9

Phases of Leg Motion 993
adjusting the correct distances of hotspot and
falloff for the light.
Ph as es of Leg M otion
A leg’s motion has four phases, beginning with
the foot on the ground. Then the foot lifts, moves
through the air, and returns to the ground again.
Biped divides this motion into four phases, as
follows:
•
Touch—Occurs at the leg keyframe where the
leg’s foot first touches the ground and always
corresponds with the start frame of a footstep
in Track View — Dope Sheet.
•
Plant—Occurs after touching, and before
lifting. It is always in between the start and end
framesofafootstepinTrackView—Dope
Sheet.
•
Lift—Occurs at the key f rame where the leg’s
foot lifts off the ground, and always corresponds
totheendframeofeachfootstepinTrackView
— Dope Sheet.
•
Move—Occurs while the foot is in the air and
is always in the intervals in between steps in
Track View — D ope Sheet. In wal king, w hi le
one foot mov es, the body is supported by the
other leg. In running or jumping, whi le a foot
moves there is a period where the body is not
suppor ted, and moves in midair.
Ph otometry
When you use photometric lights (page 2–1301),
3ds Max provides physically based simulation of
the propagation of light through an environment.
Theresultsarenotonlyhighlyrealisticrenderings,
but also accurate measurements of the distribution
of light within the scene. The measurement of light
is known as photometry.Thistopicintroducesthe
quantities used for defining and measuring light.
There are se veral theories that describe t he nature
of light. For this discussion, we define lig ht as
radiant energy capable of producing a visual
sensationinahumanobserver.Whenwedesign
a lighting system, we’re interested in evaluating
itseffectonthehumanvisualresponsesystem.
Thus photometry was developed to measure light,
taking into account the ps ychophysical aspec ts of
the human eye/brain system. Four photometric
quantities are used in the lighting simulation
system:
•Luminousflux
•Illuminance
•Luminance
•Luminousintensity
Luminous flux is the quantity of light energy per
unit time arriving, leaving, or going through a
surface.Theunitofluminousfluxisthelumen
(lm), which is used in both t he International
System (SI) of Units and in the American System
(AS) of Units. If we think of light as part icles
(photons) moving through space, then the
luminous flux of a light b e am arrivi ng at a surface
is proportional to the number of particles hitting
the sur face during a t ime interval of 1 second.
Illuminance is the luminous flux incident on
a surface of unit area. This quantit y is useful
for describing the level of illumination incident
on a surface without mak ing the measurement
dependent on the size of the sur face itself. The SI
unit of illuminance is the lux (lx), which is equal to
1 lumen per s quare meter. The corresponding AS
unitisthefootcandle(fc),equivalentto1lumen
per s quare foot.
Part of the light incident on a surface is reflected
back into the environment. The light reflected
off a surface in a particular direction is called
luminance, the quantity that is converted to
display colors to generate a realistic rendering of
the scene. Luminance is measured in candelas per