= Using OpenGL Shading Language Vertex and Fragment Shaders (Introduction) =
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== Goal ==

Replace the fixed-function pipeling lighting and texture calculations with an OpenGL Shading Language vertex and fragment program for selected nodes in the scene graph. (The procedural brick shader from 3Dlabs.)

== Overview ==

OpenGL Shading Language allows programers to write custom pixel and vertex shaders. For more information on shading languages - including minimum hardware and software requirements - visit here. The osg::Program and osg::Shader module allow osg users to apply these shaders as part of a !StateSet to selected subtrees within a scene graph. Using a custom vertex or fragment shader in !OpenSceneGraph involves the following basic classes:

1. osg::Program - application level abstraction of the OpenGL Shading Language glProgramObject. The !ProgramObject class is derived from the osg::!StateAttribute class. This means that instance of the osg::Program class can be associated with !StateSets and enabled using the !StateSet class' setAttributeAndModes() method. Enabling a program object for a stateset results in drawables within that stateset being rendered using the Program's shaders.
2. osg::Shader - application level abstraction of the OpenGL Shading Language glShaderObject. This class manages loading and compiling shader source code. Instances of the osg::Shader class can be assigned to one or more osg::Program instances. There are two types of shader objects: osg::Shader::FRAGMENT and osg::Shader::VERTEX.

== The code ==

The steps to create an application that uses an OpenGL pixel and fragment shader are as follows:

 * Create an osg::Program instance
 * Create VERTEX and FRAGMENT instances of the osg::Shader class
 * Load and compile the shader source
 * Add the shaders to the osg::Program instance
 * Associate and enable the osg::Program instance as part of a !StateSet.

The code to load and apply a basic vertex and fragment shader is as follows:

{{{
       osg::StateSet* brickState = tankNode->getOrCreateStateSet();

       osg::Program* brickProgramObject = new osg::Program;
       osg::Shader* brickVertexObject = 
          new osg::Shader( osg::Shader::VERTEX );
       osg::Shader* brickFragmentObject = 
          new osg::Shader( osg::Shader::FRAGMENT );
       brickProgramObject->addShader( brickFragmentObject );
       brickProgramObject->addShader( brickVertexObject );
       loadShaderSource( brickVertexObject, "shaders/brick.vert" );
       loadShaderSource( brickFragmentObject, "shaders/brick.frag" );

       brickState->setAttributeAndModes(brickProgramObject, osg::StateAttribute::ON);
}}}

The convenience function to load the source code for a shader and associate the compiled code with a shader object instance is as follows:

{{{
    bool loadShaderSource(osg::Shader* obj, const std::string& fileName )
    {
       std::string fqFileName = osgDB::findDataFile(fileName);
       if( fqFileName.length() == 0 )
       {
          std::cout << "File \"" << fileName << "\" not found." << std::endl;
          return false;
       }
       bool success = obj->loadShaderSourceFromFile( fqFileName.c_str());
       if ( !success  )
       {
          std::cout << "Couldn't load file: " << fileName << std::endl;
          return false;
       }
       else
       {
          return true;
       }
    }
}}}