Thursday 27 February 2014

Screencasting on Windows

I always find screencasting quite difficult to set up on Windows. Here are some notes on a recent successful attempt that uses the well-known ffmpeg library/command-line tool to record from a Direct Show screen capture device.

Set up the screen capture device

Install Screen Capture Recorder and configure by selecting the shortcut "configure by resizing a transparent window". Start broadcasting the desktop by choosing the shortcut "stream desktop local LAN", and "Start Normal 10 fps".

Capture the screen

Install 32-bit ffmpeg (even on 64-bit Windows) and capture as follows (press "q" to stop):
ffmpeg -f dshow -i video=screen-capture-recorder -r 10
       -y recording.mp4

Convert to the target format

For example, convert to .wmv and retain just the first 60 seconds (see "-ss" for skipping the start and "-q:v 10" to control the size/quality):
ffmpeg -i recording.mp4 -t 60 -b:v 1000k
       -vcodec wmv2 myfile.wmv

Monday 3 February 2014

When Python goes wrong: Off-by-one

Many niggling problems with Python were fixed in Python 3, but here's one that recently bit me in the expunged. It relates to rounding in format strings and so is a potential pitfall for anyone handling floats.

The thing is, "%f" rounds differently than "%d".
>>> "%.1f" % 1.56 ### I expect 1.6
>>> "%d" % 1.6    ### By analogy, I expect 2
The correct solution is either "%.0f" (yes - it exists) or explicitly round it yourself how you like, e.g. "%d" % int(x+0.5) or using round().

This was causing off-by-one results in my code which I fortunately identified prior to submitting a manuscript. But I'm disappointed, Python; and don't blame C.

Note: As a further issue, if you need to handle negative values also, doublecheck the rounding behaviour of your may not always be what you expect.