ManipulateLight

Implementing light

When using opengl display, GAMA provides you the possibility to manipulate one or several lights, making your display more realistic.

Index

• Light generalities
• Default light
• Custom lights

Light generalities

Before going deep into the code, here is a quick explaination about how light works in opengl. First of all, you need to know that there are 3 types of lights you can manipulate : the ambient light, the diffuse light and the specular light. Each "light" in opengl is in fact composed of those 3 types of lights.

Ambient light

The ambient light is the light of your world without any lighting. If a face of a cube is not striken by the light rays for instance, this fice will appear totally black if there is no ambient light. To make your world more realistic, it is better to have an ambient light. An ambient light has then no position or direction. It is equally distributed to all the objects of your scene.

Diffuse light

The diffuse light can be seen as the light rays : if a face of a cube is striken by the diffuse light, it will take the color of this diffuse light. You have to know that the more perpendicular the face of your object will be to the light ray, the more lightened the face will be. A diffuse light has then a direction. It can have also a position. Your have 2 categories of diffuse light : the positionnal lights, and the directionnal lights.

Positionnal lights

Those lights have a position in your world. It is the case of point lights and spot lights.

• Point lights

Points lights can be seen as a candle in your world, diffusing the light equally in all the direction.

[TODO Image]

• Spot lights

Spot lights can be seen as a torch light in your world. It needs a position, and also a direction and an angle.

[TODO Image]

Positionnal lights, as they have a position, can also have an attenuation according to the distance between the light source and the object. The value of positional lights are computed with the following formula : diffuse_light = diffuse_light * ( 1 / (1 + constante_attenuation + linear_attenuation * d + quadratic_attenuation * d)) By changing those 3 values (constante_attenuation, linear_attenuation and quadratic_attenuation), you can control the way light is diffused over your world (if your world is "foggy" for instance, you may turn your linear and quadratic attenuation on). Note that by default, all those attenuation are equal to 0.

Directionnal lights

Directionnal lights have no real "position" : they only have a direction. A directionnal light will strike all the objects of your world with the same direction. An example of directionnal light you have in the real world would be the light of the sun : the sun is so far away from us that you can consider that the rays have the same direction and the same intensity wherever they strike. Since there is no position for directionnal lights, there is no attenuation either.

Specular light

This is a more advanced concept, giving an aspect a little bit "shinny" to the objects striken by the specular light. The way specular light is distributed to the environment depends on the way you implemented your diffuse light.

[TODO Image]

Default light

In your opengl display, without specifying any light, you will have only one light, with those following properties :

Those values have been chosen in order to have the same visual effect in both opengl and java2D displays, when you display 2D objects.

Custom lights

In your opengl display, you can create up to 8 lights, giving them the properties you want.

Ambient light

In order to keep it simple, the ambient light can be set directly when you are declaring your display, through the facet ambient_light. You will have one only ambient light.

experiment my_experiment type:gui {
	output {
		display "my_display" type:opengl {
			light id:0;
		}
	}
}

Note for developpers : Note that this ambient light is set to the GL_LIGHT0. This GL_LIGHT0 only contains an ambient light, and no either diffuse nor specular light. However, you still can manipulate the diffuse light for the GL_LIGHT0 through the deprecated facet diffuse_light (not recommanded).

Diffuse and Specular light

In order to add lights, or modifying the existing lights, you can use the statement light, inside your display scope :

experiment my_experiment type:gui {
	output {
		display "my_display" type:opengl {
			light id:0;
		}
	}
}

This statement has just one non-optionnal facet : the facet "id". Through this facet, you can specify which light you want. You can control over 8 lights, through an integer value between 0 and 6. Once you are manipulating a light through the light statement, the light is turned on. To swich off the light, you have to add the facet active, and turn it to false. The light you are declaring through the light statement is in fact a "diffuse" light. You can specify the color of the diffuse light through the facet color (by default, the color will be turn to white). An other very important facet is the type facet. This facet accepts a value among direction, point and spot.

Declaring direction light

A direction light, as explayed in the first part, is a light without any position. Instead of the facet position, you will use the facet direction, giving a 3D vector.

Example of implementation :

light id:0 type:direction direction:{1,1,1} color:rgb(255,0,0);

Screenshot of this effect :

[TODO Image]

Declaring point light

A point light will need a facet position, in order to give the position of the light source.

Example of implementation of a basic point light :

light id:0 type:point position:{10,20,10} color:rgb(255,0,0);

Screenshot of this effect :

[TODO Image]

You can add, if you want, a custom attenuation of the light, through the facets linear_attenuation or quadratic_attenuation.

Example of implementation of a point light with attenuation :

light id:0 type:point position:{10,20,10} color:rgb(255,0,0) linear_attenuation:0.1;

Screenshot of this effect :

[TODO Image]

Declaring spot light

A spot light will need the facet position (a spot light is a positionnal light) and the facet direction. A spot light will also need a special facet spot_angle to determine the angle of the spot (by default, this value is set to 45 degree).

Example of implementation of a basic spot light :

light id:0 type:spot position:{0,0,10} direction:{1,1,1} color:rgb(255,0,0) spot_angle:20;

Screenshot of this effect :

[TODO Image]

Same as for point light, you can specify an attenuation for a spot light.

Example of implementation of a spot light with attenuation :

light id:0 type:spot position:{0,0,10} direction:{1,1,1} color:rgb(255,0,0) spot_angle:20;

Screenshot of this effect :

[TODO Image]

A more advanced feature allows you to add a specular light, through the facet specular_color, to give some more realism to your scene :

[TODO Image]

Note that when you are working with lights, you can display your lights through the facet draw light to help you implementing your model. The three types of lights are displayed differently :

• The point light is represented by a sphere with the color of the diffuse light you specified, in the position of your light source.
• The spot light is represented by a cone with the color of the diffuse light you specified, in the position of your light source, the orientation of your light source. The size of the base of the cone will depend of the angle you specified.
• The direction light, as it has no real position, is represented with 4 arrows striking the 4 corners of your world, with the direction of your direction light, and the color of the diffuse light you specified.

[TODO Image]

Note for developpers : Note that, since the GL_LIGHT0 is already reserved for the ambient light (only !), all the other lights (from 0 to 6) are in fact the lights from GL_LIGHT1 to GL_LIGHT7.