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<meta name="Author" content="Mark Danks">
<meta name="Author" content="IOhannes m zmölnig">
<title>Lighting</title>
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<center>
<h2>
<u>Lighting</u></h2></center>
Lighting is an important factor is how we perceive the quality of an image.
For example, without lighting and shading, a sphere would just look like
a circle. GEM provides two types of lights, a local light and world
light.
<p>OpenGL uses a vertex lighting model. This means that for every
vertex in the scene, the influence of the light is calculated. The
color for the polygon is then modified by the light value of all of the
vertices. This generally produces a very smooth effect, but you will
occasionally run into rendering artifacts, especially if you use local
lights. For example, imagine you have a local light close a large
square. The corners of the square are far away from the light, so
none of them will be lit very brightly, even though the light itself is
very close to the surface of the square.
<p>It is important to realize that lighting is an expensive operation to
use. The number of polygons that you will be able to render will
be much lower if lighting is turned on. As usual, the complexity
of the scene and the speed of your computer and graphics card will greatly
affect your frame rate.
<p>GEM has only a maximum of 8 lights at one time. If you try to
create more lights than that, you will get an error message.
<p><a href="#Activate">Activate lighting</a>
<br><a href="#world_light">[world_light]</a> - A directional light
<br><a href="#light">[light]</a> - A point light in the world
<br><a href="#Moving">Moving lights</a>
<p><img SRC="tribar.gif" height=13 width=561>
<h3>
<a NAME="Activate"></a>Activate lighting</h3>
Lighting is activated by sending a message to <i>[gemwin]</i>. If you
send "lighting 1", then lighting will be turned on. If you send "lighting
0", then lighting will be turned off. The lighting state is kept
even if you destroy the gemwin. This means that if you close a patch
and open another one, the lighting will still be the same.
<p>Individual lights can be turned on and off by sending a 1 or 0 to the
left inlet of the light object itself.
<p><img SRC="tribar.gif" height=13 width=561>
<h3>
<a NAME="world_light"></a>[world_light]</h3>
A <i>[world_light]</i> is a light which exists infintely far away.
This reduces the computation needed, so your patch can run faster, but
it means that all of the light rays are parallel. The <i>[world_light]</i>
is good for objects like the sun and other lighting affects. This
means that translating a <i>[world_light]</i> has no effect, although rotation
does.
<p>The following patch is 03.lighting/01.world_light.pd.
<center>
<p><img SRC="world_light.jpg" BORDER=1 height=152 width=370></center>
<p>The <i>[world_light]</i> has one extra inlet. The right inlet accepts
three floats to set the color of the light. A <i>[color]</i> object
would do nothing. In this case, the light is being set to purple.
The <i>[world_light]</i> also accepts a debug message. The debug message
turns on and off a graphical representation of the light in the scene.
The <i>[world_light]</i> looks like a cone. The cone shows the direction
that the light is coming from. Remember that the actual position
of the light does not matter, so geos behind the cone will still be lit.
It is the direction of the light that matters. This is why you can
rotate the light.
<p><img SRC="tribar.gif" height=13 width=561>
<h3>
<a NAME="light"></a>[light]</h3>
A <i>[light]</i> object generates a point light in the world. Because
the light is local to the scene, there is more math to generate the effect
of the light on the vertices. However, unlike a <i>[world_light]</i>,
you can translate the <i>[light]</i> object.
<p>Below is the patch 03.lighting/02.light.pd.
<center>
<p><img SRC="light.jpg" BORDER=1 height=215 width=212></center>
<p>The <i>[light]</i> object has a right inlet for the color, just light
the <i>[world_light]</i> object. As this patch shows, the light can
be moved around the scene with both <i>[rotate]</i> and <i>[translate]</i>
objects. If you were to set the translate X value equal to 1.0, then
the sphere would not be lit at all. This is because the light would
be inside of the sphere. When you turn on the debug representation,
it is a sphere with its origin where the light it. The <i>[light]</i>
object does not have any size. It exists as a point source.
<p><img SRC="tribar.gif" height=13 width=561>
<h3>
<a NAME="Moving"></a>Moving lights</h3>
The patch 03.lighting/03.controlLights.pd allows you to move a <i>[light]</i>
and <i>[world_light]</i> object in the same scene to see the difference between
the two objects.
<p>The patch 03.lighting/04.moveSpheres.pd is an example which moves
two spheres around the world. Turn on and off the individual lights
for a demonstration of a local versus infinite light.
<p>The patch 03.lighting/05.materials.pd uses the material objects to
selectively control the color of the object. Notice that the diffuse object
sets the "overall" color, while the specular objects sets the bright reflective
area where the light directly shines.
<p><img SRC="tribar.gif" height=13 width=561>
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