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#161
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[[ This message was both posted and mailed: see
the "To," "Cc," and "Newsgroups" headers for details. ]] In article , wrote: In message , Chris Cox wrote: I do know that for anyone else doing a similar experiment (inside and outside Adobe), they get the full 32769 values. I just came up with an idea to check if it was the internal representation or the "info" tool itself, and sure enough it was the info tool that was at fault. What I did was open the "levels" dialog, and set the input max to 2. Then, I moved the info tool over the pixels, and sure enough, there was not a direct correspondence between the "old" and "new" values. the first 0 became a 0; the second 0 became a 52; all numbers that are multiples of 52 were present in the new values (no gaps). The info tool is toast in 16-bit greyscale mode. OK - that must have slipped through QE somehow. (and I think I did most of my tests in RGB mode) I'll have someone double check it in the current build and fix it if it's still broken (well, as soon as I get rid of this @#!^&*^%$ cold). Chris |
#162
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Mike Engles writes:
I would have thought that photographs taken by spacecraft are to be viewed. Not just viewed, measured. For that, you need to calibrate the camera regularly, and preserve the data from it. For that, it's worth keeping the data in linear form, and using more memory and transmission time. It strikes me that if gamma encoding is necessary for terrestrial imaging to maximise the use of a limited number of bits, then that would also apply to space photography. Generally no, because the tradeoffs are different. Some cameras *do* allow you to save data in a linear losslessly compressed form called "raw", precisely when you want more control over what's done with it. If you have raw camera data, you can process it in 16-bit linear form if you want. There was a thread in the scanner group, where the expert consensus was that any imaging,storage and processing in a linear domain invited image degradation and posterisation. Any processing in *8 bit* linear invites posterization and other degradation. Using *16 bit* per sample linear avoids most of this for ordinary pictorial images. Using *floating point* linear is enough for high dynamic range images. You must distinguish between these different linear forms. Yet we find that such linear imaging,storage and processing is common in scientific digital imaging, where one would imagine that extreme accuracy was paramount. I'll bet it isn't 8 bit linear. Do they use a large number of bits to avoid problems associated with linear storage and processing? The expert consensus was that one would need 18 to 20 bit linear images to match the efficiency of a 8 bit gamma encoded image. Yes, though the 18 or 20 bit number depends on what you mean by "efficiency", and what intensity range you're trying to cover. What is sauce for the goose is sauce for the gander. Can't you see that 8-bit linear and 16-bit linear are entirely different sauces? Timo Autiokari has been saying for ages that scientific imaging was done linearly. He has been abused soundly for his claims. He's been abused for recommending 8-bit linear over 8-bit nonlinear. We have been told that no one who does serious image processing does it linearly. Oh, who said that? I do the actual signal processing in linear space (in 32-bit floating point), but often store images in 8-bit nonlinear form. There's no contradiction here; it just requires a conversion. So all the scientists of the world who regularly do their processing in a linear domain are not really serious and that they are merely FADISTS like Timo Autiokari. Again, they're not using 8-bit linear for any serious measurement data. Dave |
#163
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Mike Engles writes:
I would have thought that photographs taken by spacecraft are to be viewed. Not just viewed, measured. For that, you need to calibrate the camera regularly, and preserve the data from it. For that, it's worth keeping the data in linear form, and using more memory and transmission time. It strikes me that if gamma encoding is necessary for terrestrial imaging to maximise the use of a limited number of bits, then that would also apply to space photography. Generally no, because the tradeoffs are different. Some cameras *do* allow you to save data in a linear losslessly compressed form called "raw", precisely when you want more control over what's done with it. If you have raw camera data, you can process it in 16-bit linear form if you want. There was a thread in the scanner group, where the expert consensus was that any imaging,storage and processing in a linear domain invited image degradation and posterisation. Any processing in *8 bit* linear invites posterization and other degradation. Using *16 bit* per sample linear avoids most of this for ordinary pictorial images. Using *floating point* linear is enough for high dynamic range images. You must distinguish between these different linear forms. Yet we find that such linear imaging,storage and processing is common in scientific digital imaging, where one would imagine that extreme accuracy was paramount. I'll bet it isn't 8 bit linear. Do they use a large number of bits to avoid problems associated with linear storage and processing? The expert consensus was that one would need 18 to 20 bit linear images to match the efficiency of a 8 bit gamma encoded image. Yes, though the 18 or 20 bit number depends on what you mean by "efficiency", and what intensity range you're trying to cover. What is sauce for the goose is sauce for the gander. Can't you see that 8-bit linear and 16-bit linear are entirely different sauces? Timo Autiokari has been saying for ages that scientific imaging was done linearly. He has been abused soundly for his claims. He's been abused for recommending 8-bit linear over 8-bit nonlinear. We have been told that no one who does serious image processing does it linearly. Oh, who said that? I do the actual signal processing in linear space (in 32-bit floating point), but often store images in 8-bit nonlinear form. There's no contradiction here; it just requires a conversion. So all the scientists of the world who regularly do their processing in a linear domain are not really serious and that they are merely FADISTS like Timo Autiokari. Again, they're not using 8-bit linear for any serious measurement data. Dave |
#164
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In message ,
"David J Taylor" wrote: Matt Austern wrote: [] One of the crucial things I missed, apparently, was that we really aren't talking about a 15-bit representation. I missed the fact that the range really is, [0, 32768] not [0, 32768). As soon as you start doing any filtering operations you will can overshoot - ie. you need a range -32768..0..32767, what I would call signed 16-bit. Of course, no physical medium can depict the negative brightness levels the negative values imply. Anf what are they, anyway? Thirsty light sponges? -- John P Sheehy |
#165
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In message ,
"David J Taylor" wrote: Matt Austern wrote: [] One of the crucial things I missed, apparently, was that we really aren't talking about a 15-bit representation. I missed the fact that the range really is, [0, 32768] not [0, 32768). As soon as you start doing any filtering operations you will can overshoot - ie. you need a range -32768..0..32767, what I would call signed 16-bit. Of course, no physical medium can depict the negative brightness levels the negative values imply. Anf what are they, anyway? Thirsty light sponges? -- John P Sheehy |
#166
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#167
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Kibo informs me that Timo Autiokari stated
that: On Tue, 23 Nov 2004 10:50:44 +1100, wrote: With photography, the intention is to produce a final image that is as similar as possible to what a human eye would've seen through the viewfinder, For you information, As it happens, I've designed imaging systems, so I'm quite familiar with the differences between requirements of a scientific imaging system, vs the requirements of a device to create images intended to approximate what a human eye would see in the same situation. And for /your/ information, a photograph evokes only a very vague approximation of what an eye would've seen if it'd been in place of the camera. Even a gamma-corrected (ie; non-linear) image is just another in a long string of compromises that makes it a little easier to trick the human eye into perceiving the printed/displayed image as 'real'. what ever real life scene the human eye is viewing at, it happens that *linear* light (photons) will hit the sensors on the retina. You're ignoring the fact that most scientific imaging uses false-colouring *precisely because* the 'true' image would either be invisible, too dark, or too bright to be processed by a naked human eye. If the human eye was capable of perceiving, (for example), Doppler-shifted light from a star on the other side of the galaxy, we wouldn't need space-telescopes in the first place, would we? - We could just look out the window instead. And the human eye can't correctly image even fairly close stars - we perceive most stars as being white, (even though they are strongly coloured), because their light is too dim for our colour vision to pick it up. Fortunately, scientific imaging systems can show us their *real* colour. Closer to home, scientific instruments create images via things like soft X-rays, or infrared light - situations where the capabilities & limitations of the human eye are completely irrelevant. The particular scaling system, (whether it's linear, log, exponential, bell-shaped or whatever) that's optimal for scientific imaging has nothing whatever to do with how the eye perceives light, & everything to do with the physics of whatever it is that the device is intended to measure. Displays however are not capable to output very high luminance levels but it so happens that the eye has the iris so it can adapt to different brightness levels, The eye does a hell of a lot more to deal with large contrast ranges than just adjust the iris. For example; the retina automatically performs an astonishingly-similar analog of darkroom or PS contrast masking to 'correct' for localised highlights in the visual field that would otherwise 'blowout', just as photographers do to 'correct' photos of sunsets or other scenes with contrast ranges that are too big to print or display. therefore 1:1 linearity is not needed, just an overall linearity of the transfer function is enough. Nonlinearity in this path makes the image appearance too dark or to bright in some portion of the tonal reproduction range. For starters, the light output of a display isn't even close to being linear, nor should it be. If you actually look at the transfer graph for a calibrated monitor, you'll find that the transfer curve is exponential. It's no harder to calibrate a monitor to give a completely linear input-voltage to light output relationship, rather than a 1.8 or 2.2 gamma curve, then run an extremely accurate linear greyscale gradient across it, but it would result in precisely the *perceived* non-linearity you've just mentioned. We gamma-correct monitors for the *exact purpose* of eliminating that non-linear perception. which requires a non-linear response. False. No, I'm afraid not. You would do well to read up on how the human eye works, as well as about scientific imaging techniques, because the stuff you're saying is just plain wrong. -- W . | ,. w , "Some people are alive only because \|/ \|/ it is illegal to kill them." Perna condita delenda est ---^----^--------------------------------------------------------------- |
#168
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Kibo informs me that Timo Autiokari stated
that: On Mon, 22 Nov 2004 21:32:21 GMT, wrote: I'm not so sure that ACR works in a totally linear domain. It definitely doesn't. Images exposed with bracketing, and compensated to be the same with the exposure slider, may have equal mid-tones, but the shadows and highlights will display that a different gamma is used. If you drag the ACR exposure slider to the left, after it runs out of "hidden highlights", it stretches the highlights so that 4095 in the RAW data stays anchored at 255 in the output, and never gets darker. That is not linear exposure compensation. Nor is it an exact analog of adjusting by F-stops (ie; non-linear), which is what I'd like it to be. You should try C1 some time, its exposure compensation adjustment control is *way* more like adjusting the exposure compensation dial on a camera. Going from that to to ACR weirds me out every time. That editing operation is still applied over linear image data, even if the operation itself is not linear. Exposure adjustment in fact is a linear operation, multiplication by a factor, Incorrect. It's scaled in F-stops, which are exponential, not linear. You'll find the mathematical details in any good textbook on photography. So at the middle it will calmly snip one level away, the coding there is: ...7FF8h 7FFAh 7FFCh 7FFEh 8000h 8001h 8003h 8005h ... Due to this discontinuity 'd say that the 15th bit of Photoshop is quite un-usable for applications that require accuracy. *sigh* You're talking about the LSB of a 15 bit value sometimes skipping a value, which, (assuming that you're correct about it), is an inaccuracy of around 0.003%. To put this hypothetical error into perspective, it'd have to be at least *four times greater* to alter a 12 bit RAW image by even a single step in value - a change that would be not only be completely invisible to the human eye, but would be completely swamped by the much, much greater errors contributed by the sensor noise in the camera, *plus* the ADC error in the camera, *plus* the colour-space rounding errors in the computer, *plus* the DAC inaccuracy in your video card, *plus* the video amp inaccuracy in your monitor. The 'error' you're talking about is about as significant as an ICBM missing the targetted position by a couple of feeet. -- W . | ,. w , "Some people are alive only because \|/ \|/ it is illegal to kill them." Perna condita delenda est ---^----^--------------------------------------------------------------- |
#169
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#170
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