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

asked.

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