Auto brightness/contrast behaves differently with grab and snap

[LeahE1]

Hi all,

I've been having some problems with the auto brightness/ contrast on my FULL HD 1080P Webcam.

Basically, when I first plug it in and take snaps in MAX, the image has too much fill light. I can take loads of snaps and it won't change . But as soon as I 'grab', it adjusts. Then all of the snaps from then on are fine.

Some screenshots the problem: 

Snap                                                                                           After grab                                

Here are my settings in MAX:

:

The problem also occurs when I snap using LabVIEW IMAQ. I am developing an application that only uses snap, and don't want to have to 'grab' beforehand to get it to adjust the fill light. I guess it would be possible to set the levels manually but I would much prefer it to auto adjust like it does using grab. I'm a bit new to working with cameras so don't really know if the issue is to do with exposure or backlight compensation, or whether this is expected behaviour. Any advice would be much appreciated!

[BruceAmmons · Accepted solution]

This would be expected behavior.  The camera resets to its default values at the start of each acquisition, and it requires several images to auto adjust the brightness, etc.

Your best option would probably be to run a Grab in the background, and just pull the current image whenever you need one.  This is also usually faster than individual Snaps, since there is no camera setup time required.

Bruce

[LeahE1]

Ahh, that makes sense. I suspected it might be something like that but it surprised me that the images weren't changing even after 10 snaps.
I'll give the grab idea a try - thank you Bruce!