Hidden Surface & Lighting

Hidden Surface & Lighting Presentation modified from a presentation at OpenGL.org

Hidden Surface • Hidden Surface Removal is very important in 3D scenes • Only surfaces closest to the eye should be seen and objects that are hidden by others should be eliminated. • The use of a depth buffer facilitates hidden surface removal.

Hidden Surface Removal • To use depth-buffering, you need to enable it: • glutInitDisplayMode (GLUT_DEPTH | ..); • glEnable (GL_DEPTH_TEST);

Hidden Surface Removal • Initialize the depth buffer and color buffer by using: • glClear(GL_DEPTH_BUFFER_BIT | • GL_COLOR_BUFFER_BIT);

Hidden Surface Removal • glutInitDisplayMode (GLUT_DEPTH | ..); • glEnable (GL_DEPTH_TEST); • ... • //in display function • glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); • draw3DObjectA(); • draw3DObjectB();

Lighting Principles • Lighting simulates how objects reflect light • Lighting properties • Light’s color and position • Material composition of object • Global lighting parameters • ambient light • two sided lighting • available in RGBA mode

Lighting Properties: • Light comes from light sources that can be turned on/off • Ambient lighting comes from light that is so scattered there is no way to tell its original location • Example: Backlighting in a room

Lighting Properties: • The diffuse component of lighting is light that comes from one direction. • Once it hits a surface, it is scattered equally in all directions

Lighting Properties: • Specular light comes from a particular direction. • It bounces off the surface in a particular direction

RGB Values for Lights • A light source is characterized by the amount of red, green, & blue light it emits. • Examples: If R=G=B=1.0, the light is the brightest possible white. • If R=G=B=.5, the color is still white, but only at half intensity, so it appears gray. • If R=G=1.0 and B=0.0, the light appears yellow.

Setting Lighting Properties • glLightfv( light, property, value ); • light specifies which light • multiple lights (at least 8), starting with GL_LIGHT0 • properties • Colors for ambient, diffuse, & specular components • position and type • attenuation

Types of Lights • OpenGL supports two types of Lights • Local (Point) light sources • Infinite (Directional) light sources • Type of light controlled by w coordinate

Light Sources (cont.) • Light color properties • GL_AMBIENT • GL_DIFFUSE • GL_SPECULAR

Turning on the Lights • Flip each light’s switch glEnable( GL_LIGHTn ); • Turn on the power glEnable( GL_LIGHTING );

Controlling a Light’s Position • Modelview matrix affects a light’s position • Different effects based on whenposition is specified • eye coordinates • world coordinates • model coordinates • Push and pop matrices to uniquely control a light’s position

Advanced Lighting Features • Spotlights • localize lighting affects • GL_SPOT_DIRECTION • GL_SPOT_CUTOFF • GL_SPOT_EXPONENT

Advanced Lighting Features • Light attenuation • decrease light intensity with distance • GL_CONSTANT_ATTENUATION • GL_LINEAR_ATTENUATION • GL_QUADRATIC_ATTENUATION

Light Model Properties • glLightModelfv( property, value ); • Enabling two sided lighting GL_LIGHT_MODEL_TWO_SIDE • Global ambient color GL_LIGHT_MODEL_AMBIENT • Local viewer mode GL_LIGHT_MODEL_LOCAL_VIEWER • Separate specular color GL_LIGHT_MODEL_COLOR_CONTROL

How OpenGL Simulates Lights • Gourad lighting model • Computed at vertices • Lighting contributors • Light properties • Lighting model properties • Surface material properties

Material Properties • Material’s color depends on % of incoming R,G,B light it reflects. • Material Properties: • Ambient is color of the object from all the undirected light in a scene. • Diffuse is the base color of the object under current lighting. There must be a light shining on the object to get a diffuse contribution. • Specular is the contribution of the shiny highlights on the object. • Emission is the contribution added in if the object emits light (i.e. glows)

Material Properties • Color Components for lights mean something different than for materials. • Example: if R = 1, G = 0.5, and B = 0 for a material, that material reflects all the incoming red light, half the incoming green light, and none of the incoming blue light.

GL_DIFFUSE Base color GL_SPECULAR Highlight Color GL_AMBIENT Low-light Color GL_EMISSION Glow Color GL_SHININESS Surface Smoothness Material Properties • Define the surface properties of a primitive • glMaterialfv( face, property, value ); • separate materials for front and back

Light Material Tutorial

Light Position Tutorial

Per Vertex Poly. Frag FB Raster CPU DL Texture Pixel SurfaceNormals • 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 )orglEnable( GL_RESCALE_NORMAL )

Steps for adding lighting • 1. Define normal vectors for each vertex of every object. These normals determine the orientation of the object relative to the light source. • Create, select, and position one or more light sources. • Create and select a lighting model, which defines the level of global ambient light and the effective location of the viewpoint. • Define material properties for the objects in the scene.

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

Hidden Surface & Lighting