My Life Is A House

My daily life with all things Libre and Open

Monday, November 05, 2012

State of libwaveform after GSoC

Hi everyone, this is long waited status update of libwaveform - library, who aims to provide all necessary tools to collect, store and draw waveform data. After dealing with studies I have started to hack again. First, more about where I was at the end of the summer - I had general reading part aka WaveformReader ready, using Gstreamer level element, and also generic waveform drawing widget WaveformWidget using gathered data. However drawing just was without any zoom support. So first thing I wanted to improve and work on post summer was to provide method how to deal with zooms.

First of all I started to think about various ways how zoom level can be measured. I went wrong direct several times, but in the end I settled for same old 'pixels per second'. So for WaveformWidget object I have variables which store current, default and max/min zoom levels by this method, and methods which increases and decreases zoom level - currently by simply multiplying and dividing by two - which can be called using callbacks from clicking on buttons, for example (see demo/ to test this out). Now when I have changeable zoom level, I must decide how to draw waveform in given situation. This is what I have in mind for now.

For example, if I have waveform data reading interval 0.1 second long (or 100 000 000 ns), and I have 10 pps (pixel per second) zoom level, that means that wave will have width 1 pixel. That is not enough in a case if I have a waveform (usual one) where minimum width of wave side is 2 pixels - so I will require reading interval 0.2 seconds long from WaveformData (because of it's next reading interval upwards we calculate adding two 0.1 readings together), and we will use step of 2 pixels (because 1 sec /0.2 sec per reading is 5 readings and 10 pixels/5 readings is 2 pixels for reading).

This works nicely if I have waveform max and min width in sync with zoom levels/steps. However, I'm still struggling with scenario when you have 2.5, 3.13, e.t. pixel width if you have some odd zoom levels (for example 71 pixels/sec). There are two scenarios - either I enforce particular zoom levels, calculated "next best thing" by given numbers from user (similar like size request in Gtk+), or just warn users about not having pixel precise waveform alignment. For now I will work on second one.

Next big target is to fix Gstreamer level element to have sub-buffer intervals, otherwise for wav file instead of 100 000 000 ns interval I get 120 000 000 ns - and so on. Currently code has condition test for given interval size, but it doesn't work and actual interval gets rounded by buffers n times.

For feedback first my question is - what do you think about zoom model I offer here and how I plan to draw waveform according to it? What could be potential pitfalls (I have thought of several corner cases, but fresh view wouldn't hurt). Also I would like to know common values you use for default, min and max zoom levels.

How to get libwaveform - current development branch is 'postsummer' on It's buildable on Fedora 18 without any big dependencies (you need usual suspects from Gtk+/Glib land, also gobject-introspection-devel if not mistaken), just use --prefix=/usr with ./ so it would be installed correctly in your system. For other systems probably best is to use/install it with jhbuild. To check out recent functionality see demo/


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