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Bad pixels, what to do about them

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Bad pixels, what to do about them
« on: 18 / August / 2009, 17:59:52 »
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My SX110 has about 4 bad pixels (that I've noticed so far). I'm not sure what the best thing is for me to do, and am hoping that someone with more experience than myself can advise me. I basically have 2 possible methods of solving the problem:

1. Send the camera to Canon for repair, and hope that whatever they do gets rid of the problem.

Or

2. Do something with CHDK (and maybe other software?) to interpolate the bad pixels.

Is it very common to have this amount of bad pixels on my kind of camera? How likely is it that after the repair, there will be no dead pixels? Would Canon actually replace the sensor, or would they just do a software fix?

When a camera is running CHDK and has a properly configured badpixel file, will all the JPG's it takes have the bad pixels removed? Or do you need to be shooting RAW to do this? If the latter is the case, then I'm not sure it's worth it for me to pursue a CHDK solution, since I'd have to do post-processing of every picture I take, when all I need is the bad pixels removed.

So, what would you do?

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Offline fudgey

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Re: Bad pixels, what to do about them
« Reply #1 on: 19 / August / 2009, 16:25:28 »
The camera has a whole lot more dead pixels than that (thousands...you'll see a number by running SCRIPTS/TEST/badpixel.lua, which is also required for DNG output). Those few you're seeing in your JPEGs just happen to be new ones that weren't bad at the factory and thus aren't listed in the Camera's internal bad pixel table and hence don't get interpolated in the JPEG development process. More will likely appear as time goes by, and the number of visible bad pixels may vary with (at least) exposure time.

I'm not familiar with CHDK's bad pixel features, but I believe for CHDK RAW (and DNG?) output you can make CHDK mask all the bad pixels you wish. Hopefully there's some documentation somewhere you can follow to try it out. I don't know if that fixes JPEGs too.

I don't know what Canon's policy is (do they do it at all and do they do it for free under warranty etc), but one would think they should be able to recreate bad pixel tables in service. Four visible bad pixels is normal and they would definitely not be replacing the sensor.

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Offline Anaglyphic

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Re: Bad pixels, what to do about them
« Reply #2 on: 19 / August / 2009, 17:29:10 »
Quite so. CHDK can do it, or Canon can do it. Up to you.

I can only read English so this pertains to Canon USA service centers...  if you go to the Canon USA site and look in the FAQ under your camera for "There are dead or stuck pixels in my pictures." you'll get a canned response that says: Unfortunately, the issue you describe seems to indicate that your product requires repair service. Canon has excellent service options and we are eager to provide any assistance you may need.

The policy is they will do pixel remapping for a camera under warranty for free. You pay shipping. I think you pay shipping as a deterrent so that people won't send cameras back every time a single dead pixel shows up. And there's probably a limit on the # of times they will update the dead pixel table without a charge. (this is conjecture.)

The odds of you getting another refurb camera or your sensor replaced is highly unlikely... you will get your same camera back, with the additional pixels added to the bad pixel list. As fudgey said, your CCD has thousands of hot/dead pixels already - just they were mapped out at the assembly plant before you saw them. I'm sure they have an automated utility they use, just like you can use with the bad pixel mapping stuff for CHDK.

So if you'd like to do it yourself, I think there's two files to make; badpixel for JPGs and badpixel.bin for CRW/DNGs.
These links should help get you started, but also search the forum:

Hot Pixel Removal
Bad Pixel Removal

Hope this helps.
Since we cannot know all that there is to be known about anything,
 we ought to know a little about everything.
-- Blaise Pascal

Re: Bad pixels, what to do about them
« Reply #3 on: 20 / August / 2009, 00:45:10 »
Thanks for the helpful replies. I had no idea there were thousands of bad pixels, although I was starting to think it must be pretty common to have a few. The interesting thing is that when you talk to Canon support, they don't tell you what they do to fix the problem (i.e. a hardware or software fix). They just say they'll fix it and if there are more problems you can send it back again. It seems reasonable that they would just update a bad pixel table or something though.

I think I'll try mapping the bad pixels with CHDK, since it would probably give me the same results as servicing the camera, but for free and faster.

I was playing around with really long override exposures last night, and had the regular Canon shutter speed set to less than 1.3 seconds, which is the threshold for whether the camera does an auto dark frame subtraction. So it didn't do the noise removal afterward, and I noticed a lot more bad pixels in that image compared to others. For later pictures, I set the Canon shutter to more than 1.3'' so it would do the noise removal. I thought this was interesting, and maybe it's a good way to control whether or not the dark frame subtraction is performed. Has anybody else found that out already?


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Offline fe50

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Re: Bad pixels, what to do about them
« Reply #4 on: 20 / August / 2009, 02:37:51 »
@Gimzim

You can control the DF creation in the CHDK RAW menu
("Dark frame subtraction", in earlier CHDK versions it was called "Noise reduction");
although it's in the RAW menu, this is a general setting, not only for RAW shooting...

When you set "Auto", the default Canon threshold will be used; you can manually set it to ON or OFF.
(it's also available in scripts)

The CHDK bad/hot pixel removal "removes" (by interpolation) the pixel in RAW, DNG and also in JPG images.
If you've still lots of images with bad pixels, you can use a software like ExifTool to remove them...

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Offline fudgey

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Re: Bad pixels, what to do about them
« Reply #5 on: 22 / August / 2009, 07:35:58 »
I was playing around with really long override exposures last night, and had the regular Canon shutter speed set to less than 1.3 seconds, which is the threshold for whether the camera does an auto dark frame subtraction. So it didn't do the noise removal afterward, and I noticed a lot more bad pixels in that image compared to others. For later pictures, I set the Canon shutter to more than 1.3'' so it would do the noise removal. I thought this was interesting, and maybe it's a good way to control whether or not the dark frame subtraction is performed. Has anybody else found that out already?

As fe50 said, dark frame reduction can be overrided from RAW menu.

But are you sure the camera didn't do dark frame reduction (you can tell pretty easily because the camera takes twice the time to take the photo when it also shoots a dark frame)?

I'm asking because some cameras have more than one bad pixel list. The list it selects depends on (at least) exposure time. If this selection doesn't depend on the final, true (overrided) exposure time but instead on the exposure time selected by Canon (auto or manual) exposure, the result would be bad pixels in your situation (e.g. shooting a 5 second exposure using CHDK override when camera is set for 1/100 s exposure instead of a 5 second exposure).

(Note: I'm no expert on this, just remembering some stuff people have been discussing. This could very well be a known issue or something that should be reported as a bug, or it could be known that my guess is wrong).

Re: Bad pixels, what to do about them
« Reply #6 on: 22 / August / 2009, 11:47:37 »
But are you sure the camera didn't do dark frame reduction (you can tell pretty easily because the camera takes twice the time to take the photo when it also shoots a dark frame)?

I'm sure it didn't do the DF on that shot, because there was no Busy message at the end, and it didn't take twice as long. It was also pretty obvious how many more bad pixels there were. I can post the picture if you'd like to see it.

I'm asking because some cameras have more than one bad pixel list. The list it selects depends on (at least) exposure time. If this selection doesn't depend on the final, true (overrided) exposure time but instead on the exposure time selected by Canon (auto or manual) exposure, the result would be bad pixels in your situation (e.g. shooting a 5 second exposure using CHDK override when camera is set for 1/100 s exposure instead of a 5 second exposure).

Before this thread, I wasn't running CHDK on this camera yet. So the bad pixels I was wondering about at first weren't a result of conflicts between CHDK overrides and Canon shutter speeds. However, your explanation seems right for the circumstances that picture was taken under.

I think that the Canon firmware's choice of bad pixel list is based only on the Canon shutter speed, based on the picture I took where Canon's was less than 1.3'' and CHDK's was something like 32''. For that reason, is it a good idea, when using shutter speed overrides, to have the CHDK bad pixel lists configured? Is CHDK's bad pixel removal done instead of, or in conjunction with Canon's bad pixel removal?

The 4 bad pixels I was seeing originally (per my original post) were only for shorter exposures (like the moon at night, for example). I'm not sure where the threshold was, but at some point, probably the DF threshold of 1.3'', all of those disappeared. That makes sense with the explanation that those pixels are new since the camera left the factory, because they wouldn't be in the regular table, but they would be caught by the DF.

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Offline fe50

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Re: Bad pixels, what to do about them
« Reply #7 on: 22 / August / 2009, 15:50:54 »
I think that the Canon firmware's choice of bad pixel list is based only on the Canon shutter speed, based on the picture I took where Canon's was less than 1.3'' and CHDK's was something like 32''. For that reason, is it a good idea, when using shutter speed overrides, to have the CHDK bad pixel lists configured? Is CHDK's bad pixel removal done instead of, or in conjunction with Canon's bad pixel removal?
Most cameras have 2 lists, e.g. the SX10, so i guess the SX110 does work with 2 internal badpixel lists, too.

When you override exposure with CHDK, on most (or most newer ?) cameras the Canon exposure value will be set to the nearest Canon value by the original firmware; with overrides > 15s the Canon value is always 15s.
You can also set up the Canon exposure to 15s to make sure the Canon fw uses the correct settings.

When the CHDK DF setting is "AUTO", the camera should use the correct DF behaviour & list...
There are still some issues on some cameras, so you should do some tests to know how your SX110 reacts.
On some cameras you'll get problems when you shoot directly without focusing (half-press shutter) first, e.g. the CHDK overrides may not work correct then or RAW images will not be stored - so you should always focus first, also when shooting night images (use manual focus).

When the CHDK bad pixel removal feature (the one with the badpixel file - not the badpixel.bin for DNG creation) is enabled,
the badpixels from your (CHDK) list will always be "corrected", additionally the Canon firmware always "removes" the bad pixels from the internal fw list.
« Last Edit: 22 / August / 2009, 15:52:28 by fe50 »


Re: Bad pixels, what to do about them
« Reply #8 on: 22 / August / 2009, 18:56:49 »
When the CHDK bad pixel removal feature (the one with the badpixel file - not the badpixel.bin for DNG creation) is enabled,
the badpixels from your (CHDK) list will always be "corrected", additionally the Canon firmware always "removes" the bad pixels from the internal fw list.

Would the two lists have some overlap in them?

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Offline fe50

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Re: Bad pixels, what to do about them
« Reply #9 on: 25 / August / 2009, 17:38:24 »
When the CHDK bad pixel removal feature (the one with the badpixel file - not the badpixel.bin for DNG creation) is enabled,
the badpixels from your (CHDK) list will always be "corrected", additionally the Canon firmware always "removes" the bad pixels from the internal fw list.
Would the two lists have some overlap in them?
Which lists do you mean - "badpixel" and the Canon firmware list ? or the two Canon lists ?

 

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