ffmpeg: record x11 window

2 files changed, 27 insertions, 0 deletions

diff --git a/src/tools/README.md b/src/tools/README.md
index d6b0a21..fd8ee7e 100644
--- a/src/tools/README.md
+++ b/src/tools/README.md
@@ -14,3 +14,4 @@
 - [qemu](./qemu.md)
 - [pacman](./pacman.md)
 - [dot](./dot.md)
+- [ffmpeg](./ffmpeg.md)
diff --git a/src/tools/ffmpeg.md b/src/tools/ffmpeg.md
new file mode 100644
index 0000000..9f0c56b
--- /dev/null
+++ b/src/tools/ffmpeg.md
@@ -0,0 +1,26 @@
+# ffmpeg (1)
+
+## screen capture specific window (x11)
+
+Following snippet allows to select a window which is then captured.
+```bash
+#!/bin/bash
+
+echo "Click on window to record .."
+# Extract window size and x,y offset.
+video_args=$(xwininfo \
+    | awk '/Absolute upper-left X:/ { xoff = $4 }
+           /Absolute upper-left Y:/ { yoff=$4 }
+           /Width:/ { if ($2 % 2 == 1) { width=$2-1; } else { width=$2; } }
+           /Height:/ { if ($2 % 2 == 1) { height=$2-1; } else { height=$2; } }
+           END { printf "-video_size %dx%d -i :0.0+%d,%d", width, height, xoff, yoff  }')
+
+ffmpeg -framerate 25 -f x11grab $video_args -pix_fmt yuv420p $@ output.mp4
+```
+> Use `yuv420p` pixel format if video is played on the web
+> ([ref](https://stackoverflow.com/questions/32829514/which-pixel-format-for-web-mp4-video))
+
+The input `-i 0,0+xoff,yoff` means to capture `$DISPLAY=0.0` starting at the
+coordinate `(xoff, yoff)`, which is the left-upper corner, and the size of the
+capture is defined by the `-video_size` argument.
+