[Elphel-support] hotpixels and auto adjustments

[Andrey Filippov]

On Fri, Jul 6, 2012 at 4:04 AM, flavio soares <qazav3.0 at gmail.com> wrote:

> So, two questions:
>
> - After editing was done, I went to the RAWs to check on colour. This
> is the first time I noticed there are two hotpixels in the sensor, one
> red and the other is bluish. Is there a way of fixing it?
>

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 ask
> because this has happened in my Pentax and I remember there was a
> function somewhere and it solved for it. Does elphel has something
> like this?
>
> We did plan to implement one of such algorithms, but then decided against
it. We try to preserve as much raw sensor data as possible, leaving the
rest to the post-processing.



> - The other is that, for the same scene, the external light coming
> from a window changed between three shots. I tried but was unable to
> get the colors equal between them, and I thought it would be better if
> I kill the automated process when recording, so that I have this part
> completelly manual.


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. 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 am killing "autoexposure" when I start camogm
> via command line. I wonder if it is coming back (being called by
> another process from time to time) or if it is something else I'm
> missing. Any guesses will be very welcome.
>

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.


Andrey
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