osgSimplegl3 Example analysis

author: longlongwaytogo
date: 2022/07/12	
email: [email protected]		
repo: https://github.com/longlongwaytogo/LearnOsgGL3

brief introduction

The front is finished osg GL3 Construction of environment , Here is the main analysis osg How to combine gl3 Carry out development work .

A detailed explanation of the example

• establish osg gl3 Environmental Science

        const int width( 800 ), height( 450 );
        const std::string version( "3.0" );
        osg::ref_ptr< osg::GraphicsContext::Traits > traits = new osg::GraphicsContext::Traits();
        traits->x = 20; traits->y = 30;
        traits->width = width; traits->height = height;
        traits->windowDecoration = true;
        traits->doubleBuffer = true;
        traits->glContextVersion = version;
        traits->readDISPLAY();
        traits->setUndefinedScreenDetailsToDefaultScreen();
        osg::ref_ptr< osg::GraphicsContext > gc = osg::GraphicsContext::createGraphicsContext( traits.get() );
        if( !gc.valid() )
        {
    
            osg::notify( osg::FATAL ) << "Unable to create OpenGL v" << version << " context." << std::endl;
            return( 1 );
        }

The above code is mainly for osg::GraphicsContext::traits Object to set parameters , The key is to specify gl Version is 3.0.

• take traits Parameters are applied to the camera ：

  osg::Camera* cam = viewer.getCamera();
  cam->setGraphicsContext( gc.get() );
  

• take shader Parameters are applied to the scene root node

    void configureShaders( osg::StateSet* stateSet )
    {
        
        const std::string vertexSource =
            "#version 130 \n"
            " \n"
            "uniform mat4 osg_ModelViewProjectionMatrix; \n"
            "uniform mat3 osg_NormalMatrix; \n"
            "uniform vec3 ecLightDir; \n"
            " \n"
            "in vec4 osg_Vertex; \n"
            "in vec3 osg_Normal; \n"
            "out vec4 color; \n"
            " \n"
            "void main() \n"
            "{ \n"
            " vec3 ecNormal = normalize( osg_NormalMatrix * osg_Normal ); \n"
            " float diffuse = max( dot( ecLightDir, ecNormal ), 0. ); \n"
            " color = vec4( vec3( diffuse ), 1. ); \n"
            " \n"
            " gl_Position = osg_ModelViewProjectionMatrix * osg_Vertex; \n"
            "} \n";
        osg::Shader* vShader = new osg::Shader( osg::Shader::VERTEX, vertexSource );
  
        const std::string fragmentSource =
            "#version 130 \n"
            " \n"
            "in vec4 color; \n"
            "out vec4 fragData; \n"
            " \n"
            "void main() \n"
            "{ \n"
            " fragData = color; \n"
            "} \n";
        osg::Shader* fShader = new osg::Shader( osg::Shader::FRAGMENT, fragmentSource );
  
        osg::Program* program = new osg::Program;
        program->addShader( vShader );
        program->addShader( fShader );
        stateSet->setAttribute( program );
  
        osg::Vec3f lightDir( 0., 0.5, 1. );
        lightDir.normalize();
        stateSet->addUniform( new osg::Uniform( "ecLightDir", lightDir ) );
    }
  

  Above code , The use of shader Obviously older , It's best to use it really 3.3 The later version statement is better , This allows you to use renderdoc Perform frame grabbing debugging （renderdoc The use of will be introduced in detail in the following article ）.

• Set projection matrix and vertex attribute reference , But this code only gl3 Previous versions required , Really develop gl3 Version of osg The program does not need the following code

  // for non GL3/GL4 and non GLES2 platforms we need enable the osg_ uniforms that the shaders will use,
  // you don't need thse two lines on GL3/GL4 and GLES2 specific builds as these will be enable by default.
  gc->getState()->setUseModelViewAndProjectionUniforms(true);
  gc->getState()->setUseVertexAttributeAliasing(true);
  

• summary ：

although osgSimplegl3 An example is a demonstration gl3 Characteristic , But the environment in which it is written is not fully open GL3 Run in core mode , It's running on gl3 In compatibility mode , This can run version edition 130 Of shader Code .

This example is just a simple demonstration of how to run gl3 Environment . Yes osg Compilation is not required , Even if osg There is no setting in the compilation settings gl shader The version is higher , You can also use a higher version shader characteristic . But the suggestion is still to set gl3 Operating under the environment of gl3 Program , The main reasons are as follows ：

• First of all ： You can use higher version features ;
• second ： There can be more efficient api Interface , Can improve performance ;
• Third ： Third party debugging tools can be used （renderdoc、gpa、NSight etc. ） Debug error . Next article , Will explain osg and gl 3.3 Configuration of the above version .