What the imager has

Started Feb 11, 2014 | Discussions thread
Raist3d Forum Pro • Posts: 37,257
Thanks Dick, pretty much in agreement, most of what I was saying -

DickLyon wrote:


In the 1:1:4 arrangement, each sample layer has this property, but at different rates -- very unlike the Bayer's red and blue planes. The large area of the lower-level pixels is the ideal anti-aliasing filter for those layers; the top layer is not compromised by the extra spatial blurring in the lower layers, so it provides the extra high frequencies needed to make a full-res image.

Another good way to think of the lower levels is that they get the same four samples as the top level, and then "aggregate" or "pool" four samples into one. This is easy to simulate by processing a full-res RGB image in Photoshop or whatever.

Yup, which goes back to what I said that this is somewhat similar to what some texture compressions schemes are done in GPU / graphics. So taken an ideal Foven Merrill sensor with none of the noise issues and diffraction - the Merrill in theory would still be better, correct? Since it samples the other layers per spatial site without the averaging.

But it's the realities of the noise/other issues what makes this (new) design pull ahead, correct?

The pooling of 4 into 1 is done most efficiently in the domain of collected photo-electrons, before converting to a voltage in the readout transistor. The result is the same read noise, but four times as much signal, so about a 4X better signal-to-noise ratio. Plus with fewer plugs, transistors, wires, etc. to service the lower levels, the pixel fill factor is closer to 100% with easier microlenses, and the readout rate doesn't have to be as high. Wins all around -- except for the chroma resolution.

This is what I said elsewhere also except for the first sentence. So we actually agree. The Chroma resolution suffers, but I also expect this design to offset that tradeoff given the current Merrill issues, some of which you mention.

The main claim of Bryce Bayer, and the fact that most TV formats and image and video compression algorithms rely on, is that the visual system doesn't care nearly as much about chroma resolution as about luma resolution.

Yes, and it's the main reason why texture compression on GPU's works reasonably well.

Unfortunately, trying to exploit that factor with a one-layer mosaic sensor has these awkward aliasing problems. Doing it with the Foveon 1:1:4 arrangement works better, requiring no AA filter, no filtering compromises. So, yes, the chroma resolution is less than the luma resolution, but you'd be hard pressed to see that in images.

I would imagine this is going to depend on the image and the level of pixel peep, and each individual. Fortunately having more resolution of a sensor helps cover this also, and for publishing mediums such as web, iPad, etc. it's pretty much a non issue. But it's worth bringing up only because at least several Foveon "looK" aficionados always do this comparison at the pixel peeping level pointing out why Foveon is better even if other solutions due to the end result have their advantages also.

If you throw out the extra luma resolution and just make 5 MP images from this new camera, you'll still have super-sharp super-clean versions of what the old DP2 or SD15 could do.

I agree Dick, and it's exactly what I mentioned here:


I want to see how much of an improvement there could be in ISO using that Low resolution raw mode.

Now imagine adding 2X resolution in each dimension, but with extra luma detail only, like in a typical JPEG encoding that encodes chroma at half the sample rate of luma. Whose eyes are going to be good enough to even tell that the chroma is less sharp than the luma? It's not impossible, but hard.

I agree it will be hard. It will, depend on the person (on average that won't matter), pixel peeping, subject being photographed. It could have some slight color crawl (depending how they manage) or blur. Similar to GPU graphics texture compression schemes.

In a game though the textures are much lower rez than the Quattro Foveon, usually the graphics are moving so in general it's hard to notice. In the case of the Quattro though stills, the resolution is vastly higher than the typical game texture so that counters that (sans 100%-200% pixel peeps and some photographic subjects).

Speaking of stories from the old days, Foveon's first version of Sigma Photo Pro had a minor bug in the JPEG output, as you probably recall: our calls to the jpeg-6b library defaulted to encoding with half-res chroma. It took a while, but a user did eventually find an image where he could tell something was not perfect, by comparing to TIFF output, and another user told us how to fix it, so we did. It we could have gotten that original level of JPEG quality from the SD9 with 5 million instead of 10 million pixel sensors and data values, and could have gotten cleaner color as a result, would that have been a problem? I don't think so; except for marketing, and they had enough problems already. Same way with Sigma's new one, I expect; if 30 M values gives an image that will be virtually indistinguishable from what could be done with 60 M, but with cleaner color, will someone complain?

Probably so.

I think the big huge win is that the tradeoffs of the new design will more than offset or should more than offset the Merrill's in a significant way. It's what I am expecting (say 2 stops better ISO, superb B&W).

So, it's complicated. Yes, reduced chroma resolution is a compromise; but a very good one, well matched to human perception -- not at all like the aliasing-versus-resolution compromise that the mosaic-with-AA-filter approach has to face.

Agreed, which is what I have been saying too for the most part. The Quattro would still have more data than the Bayer to do its work. What I want to see is the bottom line improvement on the ISO. 14-bit (assuming it is really accurate to sample at that level) should benefit DR and ISO.


disclaimer: I've been away from this technology too long to have any inside knowledge. And give my apologies to Laurence for my too many words.

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Raist3d/Ricardo (Photographer, software dev.)- I photograph black cats in coal mines at night...
“The further a society drifts from truth the more it will hate those who speak it.” - George Orwell

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