Archive for 27 febrero 2012

“Dead” pixels in my IXUS digital camera (And a fix using CHDK !!)

Lunes, 27/Feb/2012 Deja un comentario

Some days ago, I made a nice trip to Thailand, and, as always, I take my IXUS 80 IS with me, to enjoy the typical photo sessions I usually do: some long exposure ones, some random shoots, etc… CHDK is installed in my SD card, so all the bunch of extra features it offers, was there for me to enjoy also.

Anyway, when coming back home, I started downloading and processing all the pics I took. Then suddently something caught my eye: Some dark (and mainly slow) photographs showed pure red and or blue pixels where they should show black colours. I checked it against 4-5 pics and then I could confirm it: These misbehaving pixels were exactly the same ones across those pics, as they appeared in the very same location on every photograph.
One extra thing that confused me a bit more, was that on some long exposure pics I also took, those rogue pixels WERE NOT THERE!!

I thought, “oh no, my wonderful IXUS’ CCD is broken, and in a very funny way!!”. As always, when something like this happens, I just google some info to see if I could do something about it. In yesterday’s googling I’ve learned some things:

1.- These pixels that show up randomly coloured, are not really broken. They’re “over sensitive” (Oh boy!)
2.- This oversensitive behaviour shows up in dark areas, and mainly doing “slow shots”, (1/3 s. etc…)
3.- All cameras aparently have this kind of pixels (there’s no warranty of a 100% funny pixel free camera).
4.- There’s a nice fix for it!! CHDK itself…

As I said before, when doing long exposure shots, these funny pixels didn’t show up. Then I started thinking, “what’s the difference between a “slow dark shoot” (night shoot against people in a club, for example) and a ULTRA SLOW shoot (+3 minutes) using a tripod against the night sky ? ”

Then some pieces of the puzzle gently fitted on my mind:
-When doing long exposure shots, I could remember the tedious “Wait, processing…” banner appearing after each exposure. That means 1 min exposure means 1 min black frame noise reduction calculation time. 5 mins exposure, 5 mins noise reduction against a black frame. And so on.
Apparently, “those funny pixels” are included in the noise reduction calculation, and thus, when substracting them from the real shot, they just disappear from the final .jpg file, as expected, and as I confirmed. Yay!!

So, when I take “normal slow shoots” at night (not long exposured ones), the “noise reduction process” is not peformed at the end of every shoot. Shouldn’t it be nice if I could force it somehow? And then, reading some CHDK forums here and there, someone hit the nail claiming a strange solution buried in the “RAW Options” submenu. Read on:

Apparently, if you enter onto that submenu (RAW Options), there’s one option there named “Black frame noise reduction” (Or similar name, mine is in Spanish) which is set to AUTO by default. Apparently, AUTO means “Noise reduction post processing will be used in long exposure shoots only”, and nowhere else.
Someone could say, OK, but why is this option “hidden” in the “RAW Options” submenu? I have no idea at all, because setting it to ON forces “Black frame noise reduction” post processing on EVERY shot, not only RAW ones, and not only “long exposure ones”.
That means, even I’m NOT enabling RAW file creation when taking pics (just HQ jpg files are OK for me right now), and even though CHDK reads and uses “RAW Option” submenu settings (even I told it not to store RAW pics on my SD card), I get bad pixel free pics (!!), even when saving them as JPG files!! And all because of the “ON” setting instead of “AUTO”.

Is there some known caveat? Well, “yes”: Now every pic gets “Black Frame Noise Calculation” times after every shoot. But there’s something nice about it: 1/3 s. pic gets 1/3 s. processing time, which is completely affordable in terms of patience waiting for the camera to be ready for the next shoot.

Now EVERY (dark) photo is JUST BLISS, with no random coloured piexels around, (and I would even say generic noise amount is lowered just a bit, I hope it’s not some form of CHDK Placebo 🙂 …

As always, CHDK: Giving proper solutions to everyday (1st world) problems!!


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