Added auto exposure support

[chrisrca]

When using some webcams the exposure setting does not correctly adjust the camera's exposure, leaving the surroundings much darker than they should be. This can be seen here as the exposure is set to -7 by default the camera shows a completely black screen.

Even after adjusting this value the room is still barely visible and not acceptable for mocap.

By allowing string inputs on the exposure field we can set the camera's exposure to automatic when "auto" is entered, switching to automatic exposure and instantly brightening the room up to an acceptable level. This field still accepts integer entries for those who want to use it.

Video of change:

[jonmatthis]

Hello! Thank you for this!

We've desperately needed auto exposure for a while, but I could never quite figure it out!

I saw that you used 0.75/0.25 to switch from auto to manual exposure (that was the trick I didn't know!), do you know if that is camera/platform/backend specific? Are you running your on Windows, Mac, or Linux?

This thread on the opencv repo discusses these settings, and I see some folks using 1 and 3 to set their exposure :

ihttps://github.com/opencv/opencv/issues/9738

Are you aware of actual documentation about CAP_PROP_AUTO_EXPOSURE setting?

In any case, thanks again for this! I will poke around with this a bit and see what I can figure out from my end.

[chrisrca]

Hi! Sorry for the delayed response.

I've only tested this on Windows although it might work on Linux and Mac as is. (I don't really know anything about about its usage on those platforms, I've just seen that some camera drivers may be needed for it which sounds like a lot of extra and possibly hardware specific stuff). Just needed it for one of my cameras and it seemed like a simple addition. I believe it should just work for any non-virtual windows camera but that would also need to be tested (it has worked for various different usb cameras that I have tried though).