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Old Feb 24, 2012, 5:05 PM   #11
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In the RGGB array, each pixel will produce the same signal, when the subject is a neutral gray, correct? With RGBW, the W pixel would saturate at a much lower light level than the other three. This should mean the s/n ratio of the color pixels will be worse, and when amplified to the corresponding luminance, will produce a pretty noisy picture.
Using the two G pixels in RGGB acually produces less Green noise than Red or Blue, due to the averaging effect. Look at a Blue sky for an example.

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Old Feb 25, 2012, 4:05 AM   #12
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In the RGGB array, each pixel will produce the same signal, when the subject is a neutral gray, correct? With RGBW, the W pixel would saturate at a much lower light level than the other three. This should mean the s/n ratio of the color pixels will be worse, and when amplified to the corresponding luminance, will produce a pretty noisy picture.
Using the two G pixels in RGGB acually produces less Green noise than Red or Blue, due to the averaging effect. Look at a Blue sky for an example.

brian
I don't think you'd cripple the RGB channels to prevent the W channel from clipping like that. What I'd do is: Where the white pixel is clipped, discard it and interpolate the hue and luminance from the adjacent RGB values. Where the white pixel isn't saturated, use the luminance from the white pixel but interpolate the hue from the adjacent RGB. So the W channel is only used to give better shadow detail and less mid-range noise. At the top end where s/n ratio is better anyway, you rely on the RGB pixels.

I think in essence you'd get a bit more chroma noise in the green channel but less luminance noise across all channels which I think is a great trade-off.
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Old Feb 25, 2012, 5:18 AM   #13
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With RGBW, the W pixel would saturate at a much lower light level than the other three.
Clipping wouldn't happen at the individual photoreceptors. Photons don't sit patiently, waiting to be converted into electrons. And certainly, any clipping that might occur somewhere in the microcircuit, that has never happened before because all the photoreceptors were filtered, yet might with a different filter because the W photoreceptor is unfiltered, would be a minor technical problem.

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This should mean the s/n ratio of the color pixels will be worse, and when amplified to the corresponding luminance, will produce a pretty noisy picture.
For the most part, noise occurs at a photoreceptor regardless of the signal received by that photoreceptor or any adjacent photoreceptors, and gets "amplified" in the A/D process descretely. Thus the signal-to-noise ratio for the R, G, and B photoreceptors would likely be the same when using an RGGB filter as when using an RGBW filter, and would start off the same for the W photoreceptor, but because of the increased signal it receives as a result of not being filtered, its signal-to-noise ratio would be considerably lower than for the R, G, and B photoreceptor. And when the adjacent R, G, and B pixels are deduced, the data from the W photoreceptor, with its much higher signal-to-noise ratio, would produce a cleaner image.

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Using the two G pixels in RGGB acually produces less Green noise than Red or Blue, due to the averaging effect. Look at a Blue sky for an example.
The noise in a blue sky is green.
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Old Feb 25, 2012, 10:54 AM   #14
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The RGGB array was used because the eye is more sensitive to yellow/green wavelengths, so using the green channel for luminance most closely matched perceived brightness. An output from the sensor giving equal weight to the red, blue and green/2, would still look somewhat chartreuse to us. To solve this, the red and blue pixels are given more weight in order to normalize color output. The actual formula for a gray pixel is something like (G+G/2)+1.45R+1.32B. (don't have the exact figures to hand, and they vary somewhat for different cameras) From this, you can see that any differences between adjacent pixels (noise), become multiplied (amplified) for the red and blue, and averaged for the green, resulting in greater red and blue noise.
Using an RGBW array would require larger multipliers for all the color channels, and more complex processing, since there would have to be a way to limit the effect of the W pixel saturating in higher light levels, possibly by varying the multipliers based on W.
Considering that the very simple interpolation formula I gave in the first example has to be performed for every pixel (several million times), imagine the processing power needed for the second condition. Not saying it can't, or won't be done, but it won't be simple.

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