Lighting

1. CSC 307 1.0
Graphics Programming
Budditha Hettige
Department of Statistics and Computer Science

2. 2
Lighting
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3. Real-World and OpenGL Lighting
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4. Using a Light Source
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5. 5
Lighting Principles
• Lighting simulates how objects reflect light
– material composition of object
– light’s color and position
– global lighting parameters
• ambient light
• two sided lighting
– available in both color index
and RGBA mode
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6. 6
How OpenGL Simulates Lights
• Phong lighting model
– Computed at vertices
• Lighting contributors
– Surface material properties
– Light properties
– Lighting model properties
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7. OpenGL Lights
 Common Properties
 Ambient
 Diffuse
 Specular
 Types
 Directional Light
 Point Light
 Spotlight
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8. Lighting….
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9. 9
Surface Normals
• Normals define how a surface reflects
light
• glNormal3f( x, y, z )
– Current normal is used to compute vertex’s
color
– Use unit normals for proper lighting
• scaling affects a normal’s length
glEnable( GL_NORMALIZE )
or
glEnable( GL_RESCALE_NORMAL )
CPUCPU DLDL
Poly.Poly.
Per
Vertex
Per
Vertex
RasterRaster FragFrag FBFB
PixelPixel
TextureTexture
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Material Properties
• Define the surface properties of a primitive
glMaterialfv( face, property, value );
– separate materials for front and back
GL_DIFFUSE Base color
GL_SPECULAR Highlight Color
GL_AMBIENT Low-light Color
GL_EMISSION Glow Color
GL_SHININESS Surface Smoothness
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Light Properties
• glLightfv( light, property, value
);
– light specifies which light
• multiple lights, starting with GL_LIGHT0
glGetIntegerv( GL_MAX_LIGHTS, &n );
– properties
• colors
• position and type
• attenuation
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12. Surface lighting
 Surface lighting properties
 Values defined in RGB colorspace
 Ambient
 Light reaching all points on a surface
 Diffuse
 Light scattering from rough surfaces
 Specular
 Light reflection from shiny surfaces
 Shininess
 Defines the spread of the specular term
 Larger values have more “mirror” like specularity
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13. Lighting model
 Light Properties
 Ambient
 Color and intensity of light that interacts with a
surface materials ambient property
 Diffuse
 Color and intensity of light that interacts with a
surface materials diffuse property
 Specular
 Color and intensity of light that interacts with a
surface materials specular property
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14. Lighting model
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Ambient Diffuse+ Combination=Specular+
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Controlling a Light’s Position
• Modelview matrix affects a light’s position
– Different effects based on when position is
specified
• eye coordinates
• world coordinates
• model coordinates
– Push and pop matrices to uniquely control a
light’s position
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16. GLUT simple geometry
 GLUT can render simple geometry
 Sphere, Box, Cone, Torus, Teapot, etc
 void glutSolidSphere(GLdouble radius,
GLint slices, GLint stacks);
 radius: radius of the sphere
 slices: number of longitudinal subdivitions
 stacks: number of latitudinal subdivitions
 void glutSolidCube(GLdouble size);
 size: length of all sides of the cube
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17. 17
Tips for Better Lighting
• Recall lighting computed only at vertices
– model tessellation heavily affects lighting
results
• better results but more geometry to process
• Use a single infinite light for fastest
lighting
– minimal computation per vertex
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18. Setting up state for lighting
 Lighting and lights are disabled by default
 Enabling lighting overrides the use of assigned colors
from glColor calls
 glShadeModel(GL_SMOOTH);
 Set Gouraud shading
 glShadeModel(GL_FLAT);
 Set Flat Shading
 glEnable(GL_LIGHTING);
 Enable fixed function lighting
 glEnable(GL_LIGHT0);
 Enable light 0 in the scene
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19. Hidden Surface removal
• First Enable Culling
– glEnable(GL_CULL_FACE);
• Define which face to cull
– glCullFace(GLenum face);
– Face: The polygon facing
• GL_FRONT, GL_BACK,
GL_FRONT_AND_BACK
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20. Assigning Material Properties
 void glMaterialfv( GLenum face,
GLenum pname,
const GLfloat * params);
 face: The geometry facing to assign
 GL_FRONT,GL_BACK, or GL_FRONT_AND_BACK
 pname: The material property to assign
 GL_AMBIENT,GL_DIFFUSE,GL_SPECULAR, and
others
 params: pointer to an array of values
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21. OpenGl calls array pointers
 Some sets of OpenGL methods take multiple
numbers and types of arguments
glVertex2i
Type of argumentNumber of arguments
glMaterialfv
Type Suffix
GLbyte b
GLshort s
i
f
d
ub
GLushort us
ui
GLint,
GLsizei
GLfloat,
GLclampf
GLdouble,
GLclampd
GLubyte,
GLboolean
GLuint,
GLenum,
GLbitfield
Method takes an array
pointer argument
GLfloat mat_ambient[] = { 0.3, 0.3, 0.3, 1.0 };
glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
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22. Assign Material Property
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//Define GLfloat arrays to hold material data
GLfloat mat_ambient[] = { 0.3, 0.3, 0.3, 1.0 };
GLfloat mat_diffuse[] = { 0.8, 0.8, 0.8, 1.0 };
GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
//Shininess will be an array of length 1
GLfloat mat_shininess[] = { 50.0 };
//Apply the material properties to the front
facing
glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);
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23. Assigning Light Properties
 Similar to Materials
 void glLightfv(GLenum light,
GLenum pname,
const GLfloat * params);
 light: The light to assign
 GL_LIGHT0, GL_LIGHT1, …, GL_LIGHT8
 pname: The light property to assign
 GL_AMBIENT,GL_DIFFUSE, GL_SPECULAR, and others
 params: pointer to an array of values
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24. Assigning Light Properties
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//Define GLfloat arrays to hold light data
GLfloat light_ambient[] = { 0.3, 0.3, 0.3, 1.0 };
GLfloat light_diffuse[] = { 0.8, 0.8, 0.8, 1.0 };
GLfloat light_specular[] = { 1.0, 1.0, 1.0, 1.0 };
//Apply the light properties to LIGHT0
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
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25. Point Light
 A light source originating from a zero-
volume point in the scene
 Casts light in all directions
//Light position
GLfloat light_position[] = { 50.0, 100.0, -50.0, 1.0
};
//Apply the light position
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
Position
For Positions this
should be 1.0
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26. Directional Light
 A light infinitely far away from the drawn
scene
 Used most often for emulating sunlight
 Distance from sun to earth is large
 Light direction can be considered the same
//Light direction down the negative x axis
GLfloat light_direction[] = { -1.0, 0.0, 0.0, 0.0 };
//Apply the light direction
glLightfv(GL_LIGHT0, GL_POSITION, light_ambient);
Direction
-should be normalized- For Directions this
should be 0.0
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27. Spotlights
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28. Spotlight
 A light source originating from a zero-
volume point in the scene
 Direction
 Direction the light is focused on
 Cutoff
 angle that defines light cone
 Exponent
 Concentration of the light
 Brightest around the center
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29. Spotlight example
//Spotlight properties
GLfloat light_position[] = { 50.0, 100.0, -50.0, 1.0 };
GLfloat light_spot_direction[] = { 0.0, 0.0, -1.0};
GLfloat light_spot_cutoff[] = { 25.0 };
GLfloat light_spot_exp[] = { 2.0 };
//Apply the light position
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION , light_spot_direction);
glLightfv(GL_LIGHT0, GL_SPOT_CUTOFF , light_spot_cufoff);
glLightfv(GL_LIGHT0, GL_SPOT_EXPONENT , light_spot_exp);
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30. Shading
• Flat Shading
– Flat shading selects the computed color of
just one vertex
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31. Shading
• Gouraud Shading
 Lighting color calculated per-vertex
 saves computation
 Vertex color is bilinearly interpolated
across polygon
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32. Flat vs Gouraud shading
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Flat Gouraud
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33. Drawing a Smooth-Shaded Triangle
// Enable smooth shading
glShadeModel(GL_SMOOTH);
// Draw the triangle
glBegin(GL_TRIANGLES);
// Red Apex
glColor3ub((GLubyte)255,(GLubyte)0,(GLubyte)0);
glVertex3f(0.0f,200.0f,0.0f);
// Green on the right bottom corner
glColor3ub((GLubyte)0,(GLubyte)255,(GLubyte)0);
glVertex3f(200.0f,-70.0f,0.0f);
// Blue on the left bottom corner
glColor3ub((GLubyte)0,(GLubyte)0,(GLubyte)255);
glVertex3f(-200.0f, -70.0f, 0.0f);
glEnd();
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