I’m getting back into photography after a few years. In the interim I dabbled a bit with music composition and production. I’m rubbish with names, so please forgive verbosity used in place of correct technical vocabulary.
My immediate problem is I want to de-yellow an urban night shot from a few years ago. (My city is switching to LED array street lights - will a night filter (physical) even work for those?)
In audio, if something has an unwanted tone, I’d use an equalizer curve to get rid of it. But first, I need to know the frequency, so I’d start by making a narrow band amplifier in the equalizer and scan until I found that ugly noise. Then I’d reverse the amplifier to get rid of that narrow band.
Can I do the same thing with photography? Could I apply a curve layer (an additional curve layer) to the image with a narrow amplifying peak to find whoch yellow frequency is the irritating one and then reverse the direction to get rid of the yellow instead of amplifying it? Would I have to fiddle with red and green channels to find the combination that makes the sodium lamp yellow?
In audio, if something has an unwanted tone, I’d use an equalizer curve to get rid of it. But first, I need to know the frequency, so I’d start by making a narrow band amplifier in the equalizer and scan until I found that ugly noise. Then I’d reverse the amplifier to get rid of that narrow band.
Can I do the same thing with photography? Could I apply a curve layer (an additional curve layer) to the image with a narrow amplifying peak to find whoch yellow frequency is the irritating one and then reverse the direction to get rid of the yellow instead of amplifying it? Would I have to fiddle with red and green channels to find the combination that makes the sodium lamp yellow?
You can’t do what you would do in audio. The problem is that cameras don’t really capture the frequency spectrum of light, just some lobes. So you can’t emulate the narrow frequency cuts sometimes done in audio.
A didymium / nightlight filter does indeed do that, but it’s tuned for the frequency of sodium so it won’t help much with the spectrum of white led.
With that said if it’s either white led of the same type or sodium vapor, you can usually get acceptable results by just adjusting white balance (which I guess could be though of as a 3 band EQ).
Mixed light is a real curse though and at least as far as I know common image processing tools don’t really offer the tools to deal with that. You can try different white balances and masks but it’s a pain.
I think the audio equivalent of what you are trying to do is to pan a single element inside a stereo mix.
You can think of RGB colour channels like LCR audio channels. A yellow image is too much level in the Red and Green channels. It like too much level in the Left and Centre. You can turn down the LC channels but it affects the whole stereo Field
Basically, I shifted WB to very cold temp (1800) and then desaturated blue color, because everything outside of this yellow light becomes really blueish at 1800.
Here is how it looks like without blue color desaturation:
Obviously, if you have some actual blue color on the subject, you will need to use some local editing, but in this particular image I was able to just desaturate blue globally on the whole picture. And of course it gets a bit more difficult if you have people on your picture because of skin tones, but still manageable.
Some time ago I asked for help how to adjust the ugly colors in a photo shot in very tricky artificial lights; I received lots of replies and many explanations that could probably help you:
PCA eliminates the “extremely narrow spectrum” issues of sodium, but retains the “almost no blue light” properties.
PCA basically takes the phosphor plus blue setup of “white” LEDs to an extreme of almost 100% phosphor emissions and no direct LED emissions. Usually considered a severely defective LED except in special applications like this.
More channel info. Most of the detail can be found toward reds. The analogy fails because you wouldn’t want to suppress good data. It is a matter of redistributing or gleaning from it. RGB:
Basically, set the whitebalance on a patch from the lightest cat fur I could find, then desaturated the blue channel almost to 0. Bit of curve to get some contrast, and 'ere y’go…
Now I’m going to add a threshold to see if I can handle extreme blues without the profile shenanigans…
Color balance - increase saturation of yellow (opposite of blue)
Color zones - in ‘saturation’ tab, isolate blues and decrease
Channel Mixer/New calibration module - decrease B slider in B channel. This will also make blues darker, so blend ‘colour’ if you don’t desire that.
Here is an example (working on the first jpeg posted by Alexander) where I have used Color Balance to remove the orange cast, then channel mixer to reduce blue saturation.
Method:
Draw colour picker rectangle on the area you want to be neutral. We have neutral when R=G=B.
First determine how bright you want the area. As the cat has white fur, and white is bright, I chose the max RGB, which is R = 136 (first screenshot). So the goal is to boost the G and B values to = 136. For B, we adjust any slider (there is no right or wrong, only what you think looks best) in B channel of channel mixer, and for G, we adjust any slider in G channel. When all three colour picker values are equal, we are done. (You’ll notice mine are not perfectly equal, I left a tiny bit more red, but close. Also, my values do not = 136. That’s because I also adjusted R channel a bit, and because I re-drew the colour picker area).
@ggbutcher I ended up guided filtering the GB channels with R. Still, the image is off because of my funky automagical method. Probably need to be less and be a traditional lad.
).
and be a traditional lad.