Besides all the back end stuff for handling high bit depth, the best feature for me is the ability to hit the / key and start typing the name of the menu item I want. Results are filtered as you type. Arrow down and hit return to select what you need.
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Someone at GIMPChat linked to this thread. Cool stuff, Boris. 
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With the new G’MIC>Color>Mixer [PCA] you can also get a result like this very fast:
Here I only used this filter with values from screenshot.
After this, one can use for example G’MIC>Color>Normalize brightness or G’MIC>Details>Tone Enhance to “amplify” this result.
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Now a few tips about masking.
Some are not new but I have to use them very often and I think that may be of interest to some GIMP users.
In particular, Dodge and Burn tool for cleaning masks and my new discovery Warp Transform tool for removing fringe has proven to be very helpful for me.
Here is our photo:
I would like to darken the cloudy sky and increase contrast for the landscape a bit. Of course this can be done more effectively with other methods, but we’ll use a mask here.
Since the clouds and landscape hardly differ in brightness we could try to make the separation with monomixer on duplicate layer (Colours – Components - Monomixer):
Increasing blue and green and decreasing red seems to be suitable for separation here.
Let’s raise the contrast now and see what happens:
Separation is better, but we still have some gray areas that need to be treated to get a clean mask.
I have intentionally not made the curve very steep to avoid oversharpening of the edge between white and black area.
Now Dodge and Burn tool becomes very handy. First we clean the white area of the mask along the edge. In the tool options we set under Type "Dodge“ and under Range „Highlights“:
Then we switch to Burn and Midtones to make the edge of the darker part of the mask black:
Note: Sometimes it is better not to darken the edge completely to avoid hard transitions!
After we have darkened the area along the edge of the mask we can paint the remaining part black. Now our mask is ready:
Now we need to turn our layer we just finished into a layer mask. To do this, we duplicate our original layer and add some layer mask from the menu. (Right click on layer and choose add layer mask). It doesn’t matter what kind of mask, we will replace it in the next step.
Activate the layer that we made above, copy it to clipboard (Ctrl-C). Click the layer mask of the duplicated layer and paste the copy from the clipboard (Ctrl-V – when Floating selection appears, click on Anchor layer).
Now the layer mask has been replaced by ours:
We don’t need the top layer any more and it can be deleted.
Now we darken the clouds and increase the contrast of the landscape and see how it looks like:
It looks good, but we have some fringing on the edge:
We can use Gaussian blur to soften the edge of the mask a little and then narrow the edge with levels.
Activate layer mask than Filters – Blur – Gaussian Blur. Size: 1-1,5 for x and y:
Than Colours – Levels. Move arrow for mid tones towards black until the white border disappears in the picture:
Now, the transition between clouds and landscape looks much better. However, there are still minor issues in some areas:
At this point the landscape shines through from the upper Layer that was darkened with the sky.
We could now extend or narrow the mask by one or two pixels, but at other places we would have changed the border as well.
Luckily we can do this locally by using the Warp Transform tool. We will now set the behavior of the Warp tool to Move pixels:
First we have to make sure that the layer mask is selected and then we just carefully pull the edge down a little bit with the brush. Now we have „pressed“ the edge of the mask down a bit.
As we can see, the edge is getting even darker now:
We have now pushed the edge of the layer mask underneath and narrowed it. That’s why we see even more edge of the dark landscape.
Now we also have to push that down, so to say to "hide it down“ behind the brighter landscape of the lower layer. So, we have to choose the layer itself (no longer the mask) and repeat the same step with Warp Transform brush – push edge down until we see no more fringing:
Now we have nice and smooth edge
Ok, enough for now. More later
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Some seriously good tips in here. I have long wondered how to get better colors in the sky when processing blue skies. Between that and @s7habo’s tip about maksing, I may take another crack at a photo that gave me a lot of grief, both because of the issues with making an accurate mask, and the poor color in the sky.
Here’s the shot. You can see artifacts where I tried to bring more blue out of the sky, and the masking around the trees is shoddy. I’m embarrassed to say how much time I spent trying to get this photo right.
One question: This is the second time in 2 days that I’ve seen someone using the PCA Mixer from G’Mic. Where do you guys learn to use these things? I tried it out and mashed a bunch of buttons for 15 minutes, couldn’t figure out what it was doing or how to control it, and thus will probably never use it. If I had some guidance or documentation, though…
This might help: Principal Component Analysis explained visually.
Basically the mixer considers 3 PCA components. The primary contains most of variation of the image, followed by the secondary and finally the tertiary. I.e., you could remove the tertiary and still see most of the original image. My first impression is that factor is the weight of the components, shift is the intensity and twist is the hue shift.
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A quick one this time:
Since GIMP 2.10 you can set blend modes for the layer groups .
This can be very useful.
An example for high pass sharpening (How to arrange the layers and which blend modes are used can be seen in the picture below):
Now, if you move the slider for Gaussian blur, you can observe the sharpening radius directly on the canvas. This way you can precisely adjust the radius (size of blur) and amount of sharpening (opacity of Group layer) before applying it.
If the sharpening is too strong, instead of linear light you can also use grain merge or overlay blend mode for the group layer.
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This is a great tip. Recently, been selectively sharpening my photos and applying different sharnesses to different parts of them. Is your procedure basically the same thing that “unsharp mask” does, but the long way around?
I love the scenic route home. 
Another way to sharpen that I use from time to time, is with the Highpass filter. Here is a screencast:
HighPass_Sharpen.mp4.7z (2.2 MB)
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Not really. Although the final effect is similar, the big difference lies in the blend modes used for both approaches and the corresponding consequence for further processing.
Difference in composition can be seen here:
As the advantage of Unsharp masking is its ability to control the contrast and width of the edge and which area outside the edge should not be affected, the strength of the high pass filter lies in the possibility of using different blend modes with corresponding influence on the end result.
Here are a few examples (left is the result of Unsharp mask for comparison and right are different blend modes for High pass filter).
Grain Merge:
Hard Light:
As you can see - in contrast to Unsharp Mask - Hard Light only influences darker areas. Accordingly there is no halos in brighter areas.
Linear light:
Overlay:
Soft light:
Vivid light:
So, each of these blend modes affects the sharpening in a different way. In my experience, depending on the image and which blend modes you use, I can often achieve better results with high pass sharpening than with Unsharf mask.
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Yes, it’s even faster .
However, in the above example, the blur layer that was used to create high pass filters is preserved, and can be used for other purposes e.g. frequency separation.
Here is another example of using one color model component , this time LCH C (ab) to brighten up the image without affecting highlights and shadows too much and with some ability to control contrasts.
In this picture I want to brighten up green trees and the house without affecting white benches and texture of the cloudy sky too much:
So, we duplicate the layer and extract the LCH C(ab) component (Colours – Components – Extract Component - LCH C (ab) ):
When zooming in we see that the result is noisy. We will use Selective Gussian Blur to remove the noise (Filter - Blur – Selective Gaussian Blur):
Blur radius 16,00 and Max. Delta 0,040 works fine:
For this layer we choose Hard light as blend mode (from drop down menu in layer window choose Hard light):
We make a new layer out of the result (right click on top layer and choose “New from Visible”):
We delete Hard Light layer and rename Visible to Screen:
Now we change blend mode of “Screen” layer to Screen:
As you can see, the photo is already a little brighter. Now we will use curves to influence the brightness and contrast even more.
Open curves tool (Colours - Curves) and move upper point to center of graph as shown in the image below:
The photo is now much brighter and we have starting point for further adjustments.
We can increase the local brightness of the image even more by drawing an arc on the right side of the curve:
If we want more contrast, we carefully drag black dot very little to the left:
Before and after:
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At first reading LCH C (ab) was confusing to me. To break it down, basically it is the C* channel (chroma) of L*C*h based on L*a*b*.
See this Wikipedia entry for more info. (Note that Wikipedia entries on colour, etc., aren’t perfect and need improvement but they are a good start.)
C is the A+B Channel? I thought that was LCH(Color). Chroma is a extrapolation of A, and B, but not a copy a transfer of A and B Channel. Am I thinking this wrong?
No. L* is the same. C*h is a polar projection of a*b*. Maybe that is why I found the naming of LCH C (ab) confusing. Moreover it is possible to get C*h from L*u*v*. Following GIMP’s naming convention, I guess C* would then be called LCH C (uv). Does that make sense?
Another quick tip.
Skin retouching with Gaussian blur in combination with HSV saturation blend mode.
Duplicate original layer, add HSV saturation blend mode to that layer, select skin area you want to clean and apply Gaussian blur
(I’ve hidden skin selection here so you can see the result better.):
Fine details are preserved and can be enhanced by sharpening the area a little bit.
Bigger skin blemishes have to be treated differently and the method is not as precise as frequency or wavelets separation, but in most cases you can get very good results quickly!
Picture used from Pixabay under CC0 License.
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Since area selection is often a bugaboo for me, I would appreciate an elaboration of your skin selection here.
@Underexposed It is just a quick tip. If you have more time, use a mask and brush in or out the areas that you would like to modify.
Hi Bill,
I’m using Free Select Tool:
How to select skin areas is described here in great article about skin retouching by @patdavid :