CS 470 Introduction to Computer Graphics

CS 470Introduction toComputer Graphics Color Light Materials

Colors • Consider— • Color, as we perceive it, is, usually, a reflection of light from a light source (the sun, a reading lamp. • One way to view the idea of color is that objects in the world do not really have inherent colors in the abstract sense. • …rather objects reflect light (to different degrees) … and… • due to properties of the objects, portions of the light is reflected and the rest is absorbed. • The part that is reflected we experience as color.

Colors • therefore, what we experience as color is, in fact, a result of the interaction between light and the material that makes up the objects. • with out light, it could be argued, there is no color. Turn out the lights, what do you see? • Light is important – even in CG, artificially colored objects appear flat, lose their “3dness”. • Light adds character to a model

Light • Light strikes an object and it reflected to our eye – how do we model this in CG? • Phong model of light – way consider and model light. • Need to consider four vectors • L

Light • Phong model – need to consider four vectors – • P – a point on the object where the light strikes. • L – a vector from the light source to P • N – the normal vector – perpendicular to the object’s surface. • R – the perfect reflection vector, exactly the same angle from N as the angle from L to N. • V – the viewer vector – some other angle from N defined by the position of the camera. N V L R P

Lights • Light – four types • Diffused reflection – reflected light spreads out from the object, • a dull look • roughly the same regardless of V • does depend on L • Specular reflection – a concentrated reflection • shininess • Best light on R • amount viewed depends on the angular difference between R and V

Light • Light – four types • Ambient reflection – generally available light – usually from reflected and reflected light from other sources • Emissive light – an object can be a light source. • think of LED, Computer Screen, Stop-light. • not effected by other light sources • not effected by its materals • not effect by the angle of V

Lights • In OpenGL lighting is part of the state. • In OpenGL lights can be a mix of different types --- lights can have multiple components. • In OpenGL you have to turn on lighting. • If you turn on lighting, regular coloring no longer works. • …so, how do you turn on lighting?

Lights glEnable(GL_LIGHTING); turns on the OpenGL functionality to use lighting. glEnable(GL_LIGHT0); Enables a specific light

Lights • Lighting modeling requires the program to provide the normal vector N void glNormal3{b|s|i|d|f}(TYPE dx, TYPE dy, TYPE dz); void glNormal3{b|s|i|d|f}v(TYPE coordvec); defines the normal from object to point dx,dy, dz

Lights • Where do you get the values for the normal vector? • calculate them • i.e. the normal for any point on a sphere is projected from the sphere’s center through that point. • …but wait…

Lights • Lighting modeling requires that the normals N are unit normal (i.e. the sum of the squares of dx, dy,dz is equal to 1. glEnable(GL_NORMALIZE); makes N unit normal Rotations and translations do not effect N length Scaling does effect the length of N

Lights void glLight{i|f}(GLenum light, GLenum param, TYPE value); sets properties of light called light param specifies the property being set or modified. value is the value or values being assigned to the property.

Lights • Some light properties – • GL_POSITION - defines the location of the light in x, y, z, w in camera coordinates. • GL_DIFFUSE – RGBA values to define the color of diffused light. • GL_SPECULAR – RGBA values to define the color of specular light • GL_AMBIENT – RGBA values to define the color of ambient light.

Lights • A note about position coordinates • use 4 dimensional coordinates (include w coordinate. • w coordinate acts as a normal/homogenization coordinate. • w=1 means that the light is a point source with a specific location. • w=0 means that the light from a direction rather than a point (some far off place)

Lights • Light defaults – • LIGHT0 – • no ambient light • diffused and specular light is white (1.0, 1.0, 1.0, 1.0) • All other lights – • everything is black (0.0, 0.0, 0.0, 1.0)

Lights • Spotlights • GL_SPOT_DIRECTION – direction to point the spotlight in dx, dy, dz coordinates • GL_SPOT_CUTOFF – the angle in degrees for the spotlight’s cone of light. • default is 180 degrees – which is not a spotlight

Materials void glMaterial{i|f}(GLenum face, GLenum name, TYPE value); void glMaterial{i|f}v(GLenum face, GLenum name, TYPE *value); defines the properties for materials of objects. face can be GL_FRONT, GL_BACK, GL_FRONT_BACK

Materials • Some material properties – • GL_DIFFUSE – RGBA values to define the color of reflected diffused light. • GL_SPECULAR – RGBA values to define the color of reflected specular light • GL_AMBIENT – RGBA values to define the color of reflected ambient light. • GL_AMBIENT_AND_DIFFUSE – allow ambient and diffused reflected light to set the same.

Materials • Some other material properties GL_EMISSION – declares the material to be a light source. Not effected by other light. GL_SHININESS – determines the reflectivity of the object. Concentrates the reflected light around R.

Materials /* brass material */ glMaterialf(GL_FRONT, GL_AMBIENT, {0.33, 0.22, 0.3, 1.0}); glMaterialf(GL_FRONT, GL_DIFFUSE, {0.78, 0.57, 0.11, 1.0}); glMaterialf(GL_FRONT, G_SPECULAR, {0.99, 0.91, 0.81, 1.0}); glMaterialf(GL_FRONT, GL_SHININESS, 78.0);

CS 470 Introduction to Computer Graphics