Technical Ramblings

For a long time, I avoided MrSID like the plague. After trying to do *anything* useful with it, I finally gave up; the requirement for old versions of gcc, non-working on 64bit, etc. really gave me a negative impression of the SDK for MrSID reading. This was especially painful when working with OpenAerialMap, since MrSID has a practical lock on the market from ortho imagery datasources. (There are exceptions to this, but they’re usually JPEG2000 data, which was even worse to work with with the tools that I use, in general.)

However, after a set of discussions yesterday, I sat down and had a bit of a discusion about it, and Frank said that MrSID building in GDAL had gotten much easier. I didn’t really believe him, but I had the DSDK handy for other reasons, and reading the build hints, it was supposed to be easy.

Thinking I was going to prove Frank wrong, I started building. I did ./configure --with-mrsid=~/Downloads/Geo_DSDK-7.0.0.2167; confirmed MrSID ‘yes’ in the output, then make.

3 minutes later, I had a gdalinfo and gdal_translate built on my Mac with MrSID support.

My historical problems with MrSID are completely irrelevant: the effort in the new SDK to support more platforms has clearly worked, and I can say that building MrSID support even on the Mac is trivial. A big thumbs up to the LizardTech folks for their effort in this regard — and to people like Frank and Michael for egging me on into learning this about the DSDK in the first place.

Yesterday, I got to sit down and do some real performance testing with the MapServer folks. After rebuilding a local copy of the Boston Freemap on my laptop, I was able to share it with Paul, who ran it through Shark to find out where the performance killers are. The one thing we found was that this 5 year old MapServer ticket was negatively affecting performance on maps with many labels: The labelling code in MapServer right now, if you’re using outlines, draws each glyph 9 times in order to get a nice outline color. After determining this, it was determined that we are going to be working with the GD maintainers to add the support described in #1243 to GD to use Freetype’s internal stroking code to get the same behavior. (At the time, in Freetype *2.0.09*, there was a bug in this code; but we’re now on 2.3.8, so that bug has been long fixed. :)) This change will likely give a 20% increase on map drawing with many outlined labels, as can be seen in maps like the Boston Freemap.

After this, we sat down with MrSID and GDAL/MapServer to figure out if there were performance problems there. One thing we found was that the MapServer code drawing one-band-at-a-time means that there is a significant performance hit. In addition, some other performance enhancement techniques are being looked into at the GDAL level by Frank, thanks to the help of LizardTech developers participating in the sprint. He’s currently looking at improving the way that GDAL reads from MrSID, and was already able to achieve a 25% speed increase by simply changing the size of the internal GDAL buffer size for reading from MrSID to GeoTIFF. More documentation and experimentation is still in order, but there are some possible optimizations to investigate there for users of the library.

We then had a great dinner at Jack Astor’s.

Thanks to our sponsors for today: Bart van den Eijnden from OSGIS.nl and Michael Gerlek from LizardTech — performance improvements in MapServer and GDAL access for label drawing and MrSID are potentially big wins for many users of MapServer.

Day 2 of the code sprint seemed to be much more productive. With much of the planning done yesterday, today groups were able to sit down and get to work.

Today, I accomplished two significant tasks:

• Setting up the new OSGeo Gallery, which is set to act as a repository for demos of OSGeo software users in the same way that the OpenLayers Gallery already does for OpenLayers. We’ve even added the first example.
• TMS Minidriver support for the GDAL WMS Driver: Sitting down and hacking out a way to access OSM tiles as a GDAL datasource, Schuyler and I built something which is reasonably simple/small — an 18k patch including examples and docs — but allows for a significant change in the ability to read tiles from existing tileset datasources on the web.

Other things happening at the sprint today were more WKT Raster discussions, liblas hacking, and single-pass MapServer discussions, as well as some profiling of MapServer performance with help from Paul and Shark. Thanks to the participation of the LizardTech folks, I think there will also be some performance testing done with MrSID rendering within MapServer, and there was — as always — more discussion of the “proj strings are expensive to look up!” discussion.

Other than that, it was a quiet day; lots of work getting done, but not much excitement in the ranks.

We then had a great dinner at Baton Rouge, and made it home.

This evening, I’ve been doing a bit more hacking, opening a GDAL Trac ticket for an issue Schuyler bumped into with the sqlite driver, and pondering the plan for OpenLayers tomorrow.

As before, a special thanks to the conference sponsors for today: Coordinate Solutions via David Lowther, and the lovely folks at SJ Geophysics Ltd.. Thanks for helping make this thing happen! I can guarantee that neither of those GDAL tickets would have happened without this time.

I’m here at the OSGeo Code Sprint in Toronto, where more than 20 OSGeo hackers have gathered to work on all things OSGeo — or at least MapServer, GDAL/OGR, and PostGIS.

For those who might not know, a code sprint is an event designed to gather a number of people working on the same software together with the intention of working together to get a large amount of development work done quickly. In this case, the sprint is a meeting of the “C tribe”: Developers working on the C-based stack in OSGeo.

After some discussion yesterday, there ended up being approximately 3 groups at the sprint:

• People targeting MapServer development
• PostGIS developers
• liblas developers

(As usual, I’m a floater, but primarily concentrating on OpenLayers; Schuyler will be joining me in this pursuit, and I’ve got another hacker coming Monday and Tuesday to sprint with us.)

The MapServer group was the most lively discussion group (and is also the largest). It sounded like there were three significant development discussions that were taking place: XML Mapfiles, integration of pluggable rendering backends, and performance enhancements, as well as work on documentation.

After a long discussion on the benefits/merits of XML mapfiles, it came down to there being one main target use case for the XML mapfile is encouraging the creation and use of more editing clients. With a format that can be easily round-tripped between client and server, you might see more editors able to really speak the same language. In order to test this hypothesis, a standard XSLT transform will be created and documented, with a tool to do the conversion; this will allow MapServer to test out the development before integrating XML mapfile support into the library itself.

I didn’t listen as closely to the pluggable renderers discussion, but I am aware that there’s a desire to improve support and reduce code duplication of various sorts, and the primary author of the AGG rendering support is here and participating in the sprint. Recently, there has been a proposal to the list to add OpenGL based rendering support to MapServer, so this is a step in that direction.

The PostGIS group was excited to have so many people in the same place at the same time, and I think came close to skipping lunch in order to get more time working together. In the end, they did go, but it seemed to be a highly productive meeting. Among some of their discussions was a small amount of discusssion on the WKTRaster project which is currently ongoing, I believe.

After our first day of coding, we headed to a Toronto Marlies hockey game. This was, for many of us, the first professional hockey we’d ever seen. (The Marlies are the equivilant of AAA baseball; one step below the major leagues.) The Canadians in the audience, especially Jeff McKenna, who played professional hockey for a time, helped keep the rest of us informed. The Marlies lost 6-1, sadly, but as a non-Canadian, I had to root a bit for the Hershey team. (Two fights did break out; pictures forthcoming.)

We finished up with a great dinner at East Side Mario’s.

A special thanks to our two sponsors for the day, Rich Greenwood of Greenwood Map and Steve Lehr from QPUBLIC! Our sprint was in a great place, very productive, and had great events, thanks to the support of these great people.

Looking forward to another great day.