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Badpixel Mapping for Different Exposures

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Offline Sess

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Badpixel Mapping for Different Exposures
« on: 21 / June / 2008, 00:27:37 »
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The badpixel mapping ability (http://chdk.wikia.com/wiki/CHDK_firmware_usage/AllBest) is one of the best features in CHDK. The ability to map out hot pixels, something you'd normally have to have Canon technicians do, is very powerful. The only problem is you can only have a single set of bad pixels mapped out at a time. This means the badpixel list is only accurate for the specific exposure used to make the corresponding dark field. As you move away from that particular exposure, the badpixel list will either start mapping out pixels that are perfectly fine or will ignore hot pixels that need to be mapped out.

I suggest an alteration to the format of the badpixel file. Instead of just listing the problem pixels like this:

281,1307=190
520,2168=1003
521,2168=695
2557,2827=1023
3796,2855=192
1133,2873=245
2593,2888=264

How about we list the shutter speed followed by the pixel info using an empty line (or whatever) to indicate a new exposure like this:

1/4
520,2168=637
521,2168=456
2557,2827=1023
1133,2873=167
2593,2888=161

1
281,1307=264
520,2168=1023
521,2168=961
2557,2827=1023
3796,2855=271
1133,2873=350
2593,2888=366

2
281,1307=420
520,2168=1023
521,2168=1023
3750,2430=215
3750,2431=162
2557,2827=1023
3796,2855=425
1133,2873=515
2593,2888=590

4
2384,7=177
1961,18=203
3917,20=200
4074,143=163
427,988=191
281,1307=696
756,1896=238
520,2168=1023
521,2168=1023
3750,2430=352
3750,2431=281
2557,2827=1023
3796,2855=726
156,2873=163
1133,2873=941
2594,2884=172
2593,2888=892
3522,3011=229
2162,3027=169

And when the camera uses a shutter speed not specified in the badpixel file it can just pick whatever shutter speed entry is closest. It would also be useful if the badpixel feature existed in the menus so settings could be changed, such as an optional upper and lower limit of shutter speeds for when the badpixel file should be used.

The badpixel mapping ability is such an amazing feature of CHDK that I'm amazed it isn't given more attention.

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Offline Sess

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Re: Badpixel Mapping for Different Exposures
« Reply #1 on: 30 / June / 2008, 15:40:42 »
Doesn't anyone have any opinions on this idea?

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Offline fudgey

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Re: Badpixel Mapping for Different Exposures
« Reply #2 on: 30 / June / 2008, 17:32:04 »
Umm...well... my initial reaction was... I never thought dead pixels would be exposure time dependent. If you get more dead pixels in a long exposure, I would have thought that was just because there are a number of pixels that behave badly and for a long exposure the probability to get a certain pixel appear dead is greater and thus more of them would be seen?

The physical pixels are the same for every exposure time after all.

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Offline RaduP

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Re: Badpixel Mapping for Different Exposures
« Reply #3 on: 01 / July / 2008, 01:03:02 »
Yeah, I think there are two kind of 'bad' pixels:
1. Truly dead (or always on) pixels that stay like that the whole time, regardless of exposure.
2. The noise pixels, which are random, and can not be mapped out.


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Offline LjL

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Re: Badpixel Mapping for Different Exposures
« Reply #4 on: 02 / July / 2008, 08:40:32 »
Unless I'm mistaken there are also the "warm" pixels - pixels that are not completely dead, but consistently show higher-than-average (or, for that matter, lower-than-average) brightness.

That varies with exposure, and I believed it was among the main reason why the Canon software automatically subtracts a dark frame on long exposure (the dark frame's length being equal to the exposure length).

For that matter, there is also the problem of vignetting (regions of pixels that are brighter/dimmer than average due to the optics), which can be corrected using a gray frame (i.e. photograph a clear sky). It would be nice if CHDK could automatically divide by a gray frame.

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Offline RaduP

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Re: Badpixel Mapping for Different Exposures
« Reply #5 on: 02 / July / 2008, 11:56:34 »
Yes, of course, but the reason why a dark frame substraction is done is because noise can vary depending not only on the exposure, but also on temperature.
So having a list of them is not practical, and instead you can just use the dark frame substraction always on.
« Last Edit: 02 / July / 2008, 12:59:12 by RaduP »

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Offline fudgey

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Re: Badpixel Mapping for Different Exposures
« Reply #6 on: 02 / July / 2008, 16:44:18 »
...and to automatically do gray frame compensation (of which I never heard of before, I'm no expert), you'd actually always have to have a very recent gray frame available, and I believe it must be shot using the same settings. A dark frame can be shot easily by shooting with shutter closed, but for a gray frame that's just not possible. Even if the sky is a good source for such a frame, it's often not available when vignetting is at its worst (for those 65 second night shots).

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Offline LjL

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Re: Badpixel Mapping for Different Exposures
« Reply #7 on: 03 / July / 2008, 09:35:37 »
Why recent? The vignetting is (hopefully) mostly due to the optics, which shouldn't change very much in time...

If a noticeable part of it is due to the sensor and variable, though, then yeah, tough luck with that.

(By the way, Wikipedia states that both optical and sensor vignetting are somewhat corrected in most digital cameras' software... I wonder if that's really the case at least on Canons...)


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Offline fudgey

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Re: Badpixel Mapping for Different Exposures
« Reply #8 on: 03 / July / 2008, 13:00:55 »
Why recent? The vignetting is (hopefully) mostly due to the optics, which shouldn't change very much in time...

Hmm... I suppose you're right, it doesn't need to be as recent as with the sensor since it's probably not random and temperature dependent, but wouldn't dust/dirt in the optics would require updates? Do you know for sure that this really is worth correcting, especially in-camera? Correction always has a risk of going all wrong and introducing more problems.

And wouldn't we need a large set of reference images for each zoom setting, maybe for each aperture/zoom combination as well? Or even more? If so, fitting these on a flash card in RAW would be kind of awkward.

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Offline LjL

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Re: Badpixel Mapping for Different Exposures
« Reply #9 on: 03 / July / 2008, 14:48:45 »
It's quite likely that we'd need shots for various apertures and zooms, yep.

It could very easily end up taking way too much memory, as you say... although if they were stored compressed (and averaged over several frame, as we don't want sheer noise to appear in them, as that's something for the dark frame to correct), it might be feasible.

Although, looking at the time it currently takes to just average two RAWs, erm...

 

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