GNU Octave compatibility (proc code)

[beaudu]

Presently WebObs is using the Matlab language for most of the built-in procs. All codes are open source, written in pure Matlab core language without the need for toolboxes, and since the WebObs package contains compiled binaries, no license is required for use (see install instructions). We point out here that the use of Matlab, apart from the fact that it was an initial and relevant choice 20 years ago, still allows today a gain in execution speed of a factor up to 5 compared to similar free interpreted languages. But the last might be useful for development or even for production when the speed is not an issue. For that reason, and in order to move towards a totally free open source project, making all the codes compatible with GNU Octave has always been a long term objective.

During the transition period of development, the coding strategy remains to make Matlab optimized code (presently using R2011b), but the same code should run under Octave and produce reasonable similar results, at least without execution errors...

The last commits 269ed9e, 2b7bf73, and 77b4f2e initiated the first efforts:

• readproc is fully functional (proc and associated nodes configuration and the data import!)
• sefran3 is working with minor graphical issues (channels banner size)
• genplot is working but only with gnuplot (see OpenGL problem below)
• hypomapis working except for timeseries graphs (still OpenGL problem)
• gridmaps is functional (only with French accent issues)

In the following comment I will list the first issues (and solutions).

[beaudu]

First list of Octave compatibility issues

OpenGL Precision issue

Main problem comes from a precision issue with time series with OpenGL graphics toolkits which use single precision: see this Octave documentation article. This affects all time series graphics based on the datenum time vectors which use double precision in Matlab. Actually there is two solutions to investigate:

• use the standard graphics toolkits gnuplot: it works for simple plots (like with superproc genplot) but has many issues with images and mapping (gnss).
• rewrite plot, datenum and datetick functions to make time values in the single precision range: the workaround suggested by Octave documentation is to subtract 2000 years to all datenum values, keeping the month, day of month and day of week the same... It might be a good solution since datetick is not fully satisfactory in automatic mode, and plot specific functions have already been developed (smartplot.m, timeplot.m, ...) but this will require some work.

2-D interpolation

An other major problem is the 2-D interpolation function griddata which is very slow in Octave. This function is necessary for mapping interpolation after lat/lon to UTM transformation for example. There is no obvious workaround...

dir

In Octave the dir function expands wildcards before execution, so dir('*') returns files and the contains of sub directories while in Matlab it returns only the list of file/directory names. This has been solved by not using wildcards and managing file and directory names outside the dir function.

Backslash character

The backslach character must be escaped in Octave, so \ have been replaced with \\. This seems to remain compatible with Matlab so far...

textscan format

textscan function seems not accepting the %*[^\n] format to ignore ends of lines. The few uses have been corrected by using a WO function readdatafile.

time precision issue

datevec might returns 1e-5 values for seconds instead of zero. This has been corrected with %02.0f format instead of %02d in some sprintf functions.

image transparency

image function seems not able to plot a RGB image with transparency (output is black background). This has been solved by not using alpha capability and coding directly the tranparency mask on the RGB matrix: I*M + (1-M).

function behavior with empty objects

Some functions do not allow empty objects (for example cat).

XML files

xmlread function needs some Java library in Octave. This has been solved by adding xerces binaries xercesImpl.jar and xml-apis.jar in the java path and loading the io package (see .octaverc file).

Character encoding

Since Matlab R2011b only accepts ISO-8859-1 character encoding, a lot of effort has been done in the WebObs project to fit this limitation. But Octave uses UTF-8 sot there is a lot of accent outcomes...