Lighting and Normals

The following methods enable you to add lighting, materials, and normal vectors to your shapes. Using these methods, models can appear shiny or light-absorbing.

Creating Light Sources

Light sources are specifications of a color, position, and direction. JSL allows for up to eight lights (numbered 0 to 7) defined by the Light command, where n is the number of the light.

Light( n, argument, value, ... value )

Note: To turn each light on, issue an Enable (Lighting) and an Enable (lightn) command, where n is the light number. Then, move the light to a position in the scene with a Light(n, POSITION, x, y, z) command.

The value of argument can be any one of those shown in Light Arguments and Default Values. The table shows default values for each argument.

Light Arguments and Default Values
Argument	Default Value	Meaning
AMBIENT	(0, 0, 0, 1)	Ambient RGBA intensity
DIFFUSE	(1, 1, 1, 1)	diffuse RGBA intensity
SPECULAR	(1, 1, 1, 1)	specular RGBA intensity
POSITION	(0, 0, 1, 0)	(x, y, z, w) position
SPOT_DIRECTION	(0, 0, -1)	(x, y, z) direction of spotlight
SPOT_EXPONENT	0	spotlight exponent
SPOT_CUTOFF	180	spotlight cutoff angle
CONSTANT_ATTENUATION	1	constant attenuation factor
LINEAR_ATTENUATION	0	linear attenuation factor
QUADRATIC_ATTENUATION	0	quadratic attenuation factor

Note: The default values for DIFFUSE and SPECULAR in this table only apply to Light 0. For other lights, the default value is (0, 0, 0, 1) for both arguments.

The first three arguments (AMBIENT, DIFFUSE, and SPECULAR) are used to color the light. DIFFUSE is the argument that is most closely associated with the physical color of the light. AMBIENT refers to the property of the light when it functions as a background light. SPECULAR alters the way a light is reflected off a surface.

Specify the position of the light using the POSITION argument. Nonzero values of the fourth (w) coordinate position the light in homogenous object coordinates.

Light in the real-world decreases in intensity as distance from the light increases. Since a directional light is infinitely far away, it does not make sense to attenuate its intensity as a function of distance. However, JSL attenuates a light source by multiplying the contribution of the source by an attenuation factor

where c = CONSTANT_ATTENUATION, l = LINEAR_ATTENUATION, and q = QUADRATIC_ATTENUATION.

To create a spotlight, limit the shape of the light to a cone. Use the SPOT_CUTOFF argument to define the side of the cone, as shown in the following illustration.

Spotlight

In addition to the cutoff angle, you can control the intensity and direction of the light distribution in the cone. SPOT_DIRECTION specifies the direction for the spotlight to point; SPOT_EXPONENT influences how concentrated the light is.

Lighting Models

Lighting models are specified with the Light Model command.

Light Model( argument, value,...,value )

Light models specify three attributes of lights.

•	The global ambient light intensity

•	Whether the viewpoint is local or is an infinite distance away

•	Whether lighting calculations should be performed differently for the front and back faces of objects.

Light Arguments and Default Values shows the three valid arguments for the Light Model command.

Light Model Arguments and Default Values
Argument	Default Value	Meaning
LIGHT_MODEL_AMBIENT	(0.2, 0.2, 0.2, 1)	Ambient RGBA intensity of the entire scene
LIGHT_MODEL_LOCAL_VIEWER	0 (false)	how specular reflection angles are computed
LIGHT_MODEL_TWO_SIDE	0 (false)	nonzero values imply two-sided lighting

Normal Vectors

Normal vectors point in a direction perpendicular to a surface. For a plane, all normals are the same. For a more complicated surface, normals are more complicated. JSL enables you to specify the normal vector for each vertex. These normals specify the orientation of the surface in space, necessary for lighting calculations. Accurate normals assure accurate lighting.

The normal vector is of length 1 and is perpendicular to the vertex. Typically, a vertex is shared between several polygons and a smooth shaded effect is desired, so the perpendicular at the vertex is calculated as a (possibly weighted) average of the polygon’s normals. It is important to calculate the “outward” normal for polygons unless two-sided shading is enabled because only the outer face of the polygon is illuminated. With a scaled polygon, the normal’s length is not 1 after scaling, and the lighting is wrong.

Normal vectors are set at the same time the surface is constructed, and are specified with the Normal command. Use the Enable(NORMALIZE) command to have the normals re-normalized to 1 each time the scene is drawn.

Shading Model

The shading model of a polygon is set using the Shade Model command.

Shade Model (mode)

where mode can be SMOOTH (the default) or FLAT. SMOOTH shading interpolates the colors of the primitive from one vertex to the next. FLAT mode duplicates the color of one vertex across the entire primitive.

The following script changes the color at each of a triangle’s vertices. The FILL shade model interpolates the color of the interior automatically.

// make a scene box...holds an OpenGL scene.

scene = SceneBox( 200, 200 );

// put the scene in a window.

NewWindow( "Shade Model", scene );

scene << clear;

scene << Ortho2D (-1,1,-1,1);

scene << Shade Model(SMOOTH);

scene << Polygon Mode (FRONT_AND_BACK, FILL);

scene << Begin(TRIANGLES);

scene << color(0, 0, 0); //black

scene << Vertex(-1, -1, 0);

scene << Color(0, 1, 1);//cyan

scene << Vertex(0, 1, 0);

scene << Color (1, 1, 1);//white

scene << Vertex(1, -1, 0);

To set the material properties of a surface, use the Material command.

Material( face, argument, value,...value )

face can be Front, Back, or Front_and_back. (Note that the material properties can be set separately for the front and back faces of a polygon.)

Material Arguments and Default Values shows the arguments and default values for Material arguments.

Material Arguments and Default Values
Argument	Default Value	Meaning
AMBIENT	(0.2, 0.2, 0.2, 1.0)	Ambient color of material
DIFFUSE	(0.8, 0.8, 0.8, 1.0)	Diffuse color of material
AMBIENT_AND_DIFFUSE		Both AMBIENT and DIFFUSE
SPECULAR	(0.0, 0.0, 0.0, 1.0)	Specular color of material
SHININESS	0	Specular exponent that can range from 0 to 128.
EMISSION	(0, 0, 0, 1)	Emissive color of material

Alpha Blending

The BlendFunc command allows for alpha blending. To use it, send a BlendFunc message to a scene, for example:

scene << BlendFunc(SRC_ALPHA, ONE_MINUS_SRC_ALPHA)

SRC_ALPHA and ONE_MINUS_SRC_ALPHA are OpenGL constants that tell BlendFunc to use alpha to blend against the existing display buffer. Disabling z-buffer testing or rendering primitives from back to front might be needed for some applications. By default, the z-buffer tests prevent anything from drawing behind a transparent polygon after it is drawn.

Complete details of all the constants available to BlendFunc (many of which are not useful to the JSL programmer) are available in the OpenGL documentation at opengl.org.

Fog

Fog enables figures to fade into the distance, making for more realistic models. All types of geometric figures can be fogged. To turn fog on, enable the FOG argument.

Enable (FOG)

Example

The following example uses several of the concepts presented in this section, including lighting, fog, and normalization. It draws a spinning cylinder that is affected by two lights.

scene = SceneBox( 300, 300 ); // make a scene box

New Window( "Cylinder", scene ); // put the scene in a window.

for (i=1, i<360, i++,

scene << Clear;

// the lens is 45 degrees, near is 3 units from the camera, far is 7.

scene << Perspective( 50, 1, 10 );

// move the world so 0,0,0 is visible in the camera