Issue
I am trying to get a very simple VideoCapture going with OpenCV, where I am able to change the resolution in between pictures.
My setup:
Already on the simplest step, OpenCV is giving me warnings I do not understand
import cv2
cap = cv2.VideoCapture(0)
Results in
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (961) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-1
I searched this warning, but I only find links to an OpenCV issue that should already be resolved (see for example here).
Anyway the warning is not prohibitive, as I am able to see images from the camera, and I am able to downsize the camera rresolution from 3264x2448 to 640x480 using cap.set(cv2.CAP_PROP_FRAME_WIDTH, 640) and cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 480).
The true issue occurs when I try to change the frame size back to the original:
print(cap.get(cv2.CAP_PROP_FRAME_WIDTH), cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
# Prints: 3264.0 2448.0
org_w = cap.get(cv2.CAP_PROP_FRAME_WIDTH)
org_h = cap.get(cv2.CAP_PROP_FRAME_HEIGHT)
cap.set(cv2.CAP_PROP_FRAME_WIDTH, 640)
cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 480)
print(cap.get(cv2.CAP_PROP_FRAME_WIDTH), cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
cap.set(cv2.CAP_PROP_FRAME_WIDTH, org_w)
cap.set(cv2.CAP_PROP_FRAME_HEIGHT, org_h)
print(cap.get(cv2.CAP_PROP_FRAME_WIDTH), cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
This results in a bunch of warnings and the width and height values are set to 0x0 instead:
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (1824) handleMessage OpenCV | GStreamer warning: Embedded video playback halted; module v4l2src0 reported: Internal data stream error.
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (536) startPipeline OpenCV | GStreamer warning: unable to start pipeline
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (1085) setProperty OpenCV | GStreamer warning: no pipeline
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (992) getProperty OpenCV | GStreamer warning: GStreamer: no pipeline
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (992) getProperty OpenCV | GStreamer warning: GStreamer: no pipeline
0.0 0.0
And then cv2.imshow fails ofc.
Is this not the proper way to change camera resolution between pictures? Should I leave the camera in the original 3264x2448 and just use cv2.resize to reduce the image size after the image is taken?
Solution
Found the solution here.
Observing that my OpenCV was built with v4l2 the issue disappeared with
cap = cv2.VideoCapture(0, cv2.CAP_V4L2)
Build info can be found with
print(cv2.getBuildInformation())
Answered By - SenneVP Answer Checked By - Candace Johnson (WPSolving Volunteer)