Graeme Nattress
RED Problem Solver
I posted this elsewhere, but thought it warrants wider reading....
Of course, 3 chip cameras often have offsets in their chips which stop them delivering properly co-sited chroma in 4:4:4. And of course, if you try to get a triple of 1920x1080 sensors to actually allow you to measure a 1920x1080 resolution you will find that you've contaminated your precious image with significant levels of aliasing artifacts. To me, the only correct engineering and aesthetically correct approach is to properly optically low pass filter to achieve significant attenuation at the full resolution of the sensor making sure that any modulation caused by aliasing is at a practical minimum. That's why we "only" get resolution out to around 3.2k (~78% of the linear resolution, a slightly higher number than I originally quoted at the start of the project, but I've learned a lot since then).
Say you have 12mp of photosites. What is the best way to arrange them?
3x2.66k chips (the prism + three chip approach)
12mp = 4.61k bayer pattern sensor (what we do, near enough)
12mp RGB stripe, 4.61k (Genesis approach)
If you properly optically low pass filter, the three chip approach will give you about 2.4k resolution in R, G and B, but it's limited to using 2/3" chips and lenses. It will give a great image, but you've limited your lens choice. You may get lower dynamic range than current HD cameras as of the finer pitch on the photosites.
The bayer pattern approach will give around 3.6k luma resolution and 2.3k chroma resolution. (RED One achieves 3.2k from a 4k sensor area) Notice how you don't loose much at all chroma resolution compared to the 3 chip approach, but as our vision is relatively insensitive to chroma resolution, all you'll see when projected is the higher luma resolution. However, if you're not careful in your demosaic algorithm, you may still get some chroma moire. We deal with this completely in our demosaic though.
The RGB stripe approach is tricky. If you don't have an OLPF, you'll get 4.61 / 3 (as you need a triple of photosites to get your pixel as the arrangement of photosites is unsuitable for the kind of demosaicing we can do with bayer patterns) = 1.53k. However, the Genesis uses a non-square pixel approach of 1920x3 horizontally and 1080X2 vertically = 12.4mp, giving a max resolution before adding the OLPF of 1.9k. That would probably be around 1.7k with a proper OLPF.
All of the above are compromises, however, I do think that above shows that the bayer pattern is the one that gives you the most perceptually relevant resolution for your budget of photosites. In the above I'm taking into account that we generally use a stronger OLPF on Bayer pattern sensors, and the effects of a known high quality demosaic algorithm.
The 3 chip approach and RGB Stripe approach above are "guestimates" only, and would vary upon specific implementation of the OLPF and sensors. I've not seen, for instance, an F35 or Genesis pointed at a zone plate, so I don't know how strongly or weakly they set their OLPF, and I don't know if they do any software processing to account for the fact that the RGB on their sensor is not co-sited as it is in the three chip approach.
Graeme
Of course, 3 chip cameras often have offsets in their chips which stop them delivering properly co-sited chroma in 4:4:4. And of course, if you try to get a triple of 1920x1080 sensors to actually allow you to measure a 1920x1080 resolution you will find that you've contaminated your precious image with significant levels of aliasing artifacts. To me, the only correct engineering and aesthetically correct approach is to properly optically low pass filter to achieve significant attenuation at the full resolution of the sensor making sure that any modulation caused by aliasing is at a practical minimum. That's why we "only" get resolution out to around 3.2k (~78% of the linear resolution, a slightly higher number than I originally quoted at the start of the project, but I've learned a lot since then).
Say you have 12mp of photosites. What is the best way to arrange them?
3x2.66k chips (the prism + three chip approach)
12mp = 4.61k bayer pattern sensor (what we do, near enough)
12mp RGB stripe, 4.61k (Genesis approach)
If you properly optically low pass filter, the three chip approach will give you about 2.4k resolution in R, G and B, but it's limited to using 2/3" chips and lenses. It will give a great image, but you've limited your lens choice. You may get lower dynamic range than current HD cameras as of the finer pitch on the photosites.
The bayer pattern approach will give around 3.6k luma resolution and 2.3k chroma resolution. (RED One achieves 3.2k from a 4k sensor area) Notice how you don't loose much at all chroma resolution compared to the 3 chip approach, but as our vision is relatively insensitive to chroma resolution, all you'll see when projected is the higher luma resolution. However, if you're not careful in your demosaic algorithm, you may still get some chroma moire. We deal with this completely in our demosaic though.
The RGB stripe approach is tricky. If you don't have an OLPF, you'll get 4.61 / 3 (as you need a triple of photosites to get your pixel as the arrangement of photosites is unsuitable for the kind of demosaicing we can do with bayer patterns) = 1.53k. However, the Genesis uses a non-square pixel approach of 1920x3 horizontally and 1080X2 vertically = 12.4mp, giving a max resolution before adding the OLPF of 1.9k. That would probably be around 1.7k with a proper OLPF.
All of the above are compromises, however, I do think that above shows that the bayer pattern is the one that gives you the most perceptually relevant resolution for your budget of photosites. In the above I'm taking into account that we generally use a stronger OLPF on Bayer pattern sensors, and the effects of a known high quality demosaic algorithm.
The 3 chip approach and RGB Stripe approach above are "guestimates" only, and would vary upon specific implementation of the OLPF and sensors. I've not seen, for instance, an F35 or Genesis pointed at a zone plate, so I don't know how strongly or weakly they set their OLPF, and I don't know if they do any software processing to account for the fact that the RGB on their sensor is not co-sited as it is in the three chip approach.
Graeme
) to look at, and rate, some of the image samples and compare their perseptual results with the previous statistical results (using left/right and both eyes?). I`ve carried out much crazier statistical image tests (in terms of 100000s of images), while experimenting with lossless fractal compression ideas I`ve had (for a digial microfiche). No one paying me either. Lots of demosaic research papers I`ve read seem to use quite small numbers of test images, and not just as "typical examples" either. I would imagine you`d want to test different colour spaces and bayer patterns too eg, luminance, chromiance, hue, CYMK, RGB etc in the search for the holy grail (unless you had a hardware specific reason not to). I`m not familiar with the science of colour beyond RGB/YUV conversion and inaccurate raytracing metaphors, but I am working on that. I hope I`m not too old to learn, I may need some geeksteroids. The unravelling of a CFA is interesting.