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Amiga Plus Extra 1996 #6
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AmigaPlus-eXtra-6-96.iso.7z
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programme
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a-render
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light.doc
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1987-03-22
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Appendicies
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Lighting Basics:
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Lighting: ( diffuse, specular, and global )
When A-Render calculates a scene, at least 1 light component must be
present. As in the real world, if there is no light, nothing can be
seen! There are several different lighting components that contribute
to the total intensity calculation for an object in the scene.
These are:
Global Lighting.
Diffuse Lighting.
Specular Lighting.
Global lighting :
In the real world, a lot of light bounces around you and illuminates
more than just what the light directly shines on. For example, look
under a table. What do you see ? Well it doesn't really matter,
the point is you can at least see under the table and see the floor,
and maybe you leaned down far enough to see the bottom of the table.
Most probably, you don't have a light under the table, but you can
still see things under it. How did the light get there for your eye
to see ? The light started at the sun and came in a window, or it
started at a lamp in your room. Then it bounced around your
environment, off a few items, and scattered all around your room to
create a general level of light. This light is called the 'global
light' because it is applied globally (universally) to all the
objects in a scene.
Now, really, the global intensity is not the same all over the room, but
the lighting is most probably pretty even. We usually use a small
amount of global lighting in an A-Render scene to be sure all surfaces
will get some light and will be visible (remember, if there is NO
light on a surface, that surface will not even be visible).
In A-Render, we assume the global lighting to be the same everywhere in
the scene, not dependent on the 'light bouncing around a room' effect.
When you specify global lighting, you specify the red, green, and blue
intensities. All objects in the scene will be lit with this global
lighting. Global lighting is usually a small to medium light
contribution to a scene.
Besides global lighting, there are two other contributors to the
lighting of a surface, diffuse, and specular lighting. Look at your
computer for this example. Look at a large flat area, like the side
of the monitor, or the top of your Amiga that is exposed around the
monitor. Make sure the area you are observing is not in shadow, it
must be directly illuminated by a light.
Diffuse :
Diffuse lighting is the major contributor to the light you see
reflecting off of an object. Looking at your Amiga, the majority of
the light you see is the diffuse lighting. It usually will illuminate
the whole object, except what's in shadow. Diffuse light reflects in
the color of the object. On your Amiga, the diffuse reflected light is
beige, so you will see the object (the Amiga) as beige. Notice that as
you move your eye around, the diffuse light does not appear to move,
maintaining an unchanging illumination on the surface.
Specular :
The specular light reflection on your Amiga is the bright but usually
small highlites you see on the edges of your Amiga. Specular lighting
is usually closer to the color of the light source than to the color
of the object. The highlights on your Amiga look white, assuming your
light source is white.
Specular lighting is usually a small contributor to the amount of
light you see reflected off an object, but it is very important to
have it present to try and generate images with a high degree of
realism. To make an object look shiny, make the specular contribution
high, and the diffuse contribution low. Set the COEFFICIENTS for this.
Notice that as you move your eye around, specular light reflections
appear to move along the edges. If your eyes are sharp, you will even
see a white 'glare' off the surface off your Amiga. This specular
light reflects in a large glare because the surface is rough and has a
random texture. Remember, specular light always depends on the
location of your eye, and will move when your eye does.