Too High CPU Footprint of OpenCV Text Overlay on FHD Video Stream - c++

Question

I want to display a FHD live-stream (25 fps) and overlay some (changing) text. For this I essentially use the code below.
Basically it is
Load frame
(cv::putText skipped here)
Display frame if it's a multiple of delay
but the code is super super slow compared to e.g. mpv and consumes way to much cpu-time (cv::useOptimized() == true).
So far delay is my inconvenient fiddle-parameter to somehow make it feasible.
delay == 1 results in 180 % CPU usage (full frame-rate)
delay == 5 results in 80 % CPU usage
But delay == 5 or 5 fps is really sluggish and actually still too much cpu load.
How can I make this code faster or otherwise better or otherwise solve the task (I'm not bound to opencv)?
P.s. Without cv::imshow the CPU usage is less than 30 %, regardless of delay.
#include <opencv2/opencv.hpp>
#include <X11/Xlib.h>
// process ever delayth frame
#define delay 5
Display* disp = XOpenDisplay(NULL);
Screen* scrn = DefaultScreenOfDisplay(disp);
int screen_height = scrn->height;
int screen_width = scrn->width;
int main(int argc, char** argv){
cv::VideoCapture cap("rtsp://url");
cv::Mat frame;
if (cap.isOpened())
cap.read(frame);
cv::namedWindow( "PREVIEW", cv::WINDOW_NORMAL );
cv::resizeWindow( "PREVIEW", screen_width, screen_height );
int framecounter = 0;
while (true){
if (cap.isOpened()){
cap.read(frame);
framecounter += 1;
// Display only delay'th frame
if (framecounter % delay == 0){
/*
* cv::putText
*/
framecounter = 0;
cv::imshow("PREVIEW", frame);
}
}
cv::waitKey(1);
}
}

Answer 1

I now found out about valgrind (repository) and gprof2dot (pip3 install --user gprof2dot):
valgrind --tool=callgrind /path/to/my/binary # Produced file callgrind.out.157532
gprof2dot --format=callgrind --output=out.dot callgrind.out.157532
dot -Tpdf out.dot -o graph.pdf
That produced a wonderful graph saying that over 60 % evaporates on cvResize.
And indeed, when I comment out cv::resizeWindow, the cpu usage lowers from 180 % to ~ 60 %.
Since the screen has a resolution of 1920 x 1200 and the stream 1920 x 1080, it essentially did nothing but burning CPU cycles.
So far, this is still fragile. As soon as I switch it to full-screen mode and back, the cpu load goes back to 180 %.
To fix this, it turned out that I can either disable resizing completely with cv::WINDOW_AUTOSIZE ...
cv::namedWindow( "PREVIEW", cv::WINDOW_AUTOSIZE );
... or -- as Micka suggested -- on OpenCV versions compiled with OpenGL support (-DWITH_OPENGL=ON, my Debian repository version was not), use ...
cv::namedWindow( "PREVIEW", cv::WINDOW_OPENGL );
... to offload the rendering to the GPU, what turns out to be even faster together with resizing (55 % CPU compared to 65 % for me).
It just does not seem to work together with cv::WINDOW_KEEPRATIO.*
Furthermore, it turns out that cv:UMat can be used as a drop-in replacement for cv:Mat which additionally boosts the performance (as seen by ps -e -o pcpu,args):
Appendix
[*] So we have to manually scale it and take care of the aspect ratio.
float screen_aspratio = (float) screen_width / screen_height;
float image_aspratio = (float) image_width / image_height;
if ( image_aspratio >= screen_aspratio ) { // width limited, center window vertically
cv::resizeWindow("PREVIEW", screen_width, screen_width / image_aspratio );
cv::moveWindow( "PREVIEW", 0, (screen_height - image_height) / 2 );
}
else { // height limited, center window horizontally
cv::resizeWindow("PREVIEW", screen_height * image_aspratio, screen_height );
cv::moveWindow( "PREVIEW", (screen_width - image_width) / 2, 0 );
}

Answer 2

One thing that pops is you're creating a new window and resizing it every time you want to display something.
move these lines
cv::namedWindow( "PREVIEW", cv::WINDOW_NORMAL );
cv::resizeWindow( "PREVIEW", screen_width, screen_height );
to before your while(true) and see it that solves this