I am using Python 3.6 on Ubuntu 18.04 on an NVIDIA Jetson TX2. All software is up to date. The Linux kernel is 4.9.140-tegra. GStreamer is version 1.14.1.
I have a UVC Capable webcam that shows the following capabilities and I need to stream with the Y16 ( GRAY16_LE ) format:
root@nvidia:/tmp# v4l2-ctl --list-formats-ext -d /dev/video1
ioctl: VIDIOC_ENUM_FMT
Index : 0
Type : Video Capture
Pixel Format: 'UYVY'
Name : UYVY 4:2:2
Size: Discrete 160x120
Interval: Discrete 0.111s (9.000 fps)
Index : 1
Type : Video Capture
Pixel Format: 'Y16 '
Name : 16-bit Greyscale
Size: Discrete 160x120
Interval: Discrete 0.111s (9.000 fps)
Size: Discrete 160x122
Interval: Discrete 0.111s (9.000 fps)
Index : 2
Type : Video Capture
Pixel Format: 'GREY'
Name : 8-bit Greyscale
Size: Discrete 160x120
Interval: Discrete 0.111s (9.000 fps)
Index : 3
Type : Video Capture
Pixel Format: 'RGBP'
Name : 16-bit RGB 5-6-5
Size: Discrete 160x120
Interval: Discrete 0.111s (9.000 fps)
Index : 4
Type : Video Capture
Pixel Format: 'BGR3'
Name : 24-bit BGR 8-8-8
Size: Discrete 160x120
Interval: Discrete 0.111s (9.000 fps)
With GStreamer from the command line, I can successfully get the device into the PLAYING state with gst-launch in the format I want:
root@nvidia:/tmp# gst-launch-1.0 v4l2src device=/dev/video1 ! video/x-raw,width=160,height=122,format='GRAY16_LE',framerate=9/1 ! fakesink
Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
I am trying to open the device with the GRAY16_LE format with Python OpenCV:
import cv2
cap = cv2.VideoCapture("v4l2src device=/dev/video1 ! video/x-raw, format=(string)GRAY16_LE, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1 ! appsink")
ret, frame = cap.read()
During elevated GStreamer debug, the capabilities are probed and printed below:
0:00:00.260383826 15768 0x49a9850 INFO v4l2 gstv4l2object.c:4136:gst_v4l2_object_probe_caps:<v4l2src0:src> probed caps: video/x-raw, format=(string)UYVY, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)BGR, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)RGB16, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)GRAY8, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)GRAY16_LE, width=(int)160, height=(int)122, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)GRAY16_LE, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1
With GStreamer debug on, the errors are shown as below:
0:00:00.260385266 15768 0x4744c00 INFO GST_STATES gstelement.c:2579:_priv_gst_element_state_changed:<v4l2src0> notifying about state-changed PAUSED to PLAYING (VOID_PENDING pending)
0:00:00.260529393 15768 0x49a9850 WARN basesrc gstbasesrc.c:3055:gst_base_src_loop:<v4l2src0> error: Internal data stream error.
0:00:00.260531697 15768 0x4744c00 INFO GST_STATES gstbin.c:2952:gst_bin_change_state_func:<pipeline0> child 'v4l2src0' changed state to 4(PLAYING) successfully
0:00:00.260589009 15768 0x49a9850 WARN basesrc gstbasesrc.c:3055:gst_base_src_loop:<v4l2src0> error: streaming stopped, reason not-negotiated (-4)
0:00:00.260660881 15768 0x4744c00 INFO GST_STATES gstbin.c:2087:gst_bin_get_state_func:<pipeline0> getting state
0:00:00.260748913 15768 0x49a9850 INFO GST_ERROR_SYSTEM gstelement.c:2145:gst_element_message_full_with_details:<v4l2src0> posting message: Internal data stream error.
0:00:00.260762833 15768 0x4744c00 INFO GST_STATES gstelement.c:2392:gst_element_get_state_func:<pipeline0> waiting for element to commit state
0:00:00.260909520 15768 0x49a9850 INFO GST_ERROR_SYSTEM gstelement.c:2172:gst_element_message_full_with_details:<v4l2src0> posted error message: Internal data stream error.
0:00:00.261064816 15768 0x4744c00 INFO GST_STATES gstbin.c:2504:gst_bin_element_set_state:<appsink0> current PAUSED pending VOID_PENDING, desired next READY
0:00:00.261189072 15768 0x49a9850 INFO task gsttask.c:316:gst_task_func:<v4l2src0:src> Task going to paused
I am using the identical format that the probe returns but the error indicates "format negotiation error". How can I fix this so my frames appear in Python?
I had to patch the cap_gstreamer.cpp file in the OpenCV 4 sources and then build and install OpenCV again.
At that point, I could capture the frames with appsink in the GRAY16_LE format.