Setting Kazam to correctly record full-screen on HiDPI displays in Ubuntu

In this new covid-19 world it happens more and more often that I need to record full-screen videos, for example for lectures. This is something that one can do live with Zoom, but that is not the most practical option for non-live recordings.

In Ubuntu there is a nice software, Kazam screencaster, that is perfect for the job, except that it does not get correctly the screen size if you have a high pixel density display (HiDPI): you end up with a video with only the top-left corner of the screen cropped.

There is a simple patch to fix that issue, which I describe here in case it’s useful to someone else and for the benefit of my future self.

First, you need to find the files gstreamer.py and prefs.py in the Kazam installation. For me they were in /usr/lib/python3/dist-packages/kazam/backend/.

Next, you have to fix these such that they take into account the screen scaling factor, which is obtained from the get_monitor_scale_factor function in the Gtk library

This cane be done by adding these lines to the file gstreamer.py.

 scale = self.video_source['scale']
 startx = startx * scale 
 starty = starty * scale 
 endx = endx * scale 
 endy = endy * scale 

They should be added around lines 120 or so, right after the properties endx and endy are set up (endy = starty + height - 1).

Next, open the file prefs.py and, around line 324, change this bit

for i in range(self.default_screen.get_n_monitors()):
   rect = self.default_screen.get_monitor_geometry(i)
   self.logger.debug("  Monitor {0} - X: {1}, Y: {2}, W: {3}, H: {4}".format(i,
                                      rect.x,
                                      rect.y,
                                      rect.width,
                                      rect.height))
                                      rect.height))

   self.screens.append({"x": rect.x,
                        "y": rect.y,
                        "width": rect.width,
                        "height": rect.height})

into this

for i in range(self.default_screen.get_n_monitors()):
     rect = self.default_screen.get_monitor_geometry(i)
     scale = self.default_screen.get_monitor_scale_factor(i)

     self.logger.debug("  Monitor {0} - X: {1}, Y: {2}, W: {3}, H: {4}, scale: {5}".format(i,
                                        rect.x,
                                        rect.y,
                                        rect.width,
                                        rect.height,
                                        scale))

    self.screens.append({"x": rect.x,
                         "y": rect.y,
                         "width": rect.width,
                         "height": rect.height,
                         "scale": scale}) 

That’s it! Restart Kazam and the next fullscreen recording should work OK.

Thanks to user sllorente for describing the patch here!