Writing a Geometry shader

The following example computes within a geometry shader the geometry of point field bars from a set of 3D points which specify the X,Y coordinate of the center of each bar and the Z coordinate of the top of each bar. The 3D point set is defined by an SoPointSet shape.

The example source code is located in $OIVHOME/src/Inventor/examples/Features/Shaders/GeometryShader.

This example is written using GLSL (Open Inventor supports only GLSL geometry shaders).

When defining a geometry shader, it is mandatory to define also a vertex
shader.

Point field bars computed within a geometry shader

: Example : Writing a geometry shader:

Contents of GeometryShader.cxx

C++ :

main( int argc, char** argv )
{
  ...
  // Initialize the vertex and geometry shader
  SoVertexShader* vertexShader = new SoVertexShader;
  SoGeometryShader* geometryShader = new SoGeometryShader;
 
  vertexShader->sourceProgram.setValue( "../../../data/Shaders/histoShaderVtx.glsl" );
  geometryShader->sourceProgram.setValue( "../../../data/Shaders/histoShaderGeom.glsl" );
 
  // Initialize and set the shader program
  SoShaderProgram* shaderProgram = new SoShaderProgram;
  shaderProgram->shaderObject.set1Value( 0, vertexShader );
  shaderProgram->shaderObject.set1Value( 1, geometryShader );
  // Define the input geometry type which is POINTS because defined by an
  // SoPointSet
  shaderProgram->geometryInputType = SoShaderProgram::POINTS_INPUT;
  // Define the output geometry type generated by the geometry shader which is
  // a triangle strip set
  shaderProgram->geometryOutputType = SoShaderProgram::TRIANGLE_STRIP_OUTPUT;
 
  // Uniform parameter to define the bar width
  HistoGeomShaderWidth = new SoShaderParameter1f;
  HistoGeomShaderWidth->name = "BarWidth";
  geometryShader->parameter.setValue( HistoGeomShaderWidth );
 
  // Input geometry shader primitive (SoPointSet)
  HistoGeomShaderShape = new SoPointSet;
  HistoGeomShaderCoord = new SoCoordinate3;
 
  // Build the scene graph
  SoSeparator* geomShaderSep = new SoSeparator;
  geomShaderSep->boundingBoxIgnoring = true;
  geomShaderSep->addChild( shaderProgram );
  geomShaderSep->addChild( HistoGeomShaderCoord );
  geomShaderSep->addChild( HistoGeomShaderShape );
  ...
}

C# :

Java :

Contents of histoShaderVtx.glsl Vertex Shader:

/* !!GLSL Vertex Shader */
 
void
main()
{
  // Transform the vertex (ModelViewProj matrix)
  gl_Position = gl_Vertex;
  gl_FrontColor = gl_FrontMaterial.diffuse;
}

Contents of histoShaderGeom.glsl Geometry Shader:

#version 120
#extension GL_EXT_geometry_shader4 : enable
 
uniform float BarWidth;
 
/******************************************************************************/
// Compute the diffuse color according to the normal.
vec4
computeDiffuseColor( vec3 normal )
{
  vec3 tNormal = normalize( gl_NormalMatrix * normal );
  return ( max( dot( tNormal, vec3( gl_LightSource[0].position ) ), 0. ) * gl_FrontColorIn[0] );
}
/******************************************************************************/
 
 
void
main( void )
{
  int i;
 
  for ( i = 0; i < gl_VerticesIn; i++ )
  {
    vec4 posIn = gl_PositionIn[i];
    vec4 position;
    position.w = posIn.w;
 
    // 1st face
    position.x = posIn.x - BarWidth;
    position.y = posIn.y - BarWidth;
    position.z = 0.0;
    gl_Position = gl_ModelViewProjectionMatrix * position;
    gl_FrontColor = computeDiffuseColor( vec3( 0, -1, 0 ) );
    EmitVertex();
 
    position.z = posIn.z;
    gl_Position = gl_ModelViewProjectionMatrix * position;
    EmitVertex();
 
    position.x = posIn.x + BarWidth;
    position.z = 0.0;
    gl_Position = gl_ModelViewProjectionMatrix * position;
    EmitVertex();
 
    position.z = posIn.z;
    gl_Position = gl_ModelViewProjectionMatrix * position;
    EmitVertex();
    EndPrimitive();
 
    // 2nd face
    ...
    ...
    EndPrimitive();
 
    // 3rd face
    ...
    ...
    EndPrimitive();
 
    // 4th face
    ...
    ...
    EndPrimitive();
 
    // 5th face
    ...
    ...
    EndPrimitive();
 
    // 6th face
    ...
    ...
    EndPrimitive();
  }
}
/******************************************************************************/

Shader usage for image base lighting and HDRI reflection (Data set courtesy DaimlerChrysler AG)