can and can’t do with transparent images.
Have a great day! 
Yeah, I was thinking that too. The erased data may also be visible when saving to a format that does not support transparency (e.g. JPEG).
Many other image editors change the erased RGB values to a solid color (e.g. black or white).
If you need to redact information from an image, it may be best to overwrite the data with a solid color before erasing it.
The GIMP save UI for PNG images has a ‘Save color values from transparent pixels’ option that sets the color of transparent pixels to black when unchecked.
Bit of a mis-post, this being G’MIC, but while we are here…
Gimp: to really, really, erase:
- Make sure your image has an alpha channel. In Layers, the layer name is lightface when this is so.
- Erase (or generally, make transparent) everything and anything that is not to be seen anymore. In Gimp, after erase, the red, green and blue components of the image are still there, but where the transparency component is present, the R, G, B, components are just masked. The transparent areas should display as a light grey/dark grey checkerboard. If it is the current background color instead, undo and re-read step 1.
- When ready, set your working colors to black foreground, white background
- Choose from menu: Layer → Transparency → Alpha to Selection. “Dancing ants” should appear around the selected (opaque) areas.
- Choose from menu: Select → Invert. “Dancing ants” should now march around the transparent areas.
- Choose from the menu: Edit → Fill with FG Color. This is black. Any non zero R, G, or B in transparent areas really, really go to zero.
If you work with fuzzy brushes, (hardness less than 100%) then you will only semi-erase at the brush edges. If that is not cool, work with the pencil instead, or set brush hardness to 100% with paint and erase tools.
So, I tried it out. Filled the transparent part with black, then erased it and the anti-erase gave me black. I’m worried about the intricate edges being deleted when adding black, but will test it out.
Thank you for the info on what that option means when saving. I have been leaving that unchecked already.
Thank you for your help with this.
Yes, it has turned into a GIMP thing, but the symptoms started in G’MIC and I didn’t know it was actually a GIMP thing. My apologies. 
Yes, I know how to set an alpha channel onto a layer. But I hadn’t known that there was a difference in the boldness of the Layer Title depending on whether or not they had an alpha channel. Thank you for sharing that.
I sometimes do an erase / clear and go ‘why did it go white?’ and then remember I had assumed it had an alpha channel already.
I work with intricate edges and sometimes keeping them ‘clean’ is hard, so will have to see if filling transparent areas with black will be a detriment to that.
I had noticed that fuzzy brushes didn’t erase right to the end and so usually use a ‘hard’ brush, but didn’t realize there was another ‘hardness’ setting on the Tool Options tab.
Thank you for your assistance with this.
Alan Melikdjanian (Captain Disillusion) to the rescue. He has recently released a video aimed primarily at videotographers, but general enough for those working with all forms of digital media. His acumen has few parallels, and every one of his videos is a showcase of his VFX skills.
The Horrors of the Alpha Channel (YouTube)
Hi Alan,
There is a lot I didn’t understand in this video. But I loved it! 
Very fun and lots of humour.
Thank you for sharing.