[Elphel-support] auto exposure on multi sensor setup & binning question.

[Elphel Support]

> 
 > Hi,
 > Currently we are testing a panoramic NC393 camera and are having problems with the auto exposure on multiple image sensors. 
 > 
 > When there is half shadow, half bright light in the collective images. The image is dark on the shadow part. Does autoexposure apply globally or should every sensor apply it's own autoexposure settings?

Jorick, with the limited dynamic range of the small format sensors it is a challenge to capture maximal data from the image, and there is no universal fits-all set of autoexposure parameters. We probably need to create some tutorial about it.

The overall strategy is to keep maximal data from each channel - EXIF data in each image contains acquisition settings so it is possible  to match individual channels after acquisition - that just may require more than 8-bit of the intensity for the intermediate data.

Each channel operates it autoexposure independently, so the output may not match in raw form - it needs post-processing. For example channels can be  combined in a single 16-bit per color panorama and then high-pass filter (with low cutoff frequency ~ 1/2000 pix) applied to reduce difference between bright and dark parts of the panorama.


 > What would be a way to get a more balanced exposure?

There are multiple parameters that control autoexposure, the main are window, level and fraction. The daemons (1 per each of the 4 channels) builds histograms for all pixels inside the selected rectangular area, and then calculates required exposure so the specified fraction of all pixels have values below the specified level.

Default settings are for images where there are no very bright objects (like the Sun) in the field of view, so if the camera is pointed there the picture will become all dark. If you change the fraction to say 95%, then up to 5% of the pixels are allowed to be above the level - that level does not have to be very high, so exceeding it do not mean necessarily overexposure. Camvc program allows you to adjust this pair (fraction/level) graphically. You can turn off autoexposure, set it manually to the desired level and then move slider for the fraction - level value will be adjusted to match it.

 > Second question, is binning supported and do we have to do any aditional steps for this? We'd like to reduce the resolution while still using the whole sensor. When I set it to 1/2 Horizontally and 1/2 Vertically, the output gets corrupted.

Binning is supported, it should all be the same as in the 353 camera. Can you please describe precisely how you've got corrupted images, we will try to reproduce (and address) the problem.

On the other hand, binning and decimation are not r3eally good in any of the color mosaic sensors as these modes reduce resolution more than twice. Because of the Bayer mosaic (1 row):
R1 G1 R2 G2 R3 G3 R4 G4

R1 will be merged with R2, R3 - with R4 and similar G1 with G2:
R1+R2, G1+G2, R3+R4, G3+G4, ...
> 
 > 
 > 
 > Also we have a problem in the Camogm recording application, we use fast recording (without partition).
 > When I press record, I get a warning about a buffer underrun. If click away the message, recording starts and the buffers stay full.
 > When I forget to OK the warning, the elphel becomes unresponsive on the http interface after a while and it looks if the recording task hangs. 
 > 
 > A reboot doesn't work as it never reboots, we had to power off the camera to get it going again. After reboot the recording is lost from what I see.

What firmware version do you have ? There were many changes in camogm earlier this year.

Andrey





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