Image Processing Reference
In-Depth Information
Fig. 12.5 The camera image
containing the graphics
projected on to the floor. The
two coordinate systems have
been added to underline the
misalignment of the
coordinate system of the
camera and the coordinate
system of the graphics. Note
the four light sources placed
on the floor at the corners of
the graphics
SB stared at Mick while his jaw started to drop and said: “I'm not worthy. That
is fucking brilliant, man.”
“I know,” Mick said, smiling.
12.2.2 Calibration
“How will the Game-block know if the player steps on the right answer?” Mick
After a moment of silence they looked at each other and realized that a calibration
was needed. They both knew the basics about calibration, but calibration involves
math, which neither of them was particular fond of.
What they had realized was that the coordinate system of the projector and the
coordinate system of the camera were different. To better understand the problem
they created Fig. 12.5 . This figure shows what the camera sees, i.e., the coordi-
nate system of the image from the camera is a standard orthogonal one: (x, y) .The
graphics projected by the projector is of course also represented in a standard coor-
dinate system denoted (x ,y ) . But since the projector is tilted and rolled a bit with
respect to the floor, graphics projected on the floor is a bit “off” in the sense that the
opposite sides are not of equal length. Moreover, the camera is also tilted and rolled
with respect to the floor adding to the “off-ness” of the graphics when captured by
the camera. To sum up, the two different coordinate systems are not aligned. But
why is this a problem?
Imagine the player is standing on the correct answer. The Game-block might
have told the graphics to place that answer at (x ,y ) = ( 100 , 200 ) . The Find Feet-
block now takes an image and (correctly) locates the position of the foot as (x, y)
( 307 , 298 ) . How can the Game-block now compare (x ,y ) and (x, y) and figure
out if the player is standing on the correct answer? It cannot. It simply does not
know how to map from one coordinate system to the other. So what a calibration
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