This illustration shows how the perceived intensity of a parallel light is influenced by the angle to the receiving surface.
Note: This document was written for Imagive v.4 DOS. Some small additions have been made describing additions made in later versions for Windows.
* Imagine is the copyrighted software from Impulse, co.
The lights created in the Detail editor can have several functions unavailable to those made in the Stage or Action editors. On the other hand, the Stage and Action lights have features which make them easier to aim. In all cases, the core functions are set within the Light Source Data requester. This appears upon creation of a light in the Stage or Action editor, or after the "light" button is pushed in the Detail Function Attribute requester.
Lights take on a variety of appearances depending in which editor they were created. Those started in Stage or Action will appear in the Stage as circles. A directional light will have a small dash through its "y" axis. Lights from the Detail editor may either be plain axes, or objects of any shape. Whatever the shape of the object, light is still emitted from the object's axis.
A light set to cast shadows will only display this effect if used in Trace mode. In Scanline, it will behave like any other light. Its rays will pass through any surface without effect.
Ambient light is a global effect. Once it is set, every object in a scene
will show the same increase in brightness. No other alterations are possible
with this illumination. Note that the effect of this is to flatten any
shading. If subtle light sources are also in use, their effect will be
cancelled. At the extreme, everything will appear as flat 2D shapes.
back to Contents
This illustration shows how the perceived intensity of a parallel light is
influenced by the angle to the receiving surface.
41K jpg
This image shows how both a monochrome and a colored diminished light source
falls off in intensity with distance. Pictured are a series of sphereical
shells ranging from a 64 unit diameter to 3072 units.
h5>Addenda:Real World Lights (1/RR) Imagine for Windows added another kind of diminishing light source. Called "real world," the intensity of the light falls off the the inverse square of distance from the light. As with the older 1/R lights, the "surface" of the light is at a radius from the point. Unlike the 1/R light, whose radius is a simple multiple of the light's intensity, the 1/RR light radius is considerabley more difficult to figure precisely off the top of one's head. One may make guestimates based on the following: Every time the intensity quadruples, the radius doubles. As it happens, a light of intensity 1 has a radius of 8 i.u. Applying the rule, intensity 4 yields a radius of 16, intensity 16: radius 32, intensity 64, radius 64, and so forth.
The fall-off is dramatically faster than 1/R lights. A source with intensity 256
will offer maximum brightness up to a radius of 128 i.u. At 512 i.u, there will
be an intensity of 128; at 1024, intensity 64; at 2048, intensity 32, and so
forth. When these lights are used, much more drastic shading effects occur.
This illustration shows the remarkable difference between an 1/RR light placed
at the center of concentric sphere sections, and an 1/R light, both having
intensity = 255. The left section is lit by 1/RR, and the right by 1/R.
Lights created in either the Action or Stage editors can be more readily aimed. Appearing as circles in the Stage editor, those which have been defined as parallel, circular or rectangular lights will havea small line pointing out of them indicating where they are aimed. If the command Display | Show Light Lines is selected from the Display menu, six lines will project from them to indicate bothe the type of light and its size and direction. The test of where the light will fall is found through the use of the command Display | Light View . A list of available actors is presented, and upon selection, that vantage point is used to create the Perspective view.
While lacking these easy aiming tools, the Detail editor light sources have several important options open to them. Any Detail object can be turned into a light simply by clicking the "Light" button, and setting the parameters in the requestor. Light will then be emitted from the position of the object's axis. However, light emitting paths and deformation tool will probabley be of little use.
Note that because the default position for the axis on most polygonal primitives is within the object's surface, they may make a very odd light source. If shadow casting is not turned on, the outside of the object will remain unlit, while everything around it will be illuminated in the manner designated in the requestor. For the object to have any shine to it, the axis must be moved outside the object. This can be used to emulate a lightbulb, for instance, if the object's specularity has been turned up, causing the glow from its own light to highlight itself. Obviously, a plain axis will not have this problem.
Other than placing the axis outside a polygonal object, shadow casting lights have three ways to radiate light. Their filter settings can be turned on, allowing the transparency to transmit a portion of the light. The object can be made into fog. If the fog setting is very low, .01 for example, the light will get out, and the appearance will turn into a bright silhouette. High fog setting will turn the object to a colored smudge, and the effect of being self-illuminated can be regained by increasing the brightness setting. Finally, one may simply cut shapes out of the surrounding polygons to form objects appropriate for lampshades or even jack-o- lanterns.
As of version 3, special "lite" textures have been available for round or rectangular lights. The "softedge" textures offers an essential improvement to shaped lights. It smudges the edge of the illuminated area, similar to the gradual fade of most light fixtures. A discussion of all the variations available from this and the other lite textures is best left to the Texture docs.
Just as Softedge smoothly fades the edges of a lighted area, the Soft
Shadows option makes the edge of a shadowed area fade away more or less
smoothly. As of version 4, these may be automatically called up from the
Detail's light requestor. The number of elemnets in these arrays are set in
Preferences. The deafualt value is 20. The density of their packing is
controlled by the size of the x axis.
back to Contents
Unfortunately, the addition of shadow-casting usually adds significantly to rendering times. In some cases, this change can be enormous. Depending on the complexity of the scene, a single shadow casting light can more than double the rendering time. One of the new "soft shadow" 20 light arrays used in the same scene can increase rendering time by 1400%.
A principle reason for this increase is the amount of time Imagine spends
anti-aliasing the pixels at the edge of the shadow. This can easily be
observed if one is using a version of Imagine which allows real time rendering
previews. As the scanline progresses across the screen, it slows immensely as
it crosses over the boundary of a shadow Although it is not an entirely
effective or desirable work around, lowering the number of anti-aliasing rays
in Preferences can somewhat lessen this slow-down.
back
to Contents
The following techniques make an assumption common to usual methods of naturalistic representation. If an image is consistent in its construction, and does not obviously diverge from usual perceptions, the viewer will tend to assume the image is "real." A simple instance of this occurs in paintings. Artist's colors did not achieve a full spectrum until near the beginning of the 20th century. And even after that, few artists ever work with more than a few hundred hues. Nevertheless, images whose range of colors generally fit ordinary experience are perceived as being "real," despite lacking the subtlety really existing. In like manner, the lighting methods presented here attempt to give approximations of genuine light that will give an semblance of reality.
Even the simplest adaptations of this set up can enhance Imagine quick
renders.
The right section of the illustration shows the effect of placing a point source at co-ordinates x = 0, y = 6000, z = 6000. While it is only set at intensity 64, at such a distance, its rays are perpendicular to much of the infinite ground, thereby giving a general illumination. There are many occassions when a few small lights set off in the distance will brighten up whole scenes.
There are some other side effects from using diminished lights. The penumbra formed by the indispensable softedge texture will be lessened within the zone of maximum brightness. Also, specular highlights on objects will be duller when the diminished light is at a distance.
The reflection and fluorescence
caused by sunlight in a daylight scene effectively makes every object a light
source. One may use a manageable number of lights if the scene is broken
into a number of principle regions. First, there is the disk of the sun.
Then there is a pool of extreme brilliance around the sun caused by the
scattering effects of the atmosphere. Next there is bluish light from the
air, which can be as bright as 20 per cent of the sun light. The horizon can
be considered as the accumulation of all light scattered by the lower
atmosphere and objects projecting up from the ground. The ground itself
colors and returns a large percentage of incident light. This grouping yields
five different kinds of lights.
Consider building a world light set in the following manner. Place an axis at
the world origin. This will be used as the parent of a group. Give it an
appropriate name, such as "center" or "focus." To facilitate placement, make
a sphere with a radius of 512 units, 36 circle sections, and 18 vertical
section. The following light axes will be placed around the edge of this. All
lights will be grouped to the center. Depending on the size of the world to
be illuminated, this group can simply be scaled up or down.
Of course, the numbers listed above for the number of sources and their
strength are just suggestions. Some "days" could use sun arrays that use the
peak 255 illumination. A world light set with a large number of very dim
sources would be useful for mimicing an over cast day. A few inactive axes
could be added for those occassions when special lighting conditions are
present. Each variation can be saved as a state. Note that morphing between
world light states during an anim is likely to create very large files, since
every pixel is likely to change as the lights alter.
When a principle light sources is broken up into smaller units, their rays
will form a broader bright area. Placing more dim lights around 60 degrees
away will keep the shading from being too drastic. The addition of other
small lights around the scene will simulate back scattered light. Because most
of these lights do not cast shadows, their effect will be applied to all
surfaces similar to the omnidirectional illumination from the atmosphere.
Remember that the final images is flat, and the depth cues provided by our
vision is lacking. Because dispersed light tends to flatten out appearances,
one may not want to approach the highly blended light present in contemporary
interiors. Leaving in more shading and retaining the sense of depth is a
good trade-off.
In general, this writer prefers to use mixtures of no fall-off and 1/R lights
for interiors. As noted above. the 1/RR light produces a more rapid transition
between light and dark. While real light does behave in this fashion, real
light always passes through an atmosphere which scatters the illumination
widely, making shading more gradual. An 1/RR light works better for areas that
will look best with strong contrasts, like cavernous rooms, or night scenes.
Sometimes aspects of ambient light are possible through the use of parallel
lights. One may make rectangular parallel lights the shape of walls having a
very short fall off distanceperhaps 2 or 3 units. Adjusted to an appropriate
intensity, these may then be located just a unit away from a similar sized
wall. In this manner, only a specific wall will be affected, and other walls
in the view can have the amount of illumination correct for them.
Essentially, one needs to create two points almost at the same place with
their y axes oriented in the same direction. Depending on the shape of the
fixture into which they will be set, these will be either round or
rectangular shapes. Then one simply needs to size the axes to control the
shape and fall off of the lights. The softedge texture should be added to
each. As with the world light discussed above, any number of variations may
be made on this and saved as states. A group with one yellowish 45 degree
spread light surrounded by an 85 degree spread with very broad penumbrae could
be called a "flood" light. A different state with 15 and 20 degree spreads of
near white light with only a tiny fuzz at the edge would make a passable
"spot" light.
Burning bulbs may be represented in several ways. Most simply, turn on the
surface brightness. However, if the light emitting axis is inside the bulb,
any shadow casting light will be trapped within during a trace. Setting the
fog length to something small like .01 will then allow the light out while
also flattening the bulb's silhouette. If you wish to fake the glare of
light visible when looking at a bulb, you may wish to move the axis just
outside the bulb. Then color the bulb a pale grey and turn specularity all
the way up. This will make a maximum intensity glint which shades rapidly off
towards the bulb's sides.
Imagine's facets are "one sided." This means that only one set of attributes
may be "painted" on any given face. Therefore, two different objects must be
made for a lamp shade that has a bright reflective substance on the inside,
and a matte finish on the outside.
In trace mode, shadow casting filters may be placed over light sources. This
can be done through raising the filter attribute from zero, placing a grey
scale brush on the surface andas a filter map, or adding the transpar.itx.
When a shadow casting light passes through a filtering surface, its intesity
is modifued, but not its color or shape. Thus, one may create soft shadings
with curved lenses, of hard "bat signal" shadows. If one wants to color the
transmitted light, the transpar.itx must be used. When added to an object, and
color of any light passing through will be modified. Used with a colored anim
brush, a movie projector could be made.
A complete description of these is more proper to discussions of Special FX,
Textures, and Brushes. One may use either the global Lensflar special effect,
or apply the Twinkle texture to specific objects that will track to the
camera. As of version 4, this effect is not among the strongest in Imagine's
bag of tricks. Consider using either a paint program or image processor to
produce more striking flare brushes.
Addendum: Imagine for Windows v 1.3+ added two features which can help fake
light beams/ glows. These are "hot" fog and overdrive. Making a fog object
"hot" intensifies the fog color either near the object center, an axis, of an
axial plane. This creates an area of apparently greater density. When
overdrive is invoked it is somewhat as if the gamma of the effected fog area
has been boosted. In combination, these improve effects like "blaster" beams,
or a glow over molten metal. While these effects do nothing to change the the
illumination of a scene, they do suggest glowing or glaring light sources.
back to
Contents
Lighting a World
The previous section attempted to recreate photographers' studio lighting.
This is appropriate for "still life' or "portrait" renders. More elaborate
light arrays are needed for landscape scenes.The question here is not how
many lights one can use, but how few. 
113K jpg
This shows large
structure, over 4000 units across, as lit by a world light array called
"mid-morning," and as lit in a quickrender.
The Sun.
Create and axis, name it "sun," and place it 60 degrees up from the horizon at
the edge of the guide sphere. Next, create four yellowish parallel lights of
low intensity, around 20, 19, 16. Set them to cast shadows. Place them
around the sun just a few degrees away. Aim one directly at the center, and
the others aligned parallel to the first. Group these to the sun, which in
turn is grouped to the world center.
Sun's halo.
Make 4 to 6 slightly brighter, even colored point sources. Place these
evenly within 15 to 20 degrees around the "sun" group. The total illumination
of both the sun and the halo sources does not need to exceed 225. It may be
desirable to make them somewhat less, so that any object with brightness
turned all the way up will then appear to be a light. These may also br
grouped to the sun.
The Sky light.
Create slightly blusih point sources, and place them in the opposite
hemisphere from the sun. Two or three thirty degrees apart from each other,
and no more than 22 units of intensity for the blue component will do.
The Horizon light.
Place these lights between 10 and 15 degress up from the horizon. There
should be at least six point sources. Depending on where the sun is in the
sky, they will be more or less evenly spread around the perimeter. They
should be fairly dim, no more than 25 intensity units. If there are more than
six, the strength should be further decreased.
The Ground Light
The last light source will represent the light reflected up from the ground.
Create a very dim parallel light source. Place it directly below the world
center. Usually this light would fade rapidly but in this example it will
remain undiminished. This lay out is intended to be general purpose, and used
with as little manual adjustment as possible.
The Principles Behind The World Lay Out.
As noted elsewhere, the basic Imagine light source does not dim with distance,
but does fade depending on the angle of the incident light. Specifically, the
light found on a surface is equal to the intensity of the projecting light
multiplied by the sine of the incident light. What this means is that any
face within 10 degrees of perpendicular will be very close to maximum
intensity. The level of illumination will only gradually decrease, and at 60
degrees, there will still be half the light. From there, the brilliance fades
rapidly and drops almost as much in the last 5 degrees as in the first 25.
TIP
One may quickly create diffuse light sources using the automatic soft shadow
casting arrays. Simply place an axis set to a soft shadow casting light in the
scene's "sky." Then scale the axis up so that it fills a large degree of the
scene circumference. This spreads out the invisible elements of the soft
shadow array. When rendering in trace, the scene will be more evenly lit,
although there may be many shadows going off in all directions. Also, note
that there will be a rendering time penalty as is always the case with soft
shadow arrays.
back to Contents
Interior Lighting
Ordinary interior lighting is likely to be the most complex situation to
mimic. The various light actors may be incandescent or floursecent fixtures,
with any number of different shades or defusers. Natural light may also be
present, more often as scattered light than direct rays of sunlight. Most
rooms are small enough that the effect of light repeatedly reflecting from
ceiling to floor to walls is readily noticable. This sort of enviroinment is
best modelled by a method called "radiosity." Due to the huge computational
expense of radiosity, Imgine does not support this. It is left to the user to
contrive plausible approximations.
77K jpg
Pictured is a rather boring room, appropriate for lectures like this,
demonstrating what can be done with a wide variety of light sources.
Handy Tools
In most cases, the following will be used in concert to provide a simulation
of interior lighting
Shaped Lights and Soft Edges
While shadow casting lights will be limited by any shape, it will be
speedier to use the round or rectangular switches whenever possible.
Fortunately, many luminaires project light in just these shapes. The
addition of the softedge lite texture allows the edges of these shapes to be
smudged in varying degrees. The penumbra variable in the texture requester
controls the amount of smudge. A setting of .1 starts the blurring at the
percent of the lights radius, while a .9 shades only the last ten per cent.
Therefore, a flood light can be made with a very large x and z axis, and a
small penumbra setting. A spot light, conversely, will have a small axes,
and a large penumbra value. A flourescent panel provides very dispersed
light, and so a rectangular light with extemely large x and z values coupled
with the tiniest penumbra value can be used.
Controlled Fall Off
The more scattered light is, the more rapidly its effect will decrease. A
globe light hung near a wall may produce 100 lumens at its own height, but as
few as 20 near the floor. To achieve such rapid decreases in Imagine, one
must use the Controlled Fall Off option. With the exception of shaped point
source lights, one needs only to adjust the length of the y axis to quickly
set the lights range. If a conic or pyramidal beam is desired, the size of
the axes needs to be set first to the desired proportions. Then all may be
simultaneously scaled, so that the shape of the beam is not changed when the
size of the y axis is altered to effect the brightness.
Ambient or Unlocated Light
Generally, the Global ambient light is of very limited use. Even a small
amount of it will cause subtle shading to disappear. It is preferable to use
a scattering of dim sources to feign this. In many cases, low intensity point
sources can be placed both within and without a room. Be sure not to allow
these to line up, because they will have a cumulative effect along the
implied axis. With careful adjustment, these small sources can lighten areas
of rooms effected by brilliant directional rays, such as the shafts of
sunlight, which tend to cause a glow in the areas they strike.
back to Contents
Making Luminares
Observation shows that many fixtures have at least two levels of light coming
from them. Primarily, there is the light allowed out by the shade, and then
there is a second broader area formed by the light that has reflected of the
interior of the shade. Therefore, at least two sources must be used to
reproduce this.
back to Contents
Additional Effects
The Other Lite Textures.
While the softedge is almost always desirable, the other textures will only
have use in specific scenes. Frnchwin can be used to break a rectangular
light into "panes," and Venlite will nicely reproduce the strips from light
passed through slits. Stobe simply flashes the light off and on during an
animation. Caustics.itx produces random and chaotic banding in light, and
Purphaze.itx creates similar banding with color
Lensflar FX and the Twinkle Texture
Lens flare is probabley the best thing to come along in computer graphics
since the chrome sphere. This effect happens most noticabley only when very
bright lights are viewed. In many cases, the flare effect is negligable, and
most lighting designers actively try to prevent it. Likewise, most
photographers avoid it when possible. However, it can be used in long focus
camera shots to accentuate depth, or to add an attention getting flash in any
scene.
Bright Fog, Linear/Radial Fade and Filter Maps
A complete description of these is more proper to discussions of Attributes,
Textures, and Brushes. Here, note that shafts and glows of lights can be
simulated with bright fog objects. For example, a long tapered cylinder
colored pale yellow or blue with a fog depth several times greater than its
width and made bright can be used as a car's headlight beam. This can be
attenuated through either the linear texture used in its filter mode, or with
a grey scale gradient brush used as a filter. The Radial.itx filter option can
be used on bright sphere to simulat a glowing ball of light.
41K jpg
A "flower" made of nothing more than portions of spheres and the effects of
lights.
back to Contents