andykitchen / swcursor

Large software cursor for screen recording on X11

swcursor

This is a simple software cursor for X11 environments, it creates an overlay window with an (large) image that follows the cursor around. You will need to arrange for your normal cursor to be hidden, or screen record using ffmpeg's kmsgrab which often does not capture harware cursors at all.

This is largely for screen recording where you would like a very large cursor that visually highlights when you click, Also when screen recording directly from the KMS scanout buffer using kmsgrab with ffmpeg, hardware cursors won't show up, so running this program you get a nice large cursor.

Installation and Usage

You will need to have working development libraries for gtk-3.0, gdk and xlib.

$ make
$ ./swcursor

# You can display any transparent PNG image you like for the cursor
$ ./swcursor grumpy-cat.png

Implementation

It's suprisingly difficult to find documentation on the interaction between GTK, GDK and X11 and there are some pitfalls, this is a reasonably minimal example of creating a GTK window and then manipulating it with lower level commands from GDK and Xlib. The non-trivial parts are:

• making the window transparent
• removing all decoration, and
• hiding the window from the window manager;
• using the X11 shape extension to make the window click-through.

The other trick is integrating with the GTK/GDK lifecycle. Here is the order I found with directed trial and error:

• window_init
  • gtk_widget_set_app_paintable
  • gdk_screen_get_rgba_visual / gtk_widget_set_visual
• window_realize
  • gtk_window_set_decorated / gdk_window_set_decorations etc.
  • gdk_window_set_override_redirect
• window_map
  • XShapeCombineRegion(..., ShapeInput, ...)

In hindsight, it may have been easier to use Xlib directly. But I also wanted to learn more about GTK and GDK when I started this project.

Screen Recording with FFMPEG

Below is the command I use to record my screen with ffmpeg when using swcursor. It should work on a recent Intel chipset running an up-to-date Linux kernel, with VAAPI installed. Read more about ffmpeg and VAAPI here.

#!/bin/sh

ffmpeg \
        -v verbose \
        -thread_queue_size 128 \
        -f pulse -i default \
        -device /dev/dri/card0 \
        -crtc_id 0 -plane_id 0 -framerate 30 \
        -f kmsgrab -i - \
        -af "asetpts=N/SR/TB" \
        -vf "hwmap=derive_device=vaapi,scale_vaapi=w=1920:h=1080:format=nv12" \
        -c:a aac \
        -c:v h264_vaapi -b:v 3200k \
        -- out.mp4

This setup is great because it reads the whole screen from video memory and compresses it directly in hardware, I can record my 4K desktop at 30fps with very few dropped frames and 20% CPU usage on one core.

You will need to set the admin capability on ffmpeg (or use sudo):

$ sudo setcap cap_sys_admin+ep /usr/bin/ffmpeg

If you want to do live steaming or have an NVIDIA Graphics Card, you might want to check out this gist and the ffmpeg wiki page on hardware acceleration.