Expanding dynamic range

[Willowroot]

I just got a K-M A2 so I've been looking around at digistuff ... ran across this interesting technique at DPReview.  Essentially what you do is use a CC040M magenta filter to limit the amount of green light that the sensor sees.  White balance with the filter on of course.  Since the green channel is the first to blow in daylight, this allows the red and blue channels to "catch up" and the practical effect is to expand the effective dynamic range of the system.

Here's the links:

thread one
thread two

Would be interested in the comments of people here  

[Ray]

Since the green channel is the first to blow in daylight, this allows the red and blue channels to "catch up" and the practical effect is to expand the effective dynamic range of the system.

It seems that bjanes is a great fan of this technique, but I'm still skeptical. The general principle for use of filters is that you use the filter to suit the scene. In this use of filters which you've referred to, the use is to suit the characteristics of the sensor, completley ignoring differences in scene content. One can't help wondering why Nikon or Canon did not reduce the sensitivity of the 'green' pixels, through one way or another, if this has the effect of increasing dynamic range. For some cameras, what that would mean is that the true ISO was brought back into line. For example, ISO 100 on the D60 is more like ISO 160. Reduce the sensitivity of the 'green' pixels and you are back to a true ISO 100 with (perhaps?) a half stop increase in dynamic range.

My gut feeling is, such an approach  (using a filter to suit the sensor rather than the scene) will result in a botched effect, the benfits of which will vary according to scene content and color values.

I've brought up before (in the 'interesting article' relating to ETTR), the issue of 'true blue' skies shifting towards cyan during overexposure. I'm going to post an example of this effect and then try to predict what might happen if I had used a magenta filter. I'm really thinking while I'm writing because I don't have any hard evidence.

The photo, by the way, is a crop of the top of a vertical shot. This is not overexposed as a result of not knowing what I was doing, but deliberately overexposed because it was a snapshot of my partner who was the focus of interest. I'm not sure why so many people have an irresistable urge to place themselves in front of all interesting, beautiful, old and valuable monuments, buildings, artifacts and simple beautiful scenes, when photos are taken. Do they feel a need to compete for the attention of the photographer, perhaps? Or do they simply want a record of the fact that they were there, in case anyone might disbelieve them.

The shot has a -2EC adjustment in ACR.

[attachment=571:attachment]

The RGB values for the 'true blue' part towards the upper left corner are R=104, G=114, B=143. There's a clear predominance of blue, as one would expect.

The RGB values within the part encircled by the red line are, R=208, G=219, B=228.

In other words, green has become more prominent as a result of the blue channel clipping. (Pure cyan is a mixture of blue and green in equal parts).

How would a magenta filter affect this? (Magenta being equal parts of blue and red).

Well, I'm relying upon logic here because I don't have a magenta filter. It seems to me, in this example, the situation would be complicated. The magenta filter would suppress the green channel which is not a problem. The problem is, the blue channel is clipping. If the exposure is the same, the blue channel will still clip despite the magenta filter. However, the green channel will be subdued so that the area enclosed by the red line in my example should (might?) begin to look more 'blue' than 'cyan'.

Is that a solution to the problem? If the exposure is the same, I've done nothing for the shadows. It's true, I've corrected a cyan shift in one part of the sky whilst simulataneously creating a magenta shift over the rest of the image.

If I can correct that magenta shift in the rest of the image successfully, then perhaps I could argue that I've been able to use a 1/2 stop or 2/3rds stop greater exposure, which would help the shadows. On the other hand, if I know what I'm doing in Photoshop, I can isolate that part of the sky which suffers a cyan shift, and change the hue in 'Hue/Sat' control.

Where's the benefit?

I should point out, in case anyone is wondering where the cyan shift is, the sRGB jpeg compression reduces this difference. I work in the ProPhoto RGB space and convert to sRGB for net display.

[BernardLanguillier]

I just got a K-M A2 so I've been looking around at digistuff ... ran across this interesting technique at DPReview.  Essentially what you do is use a CC040M magenta filter to limit the amount of green light that the sensor sees.  White balance with the filter on of course.  Since the green channel is the first to blow in daylight, this allows the red and blue channels to "catch up" and the practical effect is to expand the effective dynamic range of the system.
[a href=\"index.php?act=findpost&pid=65806\"][{POST_SNAPBACK}][/a]

What is white balance application? Basically just a shift of the values of one channel relative to the others.

If I am not mistaken, what you do here is under-expose the green channel at capture, but the white balance will basically then bring back that channel to where it belongs so as to get neutral colors. Since the white balance is just one point along the curve, I am not even convinced that you will completely get rid of the green cast with white balance click only, but let's assume that you do.

You might be gaining something in the highlights (depending on how the white balance exactly works with the channels), but you are also very likely to get a more noisy green channel, since you are doing the very opposite of the recomended expose to the right dogma.

Anyway, it might work in some cases and for some cameras whose green channel isn't too noisy in the shadows.

Cheers,
Bernard

[Ray]

You might be gaining something in the highlights (depending on how the white balance exactly works with the channels), but you are also very likely to get a more noisy green channel, since you are doing the very opposite of the recomended expose to the right dogma.
[a href=\"index.php?act=findpost&pid=65867\"][{POST_SNAPBACK}][/a]

Exactly! In my example above, comparing a cyan shift corrected in PS as opposed to a cyan shift corrected by use of a magenta filter, one has 2 shots with the same 'external' exposure. The shadows, however, having less 'internal' exposure in the green channel, with use of the magenta filter, will actually be noisier to some degree.

On the other hand, blown detail in the clouds will be brought back slightly. Net effect regarding dynamic range... a big fat zero.

[bjanes]

see following post

[bjanes]

What is white balance application? Basically just a shift of the values of one channel relative to the others.

If I am not mistaken, what you do here is under-expose the green channel at capture, but the white balance will basically then bring back that channel to where it belongs so as to get neutral colors. Since the white balance is just one point along the curve, I am not even convinced that you will completely get rid of the green cast with white balance click only, but let's assume that you do.

You might be gaining something in the highlights (depending on how the white balance exactly works with the channels), but you are also very likely to get a more noisy green channel, since you are doing the very opposite of the recomended expose to the right dogma.

Anyway, it might work in some cases and for some cameras whose green channel isn't too noisy in the shadows.

Cheers,
Bernard
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Bernard (and Ray),

To understand the use of the CC 40M filter, first look at the relative sensitivities of the channels exposed under daylight. Here is an example with the Nikon D200. The PS histogram shows the channels of the light gray patch of the Macbeth CC as they are recorded in the raw file and converted without white balance or gamma correction in DCRaw . The image is dark (gamma 1) and the colors are posterized because this is a GIF to show the menu letters without jpeg artifacts:

[attachment=575:attachment]

Here is a similar image for the EOS 1D Mark II in jpg, with exposure such as to give a value of around 240 in the white square of the color checker.

[attachment=576:attachment]

As is evident with both cameras, the green channel shows the higest response followed by blue and then red. To white balance the picture, the raw converter for the Nikon image multiplies the red channel by 1.83 and the blue channel by 1.36, assuming exposure at 5400K as indicated by the chart below:

[a href=\"http://www.pochtar.com/NikonWhiteBalanceCoeffs.htm]http://www.pochtar.com/NikonWhiteBalanceCoeffs.htm[/url]

For the Canon, the multipliers can be calculated from the raw channel data. The red multiplier is 1.92 and the blue 1.29.

Now if you expose fully to right in the green channel without the filter, the red and blue channels are not fully to the right, are effectively underexposed, and will suffer in number of tones and noise. By placing the magenta filter over the lens, you hold back some of the green light to better balance the channels, allowing ETTR with the red and blue channels also. You increase the exposure after placing the filter so that now all three channels are exposed to the right. Since the green channel is fully exposed (not underexposed), it does not suffer any increased noise or other ill effects. For WB you can either preset it with a gray card or include a neutral reference in the picture.

To see the true values in the channels on the histogram, you must view them before the white balance multiplier is applied with DCRaw (after the fact), or you can load a special white balance (UniWB) into the camera that sets the blue and red multipliers to 1.0. The camera histogram will then display the channels correctly and you can use them for ETTR if your camera has color histograms. You use this hack, then you will have to set the WB properly in the raw converter. Unfortunately, this hack confuses Adobe Camera Raw into thinking it is dealing with a double exposure with Nikon cameras, which set the WB coefficients to 1.0 in this situation. With ACR you can still WB with the eyedropper, but presets will not work correctly. Nikon Capture works normally in this situation.

This method allows better ETTR in all channels and is no cure for massive overexposure of the blue channel such as in Ray's example, which I think is irrelevant to the method. If you are exposing the blue more to the right, then there will be less headroom in the blue channel and the clipping that Ray observed would be worse, but as Bruce Fraser has pointed out, ETTR is not overexposure. Ray will have to learn to cope with high dynamic range situations.

Daylight is not the only situation where filters can improve noise and dynamic range. With tungsten (which is quite deficient in blue), the blue channel is quite underexposed and needs a multiplier of 2.29 with 3200K for the D200. In this situation, one can get better results by using an 80A filter just as with film photography. Of course, one loses 2 stops and the preview appears blue.

The CC40M filter reportedly only allows about 2/3 stop more dynamic range and may not be worth it if you are using a camera with very low noise, such as the Canons. However, in situations with very wide dynamic range such as landscapes, it may be worth a try. Personally, I do not have the magenta filter and have not tried the method. However, I think it is a very interesting concept that demonstrates some important facets about white balance.

However, I think that CC40M is too strong. If I were to buy a magenta filter for the Canon, I would get a CC20M. The green density os 0.2 or 63% transmission of green, or -2/3 EV. It would bring down the green channel to 93, placing it midway between the red and blue channels and thereby allow +2/3 EV more exposure to the  right without blowing the green channel.

[bjanes]

Exactly! In my example above, comparing a cyan shift corrected in PS as opposed to a cyan shift corrected by use of a magenta filter, one has 2 shots with the same 'external' exposure. The shadows, however, having less 'internal' exposure in the green channel, with use of the magenta filter, will actually be noisier to some degree.

On the other hand, blown detail in the clouds will be brought back slightly. Net effect regarding dynamic range... a big fat zero.
[a href=\"index.php?act=findpost&pid=65875\"][{POST_SNAPBACK}][/a]

Ray,

You do not understand this method at all. Please read my earlier post before coming to rash and erroneous conclusions and let us know what you think.

[PeterLange]

As is evident with both cameras, the green channel shows the higest response followed by blue and then red. To white balance the picture, the raw converter for the Nikon image multiplies the red channel by 1.83 and the blue channel by 1.36, assuming exposure at 5400K as indicated by the chart below:

Bill,

Would you have any insights about the reason for this response, G>B>R ?

Thanks! Peter

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[bjanes]

Bill,

Would you have any insights about the reason for this response, G>B>R ?

Thanks! Peter

--
[a href=\"index.php?act=findpost&pid=65942\"][{POST_SNAPBACK}][/a]

Peter,

That's a good question. On the DPReview Nikon forum it was suggested that the sensor has to perform well under other illuminations. However, for tungsten, more blue  sensitivity would be desirable, so if that were the case I would expect more blue response.

If I were designing a sensor, I would try to have the sensitivities equal for daylight, but I am not an engineer and do not really know the answer; however, since both Nikon and Canon have excellent engineers, and there must be an explanation rather than manufacturer stupidity.  

Bill

[Ray]

Ray,

You do not understand this method at all. Please read my earlier post before coming to rash and erroneous conclusions and let us know what you think.
[a href=\"index.php?act=findpost&pid=65917\"][{POST_SNAPBACK}][/a]

I think I do understand it. Perhaps it's you who doesn't understand the use of filters.

Let me go through it again and if I make an incorrect statement, you can point it out and explain where I'm wrong.

1. The red and blue pixels (photodiodes) appear to be less sensitive and noisier than the green pixels.

2. The green pixels are the first to clip (all else being equal, which it never is).

3. Introducing the right type of magenta filter suppresses the green component of light, and to a lesser extent, the blue channel, which allows for a greater exposure in some situations.

4. The greater exposure, partly offset by an over all reduction of light as a result of the filter, can reduce shadow noise through an increase in the red and blue channels, at the same time avoiding clipping of highlights in the green channel which, without use of a filter, would clip at the same exposure.

5. The net effect, all being well and fingers crossed etc, is a real increase in dynamic range, which as you say, might be as much as 2/3rds of a stop, but I've yet to see an example.

Now for the tricky bit. Any increase in dynamic range is totally dependent upon being able to increase exposure without clipping any of the channels. Increasing exposure is not always possible when using a filter that is not selected specifically for the scene being photographed.

This is the major flaw in the process. I'll give 2 examples which I predict would have a completely opposite effect on DR when using a magenta filter.

1. The best case scenario would be a scene with strong greens and cyans in the bright parts and saturated reds and magenta in the shadows. The suppression of green in the filter would facilitate a substantial increase in exposure without clipping the highlights, and such increased exposure would be 'real' for the red channel in the shadows since the magenta filter does not suppress red. The reds (and to a lesser extent the blues) would therefore be stronger and less noisy.

2. A worst case scenario would be a scene with strong reds in the bright parts of the image (a flaming red sunset, for example) and saturated greens in the shadows (foliage, freshly fertilised lawn, leguminous plants that get their nitrogen from the atmosphere etc).  In such a situation, there's no possibility of increasing exposure as a result of the magenta filter, because red is the critical channel that's going to be blown, with or without filter. Use of a magenta filter in these circumstances would have a disastrous effect on the predominantly green shadows because the filter reduces the intensity of green light, the opposite of what you want.

The result in this situation would be less dynamic range.

And of course, I'm sure you'll agree, there'll be a whole range of scenarios in between those 2 extremes. Some might result in a worthwhile increase with use of a filter. Some might result in zero increase in DR and some even a negative increase in DR.

It's ironic that in another thread you've made the point about the difficulty of getting sharp results with a 22mp camera, yet here you are advocating reducing shutter speed by a whole stop or more.

[BernardLanguillier]

Bernard (and Ray),

To understand the use of the CC 40M filter, first look at the relative sensitivities of the channels exposed under daylight. Here is an example with the Nikon D200. The PS histogram shows the channels of the light gray patch of the Macbeth CC as they are recorded in the raw file and converted without white balance or gamma correction in DCRaw . The image is dark (gamma 1) and the colors are posterized because this is a GIF to show the menu letters without jpeg artifacts:

[a href=\"index.php?act=findpost&pid=65916\"][{POST_SNAPBACK}][/a]

Thanks for the feedback. Interesting read.

I might give it a try during a coming shoot.

Cheers,
Bernard

[bjanes]

Now for the tricky bit. Any increase in dynamic range is totally dependent upon being able to increase exposure without clipping any of the channels. Increasing exposure is not always possible when using a filter that is not selected specifically for the scene being photographed.

This is the major flaw in the process. I'll give 2 examples which I predict would have a completely opposite effect on DR when using a magenta filter.

2. A worst case scenario would be a scene with strong reds in the bright parts of the image (a flaming red sunset, for example) and saturated greens in the shadows (foliage, freshly fertilised lawn, leguminous plants that get their nitrogen from the atmosphere etc).  In such a situation, there's no possibility of increasing exposure as a result of the magenta filter, because red is the critical channel that's going to be blown, with or without filter. Use of a magenta filter in these circumstances would have a disastrous effect on the predominantly green shadows because the filter reduces the intensity of green light, the opposite of what you want.

The result in this situation would be less dynamic range.

[a href=\"index.php?act=findpost&pid=65956\"][{POST_SNAPBACK}][/a]

In your second example let us assume that the luminance of the red is the highest in the image and you will expose so as to place the reds at the right of the histogram. I don't know what white balance you plan to use, but I would use daylight so that the warm colors of the sunset would be preserved. If you choose to use WB other than daylight, use of the magenta filter would not be appropriate and this case will not be considered further.

I don't know the multipliers for your 5D, so I will use those for the EOS1D Mark II and assume they are similar. Because the red multiplier for daylight white balance is 1.92, your reds (which are exposed to the right with white balance) are only midrange in the raw file, effectively 1 stop underexposed. They are in no danger of blowing out.

So you add the magenta filter and increase exposure by 2/3 EV. The increased exposure compensates for the reduction of green by the filter, and the amount of light falling on the green sensors is the same. The red channel is now toward the right by 2/3 EV and will have a better signal to noise ratio and dynamic range. The image is improved rather than the disaster you predicted. What you failed to do is to analyze what is going on in the raw channels.

It's ironic that in another thread you've made the point about the difficulty of getting sharp results with a 22mp camera, yet here you are advocating reducing shutter speed by a whole stop or more.
[a href=\"index.php?act=findpost&pid=65956\"][{POST_SNAPBACK}][/a]

I am assuming for a static situation such as a sunset you would be using a tripod  

[Ray]

I don't know the multipliers for your 5D, so I will use those for the EOS1D Mark II and assume they are similar. Because the red multiplier for daylight white balance is 1.92, your reds (which are exposed to the right with white balance) are only midrange in the raw file, effectively 1 stop underexposed. They are in no danger of blowing out.

If the reds are only midrange without the filter, then I am not exposing to the right. Because it's late in the evening, there may be a great expanse of green in shadow which I don't want to look noisy or totally black. I'm in a high dynamic range situation with the highlights predominantly red. I increase exposure till the red channel on my histogram is pushing against the right (the 5D has an RGB histogram). The green channel is looking pretty weak though and much of it is comprising the yellow of the sunset which is, however, more red than yellow. I sense I'm going to have more noise in the shadows than I'd like. I think of bjane's clever trick of using a magenta filter which I just happen to have in my pocket.

I slip on the magenta filter. The red channel is unaffected because of course it lets through all the red light, but the blue channel is now worse and the green channel is much worse. I can't increase exposure because the red channel is already as far to the right as it can be without clipping. The filter is not only useless in these circumstances, if I were to use it it would actually decrease dynamic range.

Is this not the case?

[Ray]

I am assuming for a static situation such as a sunset you would be using a tripod  
[a href=\"index.php?act=findpost&pid=65973\"][{POST_SNAPBACK}][/a]

Not necessarily. Sunsets can be very bright when the camera is directed at the sky and an exposure to the right can be quite short. I've taken many hand-held shots of sunsets. I generally use a tripod only when I have to, and that is mainly for stitching purposes.

[Dennis]

Would you have any insights about the reason for this response, G>B>R ?

Maybe just the inks? Some years ago, Kodak (and Nikon) experimented with CMY CFA'a for this reason. Does anybody know, why this technique died? Any major flaws?

If the reds are only midrange without the filter,

Ray, how do you meter a scene, if you want to measure only the light, and want to bann all reflective issues and influences of colors to the meter? If you would use a grey card, then the magenta filter method would be fine with you.

Colors of the subject play no role at all. It's all about adopting different sensitivities of the channels of a certain sensor to a certain light (temperature). A very easy example to understand is the situation with a tungsten lighted scene. The uncorrected image will have a orange color cast, since the spectrum of the light has much red and little blue. So the blue channel will be exposed purely, the green a bit more and the red a lot more. If you want to handle this problem with software based white balance only, you have to increase the values of the blue channel exessively and the green ones a bit less to match the red channel values in order to get a neutral grey, right? So you will introduce much noise in the final image. Therefore, it would be a way better idea, to balance the light before it reaches the sensor, eg using a blue 80B filter. This filter will block a certain amount of the red light, so that (ideally) the light after passing the filter has about equal parts of blue, green and red. Of course, you now have to expose longer or open up the aperture (or raise the ISO), and that might be unwanted or impossible in some situations. But the light is better balanced, and the image will suffer less noise. So what you do here, is to balance the incoming light. The colors of the scene are completely irrelevant.

The above proposed method is all the same, it balances light to the sensors needs. This balancing has nothing to do with colors in the scene, like green foliage or red skyes. It only has to do with the light itself and the sensor. So you'd need a different setup for each sensor and light temperature. If that's worth the hassle, is an individual question.

The other question is, if you want the light to be balanced. A beautifill colored sunset will loose everything, if you match the WB exactly to the temperature of light. So sometimes a color cast is exactly what you want.

But as a matter of principle, the filter method is absolutely correct. It's always a better idea, to balance the light before it hits the sensor, rather than balancing it afterwords by software.

[bjanes]

If the reds are only midrange without the filter, then I am not exposing to the right. Because it's late in the evening, there may be a great expanse of green in shadow which I don't want to look noisy or totally black. I'm in a high dynamic range situation with the highlights predominantly red. I increase exposure till the red channel on my histogram is pushing against the right (the 5D has an RGB histogram). The green channel is looking pretty weak though and much of it is comprising the yellow of the sunset which is, however, more red than yellow. I sense I'm going to have more noise in the shadows than I'd like. I think of bjane's clever trick of using a magenta filter which I just happen to have in my pocket.

I slip on the magenta filter. The red channel is unaffected because of course it lets through all the red light, but the blue channel is now worse and the green channel is much worse. I can't increase exposure because the red channel is already as far to the right as it can be without clipping. The filter is not only useless in these circumstances, if I were to use it it would actually decrease dynamic range.

Is this not the case?
[a href=\"index.php?act=findpost&pid=65982\"][{POST_SNAPBACK}][/a]

Here is your daylight exposure, with full exposure to the right. The white square reads RGB 240, 240, 240. Close enough? Reds are to the right. This is about what the RGB histogram of your 5D would show.

[attachment=580:attachment]

Here is the status in the raw channels, which are not shown on your camera's histogram and about which you are unaware. The red is to the left. You can either balance the channels with a filter or in software with a multiplier, as Dennis explains so well with another example. As far as the RGB channel histograms in your camera are concerned, they are not of much help in ETTR without the filter in place, since they show the histogram after the multipliers have been applied. With the filter in place, the multipliers are much closer 1 in the blue and red channels, and the histograms would more nearly represent the raw channels. Another benefit of the filter. What more can I say? In any event, I plan to say no more.

[attachment=581:attachment]

[Willowroot]

Bill,

Would you have any insights about the reason for this response, G>B>R ?

Thanks! Peter

--
[a href=\"index.php?act=findpost&pid=65942\"][{POST_SNAPBACK}][/a]

I believe it is because there are twice as many green pixels in the sensor as red or blue.  The sensor is laid out in a square grid so one colour will need to have twice as many pixels as the other two.  Green was chosen because the eye is most sensitive to green light.

That's how I understand it anyway.  Hopefully someone will correct this if needed.

[PeterLange]

... since both Nikon and Canon have excellent engineers, and there must be an explanation rather than manufacturer stupidity.   

Bill
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Bill,

Maybe it’s more a trade-off rather than an intended purpose.  Otherwise above quote could be turned around: if these excellent engineers shaped this response behaviour to G>B>R, then it could be risky to interfere (though I get your reasoning).

Just googled a little bit:

The following reference may not fit perfectly. However, it seems to say that the approach to optimize quantum efficiency can shift the CCD response away from the silicon’s preference for red, towards G > RB. [a href=\"http://www.andor.com/library/digital_cameras/?app=315]http://www.andor.com/library/digital_cameras/?app=315[/url]

Further, this could be promoted or caused alone by the anti-IR-filter in front of the imaging chip. http://www.naturfotograf.com/D200_rev05IR.html


So yes, a magenta filter seems to make sense (for daylight conditions) provided that there’s enough light to re-balance exposure; which I think was Ray’s point.


Further, I’d say that in a first order such effects of global scaling are independent from the Bayer CFA and the individual R/G/B spectral responses which shape the gamut to form a subset to CIE XYZ (ideally).
http://www.outbackphoto.com/tforum/viewtop...hp?TopicID=1929

Willowroot, If the number of green pixel would count, neighbouring green pixel would have to be coupled to produce a higher sensitivity. So the coupled pixel would produce only one signal and wouldn’t be distinguishable anymore. Does that make sense?


Now I should devinitively stop going over my head.

Peter

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[John Sheehy]

You might be gaining something in the highlights (depending on how the white balance exactly works with the channels), but you are also very likely to get a more noisy green channel, since you are doing the very opposite of the recomended expose to the right dogma.

You would expose everything to the right anyway.  Suppressing the green channel is only a relative thing, in auto-exposure modes, and calls for a different exposure index, for manual exposure.  You certainly wouldn't want to use a filter if you're already struggling with low light; that just weakens channels absolutely, and they all get noisier.

Using a magenta filter like this works well if you want full color over the maximum possible DR.  It makes the noise equal in all color channels (assuming Daylight WB), and consequently, the noise is less chromatic.

The normal color response of the cameras almost always has at least one channel that is very weak, and the noise from that channel drags the image down.

[John Sheehy]

Exactly! In my example above, comparing a cyan shift corrected in PS as opposed to a cyan shift corrected by use of a magenta filter, one has 2 shots with the same 'external' exposure. The shadows, however, having less 'internal' exposure in the green channel, with use of the magenta filter, will actually be noisier to some degree.

On the other hand, blown detail in the clouds will be brought back slightly. Net effect regarding dynamic range... a big fat zero.

Magenta is just a general correction for daylight WB, assuming white highlights.  You can take the idea all the way and filter specifically for the scene; if the blue sky is your highlight, then a red filter will help you push the blue sky closer to RAW max in all three channels, exposing the shadows in all channels as high as possible.

If the cameras had a real RAW RGB histogram, you could do this in the field with a set of filters.

Digital shadows are generally a murky abyss; anything you can do to get all three channels as high as possible is going to increase DR, and decrease readout noise, including horizontal and vertical banding artifacts.