[regression] No fonts in 4.13 / No visible text when using HighGUI / Qt / trackbar on Linux

[ladaapp]

Hi,

the latest 4.13 Linux wheels release do not contain dejavu fonts as previous version 4.12.

When using cv2.createTrackbar you will see no text at all, just an empty space where the text should be shown. You could follow the tutorial for example to reproduce https://docs.opencv.org/4.13.0/da/d6a/tutorial_trackbar.html

user@ubuntu-vm:~$ 7z l opencv_python-4.12.0.88-cp37-abi3-manylinux2014_x86_64.manylinux_2_17_x86_64.whl | grep ttf
2025-07-04 16:51:08 .....       672300       333386  cv2/qt/fonts/DejaVuSans-Bold.ttf
2025-07-04 16:51:08 .....       611212       311752  cv2/qt/fonts/DejaVuSans-BoldOblique.ttf
2025-07-04 16:51:08 .....       345204       164352  cv2/qt/fonts/DejaVuSans-ExtraLight.ttf
2025-07-04 16:51:08 .....       611556       311964  cv2/qt/fonts/DejaVuSans-Oblique.ttf
2025-07-04 16:51:08 .....       720012       361229  cv2/qt/fonts/DejaVuSans.ttf
2025-07-04 16:51:08 .....       631992       310825  cv2/qt/fonts/DejaVuSansCondensed-Bold.ttf
2025-07-04 16:51:08 .....       580168       298572  cv2/qt/fonts/DejaVuSansCondensed-BoldOblique.ttf
2025-07-04 16:51:08 .....       576004       295438  cv2/qt/fonts/DejaVuSansCondensed-Oblique.ttf
2025-07-04 16:51:08 .....       643852       314527  cv2/qt/fonts/DejaVuSansCondensed.ttf
user@ubuntu-vm:~$ 7z l opencv_python-4.13.0.92-cp37-abi3-manylinux_2_28_x86_64.whl | grep ttf
user@ubuntu-vm:~$ 

The code for this is still there

opencv-python/setup.py

Line 243 in 4ddfc01

# add fonts for Qt5

so maybe dejavu font is not installed and available anymore in the base image used in CI to run this job? I couldn't figure out for sure what image is used to build these wheels to confirm this.
Edit: the fonts are installed in the docker image and available in the location where setup.py expects it. To me it looks like the line after that is the problem as it expects to find the fonts in Qt cmake install dir but this version of Qt doesn't seem to install fonts anymore so I think this doesn't do anything. Instead we should probably just copy the .ttf from the system installation. For backwards compatibility with older Qt versions where apparently fonts where installed via cmake it could be done only if no fonts are found in qt cmake dir.

If I copy the .ttf files from previous installation of 4.12 to venv using 4.13 it works as expected.

Thanks!