Writing a Per-Pixel Lighting Shader

The following example computes per-pixel lighting (only the diffuse contribution is computed) on a sub-scene graph. Per-pixel lighting gives better lighting quality than per-vertex lighting (with classical OpenGL pipeline) because the light contributions are computed at each pixel instead of at each vertex.

The original example (located in $OIVHOME/src/Inventor/examples/Features/Shaders/PixelLighting), enables you to select when the application starts the shading language used for the per-pixel lighting.

The example described below, includes only the essential part of the complete source.

Per-pixel lighting shader (left) versus per-vertex lighting (right)

Simplified scene graph of PixelLighting.cxx

Example : Writing a per-pixel shader

Contents of PixelLighting.cxx

C++ :

void
configureShaders()
{
  // Initialize and set the vertex shader
  VertexShader = new SoVertexShader;
 
  // Initialize and set the fragment shader
  FragmentShader = new SoFragmentShader;
 
  // Specify the vertex and fragment shaders
  VertexShader->sourceProgram.setValue( "../../../data/Shaders/PixelLightingVtx.glsl" );
  FragmentShader->sourceProgram.setValue( "../../../data/Shaders/PixelLightingFrag.glsl" );
 
  // Initialize and set the shader program
  SoShaderProgram* shaderProgram = new SoShaderProgram;
  shaderProgram->shaderObject.set1Value( 0, VertexShader );
  shaderProgram->shaderObject.set1Value( 1, FragmentShader );
 
  // Add the shader program to the separator
  ShaderSep->insertChild( shaderProgram, 0 );
}
 
int
main( int argc, char*\* argv )
{
  ...
  SoSeparator* root = new SoSeparator;
  root->ref();
 
  // Point light
  PointLight = new SoPointLightManip;
 
  ShaderSep = new SoSeparator;
 
  SoSeparator* sphereSep = new SoSeparator;
  sphereSep->addChild( new SoSphere );
 
  // Configures the shader
  configureShaders();
 
  // Add the sphere to the shaders' separator to apply the shaders to it.
  ShaderSep->addChild( sphereSep );
 
  // Material applied to the model
  SoMaterial* modelMat = new SoMaterial;
  modelMat->ambientColor.setValue( 0.2f, 0.2f, 0.2f );
  modelMat->diffuseColor.setValue( 0.3f, 0.3f, 0.9f );
  modelMat->specularColor.setValue( 0.6f, 0.6f, 0.8f );
  modelMat->emissiveColor.setValue( 0.0f, 0.0f, 0.0f );
  modelMat->shininess.setValue( 0.2f );
 
  // Build the scene graph
  root->addChild( modelMat );
  root->addChild( PointLight );
  root->addChild( ShaderSep );
  ...
}

C# :

void
ConfigureShaders()
{
  // Initialize and set the vertex shader
  SoVertexShader VertexShader = new SoVertexShader();
 
  // Initialize and set the fragment shader
  SoFragmentShader FragmentShader = new SoFragmentShader();
 
  // Specify the vertex and fragment shaders
  VertexShader.sourceProgram.Value = "$OIVNETHOME/src/Inventor/examples/data/Shaders/PixelLightingVtx.glsl";
  FragmentShader.sourceProgram.Value = "$OIVNETHOME/src/Inventor/examples/data/Shaders/PixelLightingFrag.glsl";
 
  // Initialize and set the shader program
  SoShaderProgram shaderProgram = new SoShaderProgram();
  shaderProgram.shaderObject[0] = VertexShader;
  shaderProgram.shaderObject[1] = FragmentShader;
 
  // Add the shader program to the separator
  ShaderSep.InsertChild( shaderProgram, 0 );
}
 
void
CreateSample()
{
  ...
  SoSeparator root = new SoSeparator();
 
  // Point light
  SoPointLightManip PointLight = new SoPointLightManip();
 
  ShaderSep = new SoSeparator();
 
  SoSeparator sphereSep = new SoSeparator();
  SoTranslation sphereTranslation = new SoTranslation();
  sphereTranslation.translation.SetValue( 3f, 0f, -2f );
  sphereSep.AddChild( sphereTranslation );
  sphereSep.AddChild( new SoSphere() );
 
  // Configures the shader
  ConfigureShaders();
 
  // Add the sphere to the shaders' separator to apply the shaders to it.
  ShaderSep.AddChild( sphereSep );
 
  // Material applied to the model
  SoMaterial modelMat = new SoMaterial();
  modelMat.ambientColor.SetValue( 0.2f, 0.2f, 0.2f );
  modelMat.diffuseColor.SetValue( 0.3f, 0.3f, 0.9f );
  modelMat.specularColor.SetValue( 0.6f, 0.6f, 0.8f );
  modelMat.emissiveColor.SetValue( 0.0f, 0.0f, 0.0f );
  modelMat.shininess.SetValue( 0.2f );
 
  // Build the scene graph
  root.AddChild( modelMat );
  root.AddChild( PointLight );
  root.AddChild( ShaderSep );
  ...
}

Java :

Contents of PixelLightingVtx.glsl Vertex Shader:

/* !!GLSL */
 
varying vec4 eposition;
varying vec3 normal;
varying vec3 diffuseColor;
varying vec3 specularColor;
varying vec3 emissiveColor;
varying vec3 ambientColor;
varying float shininess;
 
void
main()
{
  // Position in clip space
  gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
 
  // Position in eye space
  eposition = gl_ModelViewMatrix * gl_Vertex;
 
  // Normal in eye space
  normal = mat3( gl_ModelViewMatrix ) * gl_Normal;
 
  // Retrieves diffuse, specular emissive, and ambient color from the OpenGL state.
  diffuseColor = vec3( gl_FrontMaterial.diffuse );
  specularColor = vec3( gl_FrontMaterial.specular );
  emissiveColor = vec3( gl_FrontMaterial.emission );
  ambientColor = vec3( gl_FrontMaterial.ambient );
  shininess = gl_FrontMaterial.shininess;
}

Contents of PixelLightingFrag.glsl fragment shader:

/* !!GLSL */
 
varying vec4 eposition;
varying vec3 normal;
varying vec3 diffuseColor;
varying vec3 specularColor;
varying vec3 emissiveColor;
varying vec3 ambientColor;
varying float shininess;
 
void
main()
{
  const vec3 lightColor = vec3( 1, 1, 1 );
  const vec3 globalAmbient = vec3( 0.2, 0.2, 0.2 );
 
  // Position in eye space
  vec3 P = vec3( eposition );
 
  // Normalize normal in eye space
  vec3 N = normalize( normal );
 
  // Compute the emissive term
  vec3 emissive = emissiveColor;
 
  // Compute the ambient term
  vec3 ambient = ambientColor * globalAmbient;
 
  // Compute the diffuse term
  // Normalized vector toward the light source
  vec3 L = normalize( vec3( gl_LightSource[0].position ) - P );
  float diffuseLight = max( dot( N, L ), 0 );
  vec3 diffuse = diffuseColor * lightColor * diffuseLight;
 
  // Compute the specular term
  vec3 V = normalize( -P );    // Normalized vector toward the viewpoint
  vec3 H = normalize( L + V ); // Normalized vector that is halfway between V and L
  float specularLight = pow( max( dot( N, H ), 0 ), shininess );
  if ( diffuseLight <= 0 )
    specularLight = 0;
  vec3 specular = specularColor * lightColor * specularLight;
 
  // Define the final vertex color
  gl_FragColor.xyz = emissive + ambient + diffuse + specular;
  gl_FragColor.w = 1.0;
}