Hi everybody
On Fri, 2005-04-01 at 09:28 +0200, Vincent Honnet wrote:
> Hello Manfred,
>
> That's nice but for that I need to compile an inherited class and
that's
> what I can't do for the moment.
just for kicks I tried doing that to get an idea if it's really that
hard. So here's a step-by-step description of what I did:
Alright, so we want to extend some OpenSG class. How do we do that?
Well, just like any other C++ library: we derive from a class that we
want to change, add some variables for our new data and override some
methods.
Variables in OpenSG are stored in the Base class, not in the actual
class. To create the Base class source, we need an .fcd file, which
describes the variables in the class, and the fcdEdit program to create
the actual code. So let's get it.
I downloaded and installed the current cl.net 2003 dailybuild and a
source snapshot (it's always better to have the source when you're
trying to extend something). The distribution contains an fcdEdit
executable in the main directory, so I just started that.
Given that I wanted to do something very similar to the DVRSimpleShader
I just loaded its .fcd file as a base.
(Source/System/NodeCores/Drawables/VolRen/OSGDVRSimpleShader.fcd).
Noticed that it's pretty empty. :) Changed the Name to
DVRSimpleFilterShader, that's what my new class is going to be called. I
want to use the new FC in my own program, so it will not be a system
component: uncheck System Component. We want to derive our new shader
from the DVRSimpleShader, so change the parent to DVRSimpleShader, which
is a System Component (i.e. part of the OpenSG core). Turned on "Use
Local Includes" (which forces the use of "" instead of <> for #include),
as I will have the generated files in my source directory and not in the
system search path. I want to use the FC in my own program, so set the
library to empty. Added two fields (minFilter and magFilter) to keep the
parameter for the filtering modes that I want to set (yes, I know, in
general the DVRVolume shaders are not supposed to have functional
fields, but for this example it was easier to do it this way). Use
GLenum as the type and 0 as the DefaultValue, leave Cardinality at
Single (only one value per field, not a vector), Access at Public (I
want the user to change it) and Visibility at Extern (I can't
reconstruct this from other variables and I want it to be transfered
across a cluster and written to .osb files).
Ok, save the .fcd file to the Tutorials folder of the installation (as
this is where I'm going to add the code). Pressed "Write Field Container
Code" to generate an initial version of the class's code in the
Tutorials folder. This contains some methods that all OpenSG classes
need, e.g. to register themselves with the type system and supply some
generic capabilities. This is the class that we're going to add our new
features to later. Used "Write Field Container Base Code" to create the
code for the Base class. This keeps all the variables I added. The Base
code should never be changed by the user, as you might have to
regenerate it later when we change something internal to the core
library. Wrote that to the Tutorials folder, too. Exeunt fcdEdit, we're
done with it.
Now for the actual coding. Opened the Tutorials .dsw with VS.Net 2003.
For testing I copied over testSimpleVolumeNodeRender.cpp into the
Tutorials folder and used it instead of 01hello.cpp. Ran it (after
adding "OpenSG/" to the includes). Works fine. Ok, now for the new code.
Added the files that fcdEdit generated
(OSGDVRSimpleFilterShader*.{h,inl,cpp}) to the project. Changed the test
program so it creates a SimpleFilterShader instead of the SimpleShader
and rebuild it. Compiles fine but does nothing new: no wonder, I haven't
actually changed anything yet.
So now for the last part: overriding methods. Which methods to override?
Let's look at the SimpleShader for help.
Looking at OSGDVRSimpleShader.h we see that there are 2 normal methods
(changed and dump) and 5 virtual methods for volume rendering:
initialize, activate, brickActivate, deactivate and cleanup. We will
have to change/overload some of these.
Looking at our newly generated OSGDVRSimpleFilterShader.h we see that
changed and dump have been defined by fcdEdit already, nothing to do
here. But which of the methods do we actually need to change?
Manfred was nice enough to tell us that we only need to touch
brickActivate. So let's do that: add brickActivate to the header and
copy the empty one from OSGDVRSimpleShader.cpp into our new code.
Now we add the code that Manfred mentioned, but we don't use the hard-
coded filter modes, instead we use the fields that we added to our new
class above:
glTexParameteri( GL_TEXTURE_3D, GL_TEXTURE_MIN_FILTER, getMinFilter());
glTexParameteri( GL_TEXTURE_3D, GL_TEXTURE_MAG_FILTER, getMagFilter());
Now we need to #include <OpenSG/OSGGLExt.h> to have the necessary
defines, and we're done.
Change the test program so that it sets the two variables to the values
we want (GL_NEAREST), build, run and viola: point-sampled RGBA volumes.
The code I generated is at
http://www.vrac.iastate.edu/~dreiners/SimpleFilterShader.zip
(SourceForge doesn't accept .zip attachments), you can use it as it is.
Now the question is: what's so hard about this? I think nothing, really.
The only complication compared to extending any other C++ library is
that you have to use fcdEdit to generate the initial code and the Base
files to keep the variables. But there's nothing really magic about it.
Admitted, this is pretty much the simplest possible example, but it
doesn't get much more complicated unless you want to do significant
changes.
I hope this helps more people over the hump of extending OpenSG. It's
really not much different from any other library and tools like fcdEdit
hide most of the complexity.
Have a nice weekend
Dirk
--
-- Dirk Reiners OpenSG Forum [EMAIL PROTECTED]
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