Sigma/Foveon Questions
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From: David J Taylor on 29 Apr 2010 14:16 "Alfred Molon" <alfred_molon(a)yahoo.com> wrote in message news:MPG.2643dae57b79e95198c2be(a)news.supernews.com... [] > It does, because that is exactly what Bayer does: interpolate missing > values, by assuming that they are a function of the data in the > neighbouring pixel. These are guesses, which often are wrong. You cannot > calculate the value of a random variable. > -- > > Alfred Molon If the intermediate values were random, what you say would be true, and interpolation would be a poor choice. But because of the anti-alias filter, lens PSF, scene content etc., the values are correlated with the adjacent pixels. Cheers, David
From: Alfred Molon on 30 Apr 2010 02:04 In article <hrcido$87f$1(a)news.eternal-september.org>, David J Taylor says... > If the intermediate values were random, what you say would be true, and > interpolation would be a poor choice. But because of the anti-alias > filter, lens PSF, scene content etc., the values are correlated with the > adjacent pixels. There may be some correlation which depends on the scene content, but randomness as well. An example for randomness would be a meadow with flowers at a distance - no correlation between the red of the flower and the green of the leaves. -- Alfred Molon ------------------------------ Olympus E-series DSLRs and micro 4/3 forum at http://tech.groups.yahoo.com/group/MyOlympus/ http://myolympus.org/ photo sharing site
From: David J Taylor on 30 Apr 2010 02:30 "Alfred Molon" <alfred_molon(a)yahoo.com> wrote in message news:MPG.2644914f1a258a4198c2c0(a)news.supernews.com... > In article <hrcido$87f$1(a)news.eternal-september.org>, David J Taylor > says... >> If the intermediate values were random, what you say would be true, and >> interpolation would be a poor choice. But because of the anti-alias >> filter, lens PSF, scene content etc., the values are correlated with >> the >> adjacent pixels. > > There may be some correlation which depends on the scene content, but > randomness as well. An example for randomness would be a meadow with > flowers at a distance - no correlation between the red of the flower and > the green of the leaves. > -- > > Alfred Molon The amount of correlation will depend on the lens MTF - what the point-spread function is, and how the anti-alias filter spreads the image. That's independent of scene content. In the example you cite, any atmospheric effects over the period of the exposure time would also contribute to blurring of distant flowers. When viewing a print with "distant flowers", the eye can't resolve the colour detail in any case, if by distant you mean "covering a small number of pixels". I don't think that anyone is saying that Bayer is the perfect solution - it isn't - but it is one which works very well in practice. Cheers, David
From: nospam on 30 Apr 2010 04:49 In article <MPG.2644914f1a258a4198c2c0(a)news.supernews.com>, Alfred Molon <alfred_molon(a)yahoo.com> wrote: > > If the intermediate values were random, what you say would be true, and > > interpolation would be a poor choice. But because of the anti-alias > > filter, lens PSF, scene content etc., the values are correlated with the > > adjacent pixels. > > There may be some correlation which depends on the scene content, but > randomness as well. An example for randomness would be a meadow with > flowers at a distance - no correlation between the red of the flower and > the green of the leaves. except that the red flower and green leaves will span more than just one single pixel, so the correct values can be calculated by looking at the neighboring pixels.
From: Alfred Molon on 30 Apr 2010 08:25
In article <290420101341351528%nospam(a)nospam.invalid>, nospam says... > there is no perhaps about it. the accuracy can be measured. delta-e > (colour errors) for foveon is higher, or less accurate. > > or just look at the images. they're full of weird colour casts, > including yellow skin, cyan skies, etc. and it's not consistent even > among multiple foveon cameras. since there's a huge variability, it > *can't* be more accurate. The big issue with Bayer is the effective resolution which is way lower than the nominal one, because a Bayer sensor only captures 1/3 of the needed information. > so is bayer. 1/3 the data for an almost perfect reproduction. No - Bayer is not a data compression method. And reproduction is not almost perfect. > > > foveon is the only existing full colour sensor. > > > > So? Full colour cameras exist, which is what matters. > > but not a full colour sensor. Wrong. Foveon is a full colour sensor. <snip> > many drawbacks for very little gain. 50% more effective resolution is a huge gain. > interpolation is *not* guessing. It is, because the value in the middle does not depend on the neighbouring values. > > which often are wrong. > > what is the error rate? exactly how often is it wrong? You can see the impact on the resolution: the Foveon 4.6MP performing as well (resolution-wise) as an 7-8 MP Bayer sensor, according to the reviews. > > You cannot calculate the value of a random variable. > > it's not random. The colour of one pixel does not depend on the colour of neighbouring pixels. And there are scenes with pixel level colour changes (meadow with flowers at a distance is an example). In practice, real world scenes often have large areas of (almost) homogeneous colour, which is why the performance of Bayer sensors is not as abysmal as the lack of 2/3 of the needed information would imply. Which is why the effective resolution of a Bayer sensors averages somewhere arounc 60%-70% of the nominal, which is very good given that just 1/3 of the needed data is captured. However there is the huge issue of colour aliasing and the need to use AA filters tuned to 1/2 of the pixel count resolution, which is why Bayer images are so soft at the pixel level. -- Alfred Molon ------------------------------ Olympus E-series DSLRs and micro 4/3 forum at http://tech.groups.yahoo.com/group/MyOlympus/ http://myolympus.org/ photo sharing site |