Does "bad pixel removal" function have any sense? - General Help and Assistance on using CHDK stable releases - CHDK Forum
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Does "bad pixel removal" function have any sense?

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Offline harvald

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Does "bad pixel removal" function have any sense?
« on: 09 / August / 2008, 17:34:38 »
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I read everything i could find about this function. I don't understand one thing. How it can use list with correction values for bad (hot) pixels if in different temperature CCD matrix generate different noise? With that i understood the higher is temperature, the more noise it produces.

If i understand right then this function only really works when temperatures of CCD when making real photo and black one are the same. Dark frame subtraction make this two shots one after another, and temperature is about the same, so it works just fine, but it doubles time of making photo. Bad pixel removal is faster, but almost never work good (if i understand this right ;P)
« Last Edit: 09 / August / 2008, 17:46:53 by harvald »
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Offline wontolla

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Re: Does "bad pixel removal" function have any sense?
« Reply #1 on: 09 / August / 2008, 18:47:51 »
Sometimes, sensors have one or several pixels that just don't work (hot or dead pixels: when a pixel fails to sense light levels correctly). CCD temperature is not involved with this anomaly.

The "bad pixel removal" helps to repair this problem. Similar to sending the camera to Canon to remap bad pixels. But it has nothing to do with noise.

If you have bad pixels (white or red dots on the JPG better noticed when zooming) use this feature, otherwise is not necessary.


Disclaimer: The above information is based on surfing the internet, I have no real experience since I never had bad pixels (or I'm too blind to spot them).
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Offline harvald

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Re: Does "bad pixel removal" function have any sense?
« Reply #2 on: 09 / August / 2008, 18:53:55 »
If this is correct then "dark frame subtraction" do this as well but it takes longer.
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Offline dzsemx

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Re: Does "bad pixel removal" function have any sense?
« Reply #3 on: 10 / August / 2008, 03:12:57 »
yes, darkframe substraction can help and it's very effective.... it removes static pattern noise too
« Last Edit: 10 / August / 2008, 03:19:12 by dzsemx »
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Offline LjL

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Re: Does "bad pixel removal" function have any sense?
« Reply #4 on: 12 / August / 2008, 09:50:52 »
Dark frame subtraction is, however, not performed automatically on short exposures (while on the other hand, hot pixels are still there); CHDK can be used to force it, but I'm not sure about the implications of that, since the Canon routines likely weren't intended to be used for short exposures, and reading these very forums, people's opinion seemed to differ on the effectiveness of Canon's dark frame subtraction.
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