Done using GIMP 2.10.20 on Kubuntu Linux 18.04 LTS
There are three more Normal Layer Modes to consider, Erase, Merge, and Split.
Erase will erase from the bottom layer any pixel for which there is a pixel in the top layer. If the two layers are the same size, using Erase mode on the Top layer would simply make the bottom layer completely transparent by erasing all of the pixels. To be of any use (I would think) you would need to have the Top layer contain some kind of shape on a transparent background. Then using the Erase mode would invert that on the Bottom layer, where the opaque shape would become transparent. This could create something interesting, though anything I have been able to think of could be done in other ways, and probably more simply.
To illustrate this process I decided to try adding a new font since we haven’t done that recently. I went to Font Library, which has lots of Open Font Licensed fonts, as we discussed previously in Free, Public Domain and Creative Commons Assets. This license is considered free by the Free Software Foundation, which is a pretty good credential, so when I go looking for fonts I look for OFL-licensed fonts by preference. I can use them anywhere without worrying about royalty issues. The font I chose was in the Dingbats section and is called FivefoldOrnamentsEtc. It has some interesting symbols, so I downloaded the zip file containing the font, moved it to my ~/.fonts directory (making it available to any application on my Kubuntu box), and then clicked the Fonts tab on the Upper Right (where you see your tabs for Brushes, Patterns, Gradients, and so on). I right-clicked on a font at random, then selected Rescan Font List. This made my new font a selection, so I clicked on it to select it.
I then created a New Layer filled with transparency, selected my text tool , drew a text box, and typed some letters (A,B,C,D,E) which showed up as symbols. I then moved the borders of the text box to just touch the symbols, and centered the box using the Alignment tools (Tools –>Transform Tools–>Align, or click on the Move tool and select Alignment). This is what resulted:
With this as the Top layer, I selected Erase mode, and got this result:
Note that you could then place a layer under the image that would show through. I decided to do this with a gradient, and picked Full Saturation Spectrum CW. I created new layer, and used the Gradient tool to fill it, and moved it to the bottom, beneath the toy image layer. At first this had no effect, but I created a Layer Group, moved the Toy image and the Text layer with the symbols into this group, and then everything worked:
This is an interesting effect, though you might not need to use it often.
Another interesting feature is that this can reverse a Layer Mask effect. Recall that Layer Masks work by creating transparency, and so does this Layer Mode, and it is like a double-negative. To see how this works, select the Top layer, and add a Layer Mask using button on the bottom of the Layers window. Make sure that you select White (Full Opacity), and do not leave a check mark in the Invert Mask box. Then make your Foreground color White, and select the Paintbrush tool. In normal Layer Mask situations, painting the Layer mask with White would make the layer opaque, thus hiding anything underneath it. But with Layer Mode set to Erase, painting with the White color actually makes the top layer transparent where you paint, and reveals the layer below it.
Merge and Split
These are obviously opposites in some sense, but there is no documentation of them right now, and I have not succeeded in working out what is going on with them. I did work it out for Color Erase and Erase, but these have me stumped. If someone out there knows how these work, please let me know. I know that Michael Davies touched on them briefly in a YouTube video, but I could not replicate his results. And I can’t find anything on GIMP Forum either. This may happen again with undocumented layer modes since there are more of them ahead. When I can work it out through experimentation, I will explain what I discover, but when I can’t, we’ll skip it and move on. Bear in mind that documentation is important, but it is usually the least desirable job on any project. I learned this a Project Manager, when documenting what the project did was like pulling teeth. And then the Agile practice came along and made it sound like not documenting anything was a virtue. My personal view is that you should document like the person who will have to take your place is a homicidal maniac who has your home address. And I practice what I preach. When I was working on projects at Saint Joseph Mercy Hospital, I got rave reviews from the IT staff who had to support the systems I installed because they knew I would document them carefully and completely
This will be a shorter tutorial than usual, since I would rather start fresh on the next group of Layer Modes, the Lighten Only group.