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chapter 6.2a

Objectives

• Introduce the OpenGL shading functions
• Discuss polygonal shading
  • Flat
  • Smooth
  • Gouraud

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Steps in OpenGL shading

• Enable shading and select model
• Specify normals
• Specify material properties
• Specify lights

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Normals

• In OpenGL the normal vector is part of the state
• Set by glNormal*()

  glNormal3f(x, y, z);

  glNormal3fv(p);

• Usually we want to set the normal to have unit length so cosine calculations are correct
  • Length can be affected by transformations
  • Note that scaling does not preserved length
  • glEnable(GL_NORMALIZE) allows for autonormalization at a performance penalty

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Enabling Shading

• Shading calculations are enabled by
  • glEnable(GL_LIGHTING)
  • Once lighting is enabled, glColor() ignored
• Must enable each light source individually
  • glEnable(GL_LIGHTi) i=0,1…..
• Can choose light model parameters
  • glLightModeli(parameter, GL_TRUE)
    • GL_LIGHT_MODEL_LOCAL_VIEWER do not use simplifying distant viewer assumption in calculation
    • GL_LIGHT_MODEL_TWO_SIDED shades both sides of polygons independently
• See code light.c 

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Defining a Point Light Source

• For each light source, we can set an RGB for the diffuse, specular, and ambient parts, and the position

  GL float diffuse0[]={1.0, 0.0, 0.0, 1.0};
GL float ambient0[]={1.0, 0.0, 0.0, 1.0};
GL float specular0[]={1.0, 0.0, 0.0, 1.0};
Glfloat light0_pos[]={1.0, 2.0, 3,0, 1.0};
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glLightv(GL_LIGHT0, GL_POSITION, light0_pos);
glLightv(GL_LIGHT0, GL_AMBIENT, ambient0);
glLightv(GL_LIGHT0, GL_DIFFUSE, diffuse0);
glLightv(GL_LIGHT0, GL_SPECULAR, specular0);

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Distance and Direction

• The source colors are specified in RGBA
• The position is given in homogeneous coordinates
  • If w =1.0, we are specifying a finite location
  • If w =0.0, we are specifying a parallel source with the given direction vector
• The coefficients in the distance terms are by default a=1.0 (constant terms), b=c=0.0 (linear and quadratic terms).
• Change by

  a= 0.80;
glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, a);

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Spotlights

• Use glLightv to set
  • Direction
    • GL_SPOT_DIRECTION
  • Cutoff
    • GL_SPOT_CUTOFF
  • Attenuation
    • GL_SPOT_EXPONENT
      • Proportional to cos

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Global Ambient Light

• Ambient light depends on color of light sources
  • A red light in a white room will cause a red ambient term that disappears when the light is turned off
• OpenGL allows a global ambient term that is often helpful
  • glLightModelfv(GL_LIGHT_MODEL_AMBIENT, global_ambient)

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Moving Light Sources

• Light sources are geometric objects whose positions or directions are affected by the model-view matrix
• Depending on where we place the position (direction) setting function, we can
  • Move the light source(s) with the object(s)
  • Fix the object(s) and move the light source(s)
  • Fix the light source(s) and move the object(s)
  • Move the light source(s) and object(s) independently

• movelight.c 

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Material Properties

• Material properties are also part of the OpenGL state and match the terms in the Phong model
• Set by glMaterialv()

  GLfloat ambient[] = {0.2, 0.2, 0.2, 1.0};
GLfloat diffuse[] = {1.0, 0.8, 0.0, 1.0};
GLfloat specular[] = {1.0, 1.0, 1.0, 1.0};
GLfloat shine = 100.0
glMaterialf(GL_FRONT, GL_AMBIENT, ambient);
glMaterialf(GL_FRONT, GL_DIFFUSE, diffuse);
glMaterialf(GL_FRONT, GL_SPECULAR, specular);
glMaterialf(GL_FRONT, GL_SHININESS, shine);

• See code colormat.c 

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Front and Back Faces

• The default is to shade only front faces which works correctly for convex objects
• If we set two sided lighting, OpenGL will shade both sides of a surface
• Each side can have its own properties which are set by using GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK in glMaterialf

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Emissive Term

• We can simulate a light source in OpenGL by giving a material an emissive component
• This color is unaffected by any sources or transformations

  GLfloat emission[] = 0.0, 0.3, 0.3, 1.0);
glMaterialf(GL_FRONT, GL_EMISSION, emission);

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Transparency

• Material properties are specified as RGBA values
• The A value can be used to make the surface translucent
• The default is that all surfaces are opaque regardless of A
• Later we will enable blending and use this feature

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Efficiency

• Because material properties are part of the state, if we change materials for many surfaces, we can affect performance
• We can make the code cleaner by defining a material structure and setting all materials during initialization

typedef struct materialStruct {
GLfloat ambient[4];
GLfloat diffuse[4];
GLfloat specular[4];
GLfloat shineness;
} MaterialStruct;

• We can then select a material by a pointer

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Polygonal Shading

• Shading calculations are done for each vertex
  • Vertex colors become vertex shades
• By default, vertex colors are interpolated across the polygon

glShadeModel(GL_SMOOTH);

• If we use glShadeModel(GL_FLAT); the color at the first vertex will determine the color of the whole polygon

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Polygon Normals

• Polygons have a single normal
  • Shades at the vertices as computed by the Phong model can be almost the same
  • Identical for a distant viewer (default) or if there is no specular component
• Consider model of sphere
• Want different normals at each vertex even though this concept is not quite correct mathematically

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Smooth Shading

• We can set a new normal at each vertex
• Easy for sphere model
  • If centered at origin n = p
• Now smooth shading works

Note silhouette edge




• See sphere.c , smooth.c 

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Mesh Shading

• The previous example is not general because we knew the normal at each vertex analytically
• For polygonal models, Gouraud proposed we use the average of normals around a mesh vertex

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Gouraud and Phong Shading

• Gouraud Shading
  • Find average normal at each vertex (vertex normals)
  • Apply Phong model at each vertex
  • Interpolate vertex shades across each polygon
• Phong shading
  • Find vertex normals
  • Interpolate vertex normals across edges
  • Find shades along edges
  • Interpolate edge shades across polygons

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Comparison

• If the polygon mesh approximates surfaces with a high curvature, Phong shading may look smooth while Gouraud shading may show edges
• Phong shading requires much more work than Gouraud shading
  • Usually not available in real time systems
• Both need data structures to represent meshes so we can obtain vertex normals

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