A bit how to use the channel mixer to create a mask, with some pictures of the color cube.
First I cropped most of the picture, so I only have to care for the interesting parts with the interesting colors.
Cut.jpg
Then I looked a while at the color cube. The bridge has sandy colors, sky is blue, both appear as separate clouds in the cube. And the clouds touch each other, where all the unsharp edges create mixed colors.
Here I viewed the cube exactly from one side. From this angle, colors are ordered by their green content from top to bottom, and by teir red content from left to right. All colors on the dotted line have the same amount of green. In a mask that was created from the green channel, they would all be mapped to the same position on the green arrow and become same grey in the mask. No way to tell bridge and sky apart anymore. Only some parts of the sky stand out and are clearly brighter than the bridge. These parts are just good for a mask, the rest not so much.
Green channel.png
Red channel is worst, any level of red can be sky or bridge, while blue is somewhat better than Green.
There are more ways to map the colors to one "channel" than just using RGB. Combining RGB with 33% each makes a simple brightness, same idea as using the L channel. If you check the channels manually, then it's worth a try.
All channels.jpg
But there is something like a best channel that can split bridge and sky and map them to different greys. That's what the last cube shows. Calculating that magic arrow that maps the objects best is not difficult:
- Take RGB out of the blue cloud
- Subtract RGB of the opposite sandy cloud
- And that are the numbers that the channel mixer needs. Maybe some additional factor...
But:
- Good blue and opposite sand are easy to tell from the color cube, but less easy from just the picture.
- Pick wrong blue and sand, and the result isn't that great anymore. Not as bad as the simple RGB channels, but the result will have overlapping in greys again.
- Dark blue is part of the bridge, not the sky. So even though it is a great picture for this method, the obvious choice for the 2 colors would not work.
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