I haven’t seen an example and explanation of how to get red in the sky and grass image that I think explains it well enough, so I thought I would offer this:
When you set blue to 0 in the blue channel, that leaves only green and red in the image. Red and green combine to form yellow, or some shade of red and yellow or green and yellow. The orange comes from there being some green mixed with the red. So you want to get rid of green in the sky, but leave it in the grass. So my first move was to set blue to -1 in the green channel. That made the sky quite red, but the highest value of red I could find was an R value of 212. By setting blue to 2 in the red channel, I made the bottom portion of the sky R 255.
I’ve played around with all kinds of combinations, and I could never get red at the top of the image to 255, and I could never get rid of a small amount of green, I did get it very low by zeroing green, but then the grass was red.
You can’t do this without a mask, because the sky is not only blue, but has the proportions of green. Also the grass is yellow, which means a combination of red and green. So if you remove green, the yellow part of the grass will be red.
In fact, it was intended that way, and you actually did it. That the sky isn’t 100% red doesn’t matter. More important was understanding the logic of how the channel mixer works. You understood that, which fulfilled the purpose of the game.
I forgot to say that if you understand the logic, you can put it into practice and do corrections quite well. For example, if this photo was taken early in the evening at sunset and I had the impression that there was still too much blue in the sky, I could use the channel mixer to make the sky a little more “reddish”.
I may be a little late here, but I had written up something similar a while back on a similar question for RawTherapee. My GIMP example was here (maybe it can help in some way?):
I thought I had gray/lightness blend figured out, but no. I reasoned that in gray channel, when you increase a color, that lightens it because you are saturating it, i.e., removing gray from the color. Adding lightness blend to the equation: increasing red makes areas with red lighter. But using the jpg of the original road, field, and sky scene and only increasing red to 1.163–no other changes–darkened the sky. How does that happen? If there is red in the sky, shouldn’t it make it lighter?
My bad. I should have opened the reference to see what post you were referring to. I thought you’d respond to my last post.
In gray mode with the color mixer in darktble, you combine the color channels from zero. This means that if you increase the red channel, you will actually only see the red channel. As long as you don’t increase the values for other channels, they will remain off at first. Since the sky is blue, and you have not increased the blue channel, the sky is dark.
Yes, but you see a darker sky, because blue channel is not yet on and blue is the predominant color of the sky.
In the case of the sky, all RGB values drop, but in the case of the road, all go up. Can you explain what makes the difference? Is it because red is below 128 in the sky and above it in the road?
Maybe you are confused by the fact that nothing happens before you move the slider. That means you see the image as a combination of all three color channels. The moment you move the first color channel, the gray mode is activated. Try it without Lightniss blend mode to see what I mean.
Here is the explanation of the Blend Mode “lightness” in darktable : "This blend mode mixes lightness from the input and output images. Color data (chroma and hue) are taken unaltered from the input image. " https://www.darktable.org/usermanual/en/blending_operators.html How exactly the values will be affected by this blend mode, I don’t know. Maybe you should ask the darktable developers.
Yes, I have been confused by the behavior of the module if you reset it. When you reset, the colored image shows up in its unmodified state, and I thought that is where I was starting from. But if you move one of the sliders off zero ever so slightly, the image goes black. That, as you say, is the real starting point.
As you move the red slider up, the sky, the whole image, in fact, becomes lighter. But because the maximum is 2, the sky stays darker than the original. Using the green slider, moving it up a little leaves the image darker, but if you move the slider up more, the image crosses over from being darker to being lighter than the original–before the slider reaches 2.
???
