CU Amiga Super CD-ROM 27/CU Amiga Magazine's Super CD-ROM 27 (1998)(EMAP Images)(GB)[!][issue 1998-10].iso/GoodBye/JohnKennedy/February96.lha/TUT9.DOC

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Imagine tutorial

Don't be in the dark:prepare to be illuminated with this
look at Imagine's many light sources.


Getting the lighting right is an important part of creating
good images. This month we'll take a look at how the
position and type of light sources can make the different
between a good render, and one which looks extremely flat
and unrealistic.


Positioning Light Sources


There must be at least one light source present in every
scene before Imagine will even start rendering. For best
results, you'll probably want at least two lights present.
Imagine also has a special form of light source called
"ambient lighting". This is a general light applied equally
to all objects from all directions.

Space scenes are a little different, and you have to decided
whether you want to go for realism (like Babylon5) or simply
what looks best (like Star Trek TNG). Realism requires that
you have a single light source -- usually the sun -- and no
ambient lighting. Although closest to what a giant space
ship floating in deep space would actually look like, this
can be pretty dull looking as a lot of the objects will be
in shadow and lacking detail. The creators of Babylon5 get
around this by ensuring that their star backdrops contain
lots of purple and blue nebula rather than simple darkness.

The ships in Star Trek all seem to manage to look perfectly
lit from all directions, and some even are fitted with their
own light sources to provide illumination.

Back to slightly more practical examples, and there are
generally accepted ways to position light sources for best
effect. A simple scene, say a camera and one object,
requires two light sources. The first is far away and this
provides a "fill" light. The second light is placed close to
the camera to provide the main illumination. You can see the
effects of these lights in the follow illustrations. By the
way, when using digitised backdrop images as shown here, you
should try and arrange the lights so that they cast shadows
in the same way as the sun does in the scanned image.


im9_1.jpg

Here there is only one light source, and it's placed behind
the object. Although dramatic, this is an effect you
probably won't use very often...

im9_2.jpg

Here the single light source has been moved some distance
away, high in the sky as though it were the sun. Notice how
bland the landscape looks.

im9_3.jpg

Now the light has been brought next to the camera, and a
second source placed in the distance to provide a "fill".
The shadows have added depth to the landscape, and
illuminate the detail on the cow object.

im9_4.jpg

Now ambient lighting has been turned on too: notice how the
darker shadows are starting to appear grey. Too much ambient
light and they'll start to appear washed out.




Making invisible light beams visible

It's often desirable to make beams of light visible, so that
the beam itself can be observed. This is quite an easy
effect to achieve, and makes use of the "fog length" of an
object.

Lets say for example you want to make a space ship emit a
beam of light. (Obviously this is what is required to create
a Class 3 tractor beam).

1. In the detail editor, create an object which will be the
beam of light. Create a cone or cylinder depending on what
you want the finished beam to look like.

im9_5.iff

2. Alter the colour of the beam to be white or some other
very light tint. Now adjust the Fog Length. The value will
vary, but try setting it to twice the width of the beam
object to start with.

im9_6.iff

3. Now in the Stage Editor create the light source and position
the beam where you want it. I've added a few other objects
here.

im9_7.iff

4. Render the scene in scanline or trace modes. Here we have
one possible solution to the BSE problem: selling cattle to
alien invaders for their own bizarre breeding experiments.

im9_8.jpg




Different type of Light sources

Imagine 3 introduced the idea of having light sources which
are not necessarily point sources which are spherical i.e.
sending out light in all directions. It's now possible to
control the direction the light travels, and also define the
light beam as either round or rectangular.

There is now other types of light source: a source which
emits parellel rays. Parallel rays are like those from the
sun, and illuminate all objects equally. To select the type
of light source you'll need to create a normal light and
then go to the Action editor and look at the object's
"Actor" line.

im9_9.iff

When you create light sources with either round or
rectangular settings and use the Display menu to switch on
the "Light Lines" option you'll see which direction the
lights are pointing. There are no lines drawn for the
default spherical light source because the light is pointing
at all directions at once.


Normal light source: im9_10.jpg
Round light source: im9_11.jpg
Rectangular light source: im9_12.jpg

You can combine the light beams with foggy objects as
before, to create rather cool searchlight effects. You'll
need to match the shape of the light beam objects to suit
the light source.

In the real world, lights cast shadows and Imagine will do
this too. You can switch shadows on and off from the Action
editor Actor requestor as before. With the shadows turned
on, rendering times are increased dramatically so only use
them when strictly necessary, and only use shadows with the
light sources which required them -- not all of them. This
last image features a round light source with shadows
switched on. There is a fill light in the distance, and
ambient lighting is also turned on. The image is 320 by 256
pixels and took X minutes to render in scanline mode on an
68040 based A4000.


-- end ---