9

Speed By Surface O perator 171
musthaveShowMapInViewportturnedonin
the Material Editor.
The options are as follows:
•
Grayscale M ultiplier—Lets material luminance
control speed, with darker areas producing
slower particles and lighter areas producing
faster ones. The software multiplies the
luminance of the material near each particle,
converted to a percentage, by the particle’s
current speed. A luminance of 0 converts to
0%, of 128 converts to 50%, and of 255 converts
to 100%.
For example, if the speed of a particle traveling
at 50 units per second is influenced by a pixel
whose luminance is 90, the resulting speed is
90/255*50, or about 17.6 u nits per second.
•
Signed Gr ays cale—Works like Grayscale
Multiplier, but the multiplier can be negative
as well, causing reversal of motion. Signed
Grayscale uses a material luminance value of
128 as the midpoint, and assigns it a multiplier
of 0%. Luminance va lues from 0 to 127 result in
multipliers of -100% to about -1%, respectively,
and values of 129 to 255 result in multipliers of
about 1% to 100% , respect ively.
•
RGB as World XYZ Mult.—Works like Grayscale
Multiplier, but uses the intensity of the
material’s red, green, and blue channels to
affect particle speed on the world X, Y, and
Z axes, respectively. So, for example, if the
materialpixelispurered,thatis,itsRGB
value is (255,0,0), then the particle will retain
its current speed on the world X axis, but its
speed on the Y and Z axes will be reduced to 0.
Similarly, a medium-yellow pixel (128,128,0)
willcausespeedontheworldXandYaxesto
be reduced by half, and will cut speed on the
Zaxisto0.
•
RGB as Local XYZ Mult.—Works l ike RGB a s
Worl d X YZ Mu lt. , b ut u s es t he o bject’s l o c a l
coordinates rather than world coordinates.
Use Sub-Material—When on, uses a sub-material
from the Multi/Sub-Object material assigned to
the Surface Geometry object to define speed.
This option al lows usage of “inv isible” materials
for control ling particle speed. If the emitter uses
a Multi/Sub-Object material but its geometry
doesn’t use the ID that corresponds one of the
sub-materials, the sub-material doesn’t appear.
However,theoperatorcanuseittocalculate
the density of particle placement. The software
assumes the material to be applied to the entire
object surface.
Mtl ID—Specifies the material ID of the
sub-material to be used for particle speed cont rol.
Direction group
The Direction drop-down list lets you specify
which way the particles go after they’re born.
Default=Surface Normals. In most cases,
the actual direction also depends on the icon
orientation. The primary exception is when
Position > Location is set to Pivot.
Par ticle movement is always in a s traight line
unless influenced by other factors.
Sur face Normals—Each particle moves along a line
perpendicular to the nearest face. The direction
the surface faces doesn’t matter.
Out Of Surfa ce—Particles move away from the
closest face.
Tip: You can use this option to confine pa rticles to
the interior of an object. Position the emitter inside
the object, designate the object as the Surface
Geometry, choose Control Speed Continuously,
and choose Out Of Surface. Each time a particle
comes close to a surface, it turns to travel directly
away from the surface. Control the particles’ travel
range with the Speed sett ing, but keep in mind that
particles moving very fast might “escape” their
container. If this happens, increase the Accel Limit
setting; this lets the particles turn more quickly.