nandor65 wrote:I use default values of the v4l2 driver. so scene_mode=0. exposure_auto_priority? I do not know about this control.
I set width and height and fps from OpenCV like this:
where width is 1280 pixel and height is 720 pixel, so 2x2 binning is applied and max fps is 49.
I capture streams of outside vehicle traffic and try to recognise licence plates. When its daytime images of digits on the plates are sharp, when it gets darker digits get blurred. I assume because auto exposure algorithm reduces the fps. I do not mind if entire image gets darker, unless plates are human readable.
OK, so frame rate is set to be camera_fps (do check your OpenCV version number. Early versions didn't actually set this). Without setting exposure_auto_priority, AE will never change this frame rate - you will get a frame every 1/fps seconds. AE can request an exposure time that is < 1/fps seconds, but never exceed it. The exposure mode (night, sport, etc) varies other parameters such as whether to increase gain or exposure time first, and what maximum value to adopt for each.
As it gets dark then yes it will increase the exposure time (up to the defined limits) to achieve a better scene.
Motion blur on licence plate reading is a real problem. From a previous job I seem to recall them having a special 1/1000sec (1ms) exposure time, even though this was on PAL CCTV cameras where they were inherently 50Hz (and interlaced).
Your best bet is IR illumination and the Pi NoIR. All number plates in the UK (except the original white on black ones) are required to be IR reflective, therefore adding IR illumination improves readability of the plates without blinding the driver. You are highly unlikely to be able to get a readable plate from a moving vehicle at night without external illumination.
The sport exposure mode will crank up gain in preference to exposure time, but it is unlikely to give enough to make number plates readable.
For reference: V4L2_CID_EXPOSURE_AUTO_PRIORITY as documented at http://hverkuil.home.xs4all.nl/spec/uap ... trols.html
When V4L2_CID_EXPOSURE_AUTO is set to AUTO or APERTURE_PRIORITY, this control determines if the device may dynamically vary the frame rate. By default this feature is disabled (0) and the frame rate must remain constant.
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