How to modify the frame rate of the v4l2 camera

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Thanks for your work,I use the ros node for video reading, video_source.launch.I saw the place to modify the csi camera in line 360 in gstCamera.cpp, but I don’t know how to modify the frame rate of the v4l2 camera (I use a usb camera), the default is the highest frame rate, in fact I only need about 10 frames to read , Please tell me how to modify the frame rate of the v4l2 camera. Thanks for all helpful suggestions.