[Elphel-support] hotpixels and auto adjustments

[flavio soares]

Hi, Andrey,

Thanks for the replies!

Flavio, there may be some bad pixels on Aptina sensors, but most likely the
> ones you see are "warm", not "hot". The difference is that "warm" have
> significantly increased dark current, and hot are always saturated. You can
> distinguish them if make a shot with shot exposure (i.e. 1ms or less) - the
> hot pixels will stay, the warm - go away.
>

I'll do that then. 1ms exposure would be a shutter of 1/1000, to I guess
I'll need quite some light, is that correct? Maybe pointing the camera to a
light source could do that. So, the warm pixels will go away by doing this?
This info is important, I'll add it to the guide, together with the
exposure values cheatsheet!


> Autoexposure and auto white balance are rather flexible and separate
> algorithms, white balance is needed only if the illumination changes (i.e.
> lamps/day light). The white balance algorithm requires something white in
> the scene (in the autoexposure/white balance WOI - it is shown as yellow
> rectangle in camera GUI). It balances colors to make the brightest area
> white, so it fails if there is nothing white or there are over-exposed
> areas in the view.
>

Mmm, interesting. I guess my problem was related to the auto WB then!


> And all the color gains and exposure are saved in the frame Exif headers,
> so it is possible to use that data in post-processing and do final color
> correction there. So it is sufficient if these automatic algorithms keep
> each color channel in reasonable range (avoiding over and under exposure).
> If you have JP4/JP46 recording you can try our ImageJ plugin that reads the
> Exif header (including MakerNote that has analog gains) and "un-applies"
> them.
>

I didn't know about this plugin. I was using movie2dng to convert the
images to DNG, but it doesn't bring much Exif information I guess. For the
WB, for example, it does give me the camera values (in ufraw) and allows me
to do manual messing about. But it doesn't bring preset values for daylight
or tungsten, for example (I'm not sure it should bring though). This info
at the link may be very useful also: "For the best performance you may
acquire images with the gamma set to 0.5 (sensor noise matched value) and
analog gains set for the white balance (automatic or manual), so pixels of
each color will use all the output range of 8 bits (compressed)."


>
> If you are really "killing" autoexposure daemon - it will never come back.
> There are better ways to control (and turn off) autoexposure - either from
> the camera GUI or using autocaqmpars.php (it is linked from the camera
> "home" web page). You may select there the parameter group and open the
> parameters to view/edit. Each parameter has a short description as a "tool
> tip" - appears when you put the mouse cursor over the parameter name.
>

I started killing the process because I tried turning it off in
autocampars, but whenever I turned to camvc, it kept readjusting the
exposure (there is a tab with a green tick on it, and it kept coming back
when I turned the camera on/off). Maybe I was trying to fiddle with the
wrong parameter. But, probably, I lacked attention and what was changing in
the image was the auto WB, I'll go back to it to see what happens!

thanks, andrey!
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