Add parameter to skip frames

[cosama]

We are having a bunch of cameras that we typically run at lower frame rates (5-10Hz). After looking into this for quite some time, I realized that those camera's do not support these frame rates and that the timestamp is often off considerably, so the kernel must write weird information in the data buffer. I don't know exactly what is happening, but the effect disappears when running at a supported frame rate (30Hz in our case). So I added an optional parameter (stride) to the driver that can be used to only read every Nth frame. It reduces the load of the driver considerably (100%CPU at 30HZ vs 40%CPU at 10Hz). Would be cool if it can be merged, might be useful to others too.

I tested the driver and it works as expected.

[cosama]

Here some more output running the driver at 30Hz and a stride of 3:

usb_camera     | [WARN] [1748457339.410597293]: NOTE: the parameters generated for V4L intrinsic camera controls will be placed under namespace 'intrinsic_controls'
usb_camera     | [INFO] [1748457339.331578693]: Initializing ROS V4L USB camera 'camera' (/dev/video0) at 1280x720 via mmap (uyvy) at 30 FPS and 3 framestride
usb_camera     | [INFO] [1748457339.335379644]: camera calibration URL: file:///workspace/config/camera_calibration/ost.yaml
usb_camera     | [INFO] [1748457339.335940934]: Advertising std_srvs::Empty start service under name 'start_capture'
usb_camera     | [INFO] [1748457339.336392678]: Advertising std_srvs::Empty suspension service under name 'stop_capture'
usb_camera     | [INFO] [1748457339.336725873]: Advertising std_srvs::Trigger supported formats information service under name 'supported_formats'
usb_camera     | [INFO] [1748457339.337139135]: Advertising std_srvs::Trigger supported V4L controls information service under name 'supported_controls'
usb_camera     | Opening streaming device /dev/video0
usb_camera     | Video4Linux: IOCTL is not supported
usb_camera     | Video4linux: Querying V4L2 driver for available controls (register base 0x980900, 0..99)
usb_camera     | Sorting control names:
usb_camera     | 	exposure_auto
usb_camera     | 	white_balance_temperature_auto
usb_camera     | 	brightness
usb_camera     | 	contrast
usb_camera     | 	saturation
usb_camera     | 	gamma
usb_camera     | 	gain
usb_camera     | 	power_line_frequency
usb_camera     | 	white_balance_temperature
usb_camera     | 	sharpness
usb_camera     | 	exposure_absolute
usb_camera     | [INFO] [1748457339.410730837]: Use 'intrinsic_controls/ignore' list to enumerate the controls provoking errors or the ones you just want to keep untouched
usb_camera     | [INFO] [1748457339.410765712]: Attempting to generate ROS parameter for V4L2 control 'exposure_auto':
usb_camera     | 	Exposure, Auto, min = 0, max = 3, step = 1, flags = 0x0 [ 0: Auto Mode 1: Manual Mode ] [0]
usb_camera     | [INFO] [1748457339.411878690]: Parameter intrinsic_controls/exposure_auto exposed with value 0
usb_camera     | [INFO] [1748457339.411930374]: Attempting to generate ROS parameter for V4L2 control 'white_balance_temperature_auto':
usb_camera     | 	White Balance Temperature, Auto, min = 0, max = 1, step = 1, flags = 0x0 [1] (bool 1/0)
usb_camera     | [INFO] [1748457339.412993571]: Parameter intrinsic_controls/white_balance_temperature_auto exposed with value 1
usb_camera     | [INFO] [1748457339.413077543]: Attempting to generate ROS parameter for V4L2 control 'brightness':
usb_camera     | 	Brightness, min = -15, max = 15, step = 1, flags = 0x0 [0]
usb_camera     | [INFO] [1748457339.413850186]: Parameter intrinsic_controls/brightness exposed with value 0
usb_camera     | [INFO] [1748457339.413934538]: Attempting to generate ROS parameter for V4L2 control 'contrast':
usb_camera     | 	Contrast, min = 0, max = 30, step = 1, flags = 0x0 [10]
usb_camera     | [INFO] [1748457339.414654362]: Parameter intrinsic_controls/contrast exposed with value 10
usb_camera     | [INFO] [1748457339.414737896]: Attempting to generate ROS parameter for V4L2 control 'saturation':
usb_camera     | 	Saturation, min = 0, max = 60, step = 1, flags = 0x0 [16]
usb_camera     | [INFO] [1748457339.415433016]: Parameter intrinsic_controls/saturation exposed with value 16
usb_camera     | [INFO] [1748457339.415515211]: Attempting to generate ROS parameter for V4L2 control 'gamma':
usb_camera     | 	Gamma, min = 40, max = 500, step = 1, flags = 0x0 [220]
usb_camera     | [INFO] [1748457339.416182891]: Parameter intrinsic_controls/gamma exposed with value 220
usb_camera     | [INFO] [1748457339.416259994]: Attempting to generate ROS parameter for V4L2 control 'gain':
usb_camera     | 	Gain, min = 1, max = 100, step = 1, flags = 0x0 [1]
usb_camera     | [INFO] [1748457339.416924698]: Parameter intrinsic_controls/gain exposed with value 1
usb_camera     | [INFO] [1748457339.417001491]: Attempting to generate ROS parameter for V4L2 control 'power_line_frequency':
usb_camera     | 	Power Line Frequency, min = 0, max = 2, step = 1, flags = 0x0 [ 0: Disabled 1: 50 Hz 2: 60 Hz ] [0]
usb_camera     | [INFO] [1748457339.417628956]: Parameter intrinsic_controls/power_line_frequency exposed with value 0
usb_camera     | [INFO] [1748457339.417706149]: Attempting to generate ROS parameter for V4L2 control 'white_balance_temperature':
usb_camera     | 	White Balance Temperature, min = 1000, max = 10000, step = 50, flags = 0x10 [4500]
usb_camera     | [INFO] [1748457339.418275380]: Parameter intrinsic_controls/white_balance_temperature exposed with value 4500
usb_camera     | [INFO] [1748457339.418326150]: Attempting to generate ROS parameter for V4L2 control 'sharpness':
usb_camera     | 	Sharpness, min = 0, max = 127, step = 1, flags = 0x0 [16]

and checking the frame rate:

# rostopic list
/camera/camera_info
/camera/image_raw
/rosout
/rosout_agg
root@slamp:/workspace# rostopic hz /camera/image_raw
subscribed to [/camera/image_raw]
average rate: 9.937
	min: 0.098s max: 0.106s std dev: 0.00234s window: 10
average rate: 9.970
	min: 0.095s max: 0.106s std dev: 0.00239s window: 20
average rate: 9.984
	min: 0.095s max: 0.106s std dev: 0.00197s window: 30
average rate: 9.985
	min: 0.087s max: 0.114s std dev: 0.00354s window: 40
average rate: 9.991
	min: 0.087s max: 0.114s std dev: 0.00320s window: 50
average rate: 9.993
	min: 0.087s max: 0.114s std dev: 0.00303s window: 60
average rate: 9.994
	min: 0.087s max: 0.114s std dev: 0.00282s window: 70
average rate: 9.994
	min: 0.087s max: 0.114s std dev: 0.00326s window: 80
average rate: 9.995
	min: 0.087s max: 0.114s std dev: 0.00308s window: 90
average rate: 9.995
	min: 0.087s max: 0.114s std dev: 0.00301s window: 100
average rate: 9.996