[osg-users] [osgPlugins] Very confused about plugins

[Bram Vaessen]

Hi,

I'm having issues with loading png. It just says: "Warning: Could not find plugin to read objects from file "GUI/canvas.png".

The same line replaced with .jpg works fine. I built OSG 3.2 from source some time ago (using some tutorial, and CMake and such), and I recall that I tried to enable both jpg and png. Somehow png doesn't work.

Now the real problem is: I have absolutely no clue to how this whole build system and plugin system works, and I can't seem to find decent information on it... only this: http://trac.openscenegraph.org/projects/osg//wiki/Support/UserGuides/Plugins

also: when I try to load a non-existing file as an image, I get the exact same error. There is no way for a 'noob' like me (at least a noob in building systems) to understand or find out what the problem is. It looks like there is a libpng13d.lib in the lib dir, and I found a osgdb_png.dll and osgdb_png.dll somewhere that I copied to my projects working directory, but that doesn't seem to be enough...

Honestly, I'm desperately confused. I have to admit that I just don't know that much about cmake, libraries and dll's etc. just know the basics, but there's a huge information gap between some very general information that you can find on cmake and osg plugins, and how the actual build system works in reality. There also doesn't seem info on how you can track down errors like plugins that are not loading. I also looked on the forums and there are some general tips like: you have to rebuild the png library... but then I'm like... ok ... how??? is that part of the osg source? I have no clue how to do this :(

I know that with some random hints from users I may be able to fix this one problem this time, but then again I still wouldn't know what happened or how to fix the next problem with plugins.

Isn't there more information on this? Or can someone explain it here how it all works?

Like:

-What are the steps that are involved in building the libraries, and loading them as plugins?
-In each of these steps what files are supposed to be created and how can you check if that happened etc.?
-how can you get more error information from the failing the loading of the plugin?

I would be very very grateful for any information!

Thank you!

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[Farshid Lashkari]

Hi Bram,

First off, I feel your pain.

Is "osgdb_png.dll" in your "osgPlugins-XXX" subfolder? If so, then the DLL might be dynamically linking to the libpng library, instead of statically linking. If this is the case, then you will need to place the "libpng13.dll" file in your application folder (next to your exe).

When dealing with DLL issues like this, I find the Dependency Walker (http://www.dependencywalker.com/) application invaluable. Opening an exe/dll file in the application will list all its DLL dependencies and let you know which ones are missing or failed to load.

Regarding the error message, I agree that they could be a little more clear. The error message does not distinguish between failing to find the DLL vs failing to load the DLL.

Cheers,

Farshid

[Robert Osfield]

Hi Bram,

On 16 July 2014 19:58, Bram Vaessen <bram.v...@gmail.com> wrote:

-What are the steps that are involved in building the libraries, and loading them as plugins?

Have look at the Getting started and Platform specifics documentation on the website:

   www.openscenegraph.org/index.php/documentation/getting-started

   www.openscenegraph.org/index.php/documentation/platform-specifics

The books will also provide guidance.

You should just need to provide the 3rd party dependencies and make sure that CMake finds them OK and then build the OSG.

Loading wise you need to make sure either the PATH or OSG_LIBRARY_PATH env var points to the parent directory of the osgPlugins-version directory that you have built or installed. The OSG will then at runtime search for the plugins and load them dynamically.  

To see where the OSG is searrching for the plugins enable verbose debug out by settings the env var OSG_NOTIFY_LEVEL to DEBUG and then look at the console output, this will list what plugins it's trying to load and what files it's trying to load.

The env var OSG_FILE_PATH is used to guide the searching of files.

You can also set the library and file search paths programatically via osgDB::setDataFilePathList(..) and setLibraryFilePathList(..). See the include osgDB/FileUtils.

-In each of these steps what files are supposed to be created and how can you check if that happened etc.?

All the plugins will be built as .dll's under windows and .so's under other platforms and will be placed in a osgPlugins-version directory.  The OSG will search for osgPlugins-version/osgdb_extension.dll/.so directory first by default to avoid issues with different versions of the OSG/plugins being binary incompatible (and issue known widely in the compute industry as DLL hell.)  For this reason you should set the library path to the parent directory where the osgPlugins-version directory rather than to the directory itself, otherwise you could end up finding plugins for a different version of the OSG.

 

-how can you get more error information from the failing the loading of the plugin?

setting the OSG_NOTIFY_LEVEL to DEBUG will tell the OSG to output lots of info to console, search through this to find references to the file loading.

--

As a general note, the OSG plugin system uses the Chain of Responsibility Design Pattern, this scheme means that it tries all loaded plugins to see if they can load a file before checking the file system for plugins that may be able to load the plugin.  This scheme means that you can provide a single plugin to load multiple formats, such as used in the Quicktime plugin.  Another aspect to the scheme is that finding and loading of files is deferred to the plugins, something that is important when handling files accessed by http etc.  This loose coupling is very powerful, but it does mean there isn't always 1:1 mapping between a file format extension and a plugin, which is part of the reason why the OSG error reporting system can't always be specific about what error there might be.

Robert.

[Bram Vaessen]

Thanks Robertos and Farshid.

Robertos, when I said: "What are the steps that are involved in building the libraries, and loading them as plugins? "

I actually meant to ask this from the perspective of the 'system', and not the steps that a user has to take to make it work. This demonstrates what I mean very nicely though: most of the documentation involves a set of steps that you have to take. Not that tutorials are a bad thing, obviously this is exactly what most new users are looking for... but still it doesn't explain what is happening exactly happening in those steps, and if any of the steps fails (or even worse, you'll get an error at the end and you don't really know which step failed), you are stuck to asking other experts or trying random things :)

> The OSG will search for osgPlugins-version/osgdb_extension.dll/.so directory first by default to avoid issues with different versions of the OSG/plugins being binary incompatible (and issue known widely in the compute industry as DLL hell.) For this reason you should set the library path to the parent directory where the osgPlugins-version directory rather than to the directory itself, otherwise you could end up finding plugins for a different version of the OSG.

This is more the kind of information I was hoping to find, and the general note too, and the OSG_NOTIFY_LEVEL to DEBUG is also a very good tip, thanks!!!

Farshid: thanks it was indeed a problem with the dll's, I copied them all again, and there must have been an older version of soms png dll in my project probably, cause I only overwritten already existing files, but still it made a difference :)

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Read this topic online here:

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