Image
warping (geometric correction) is used to make an image look visually
correct when
it is projected onto a non-planar screen. This
process is also referred to as distortion
correction or geometric correction. The
image is warped so when it is projected onto the screen it is visually
correct.
Forms
of image warping can be found in many projector models, with a function
known
as keystone correction being the most common.
Keystone correction (trapezoidal in nature) is
used when
projectors have been mounted at an angle (generally pointing down from a ceiling to a screen), and the opposing angular
correction
needs to be applied in order for the image to be correct. Whilst
most
current projectors
offer this feature, in many areas, advanced functions are required. For
example, advanced off-axis correction and projection on
curved and
other unusual surfaces. For
these applications,
geometric correction tools such as Sol7 can be used.
Areas
where geometric correction is required include:
- advanced off-axis correction where projector placement is awkward and
needs an
advanced mapping over the keystone function in a projector;
- for
non-planar screens such a curved screens and hemispherical domes;
- for
projecting one image from one projector onto more than one surface;
- for
unusual projection applications onto custom designed screens.
Figure
1: An example of a cylindrical correction
Figure
2: An example of a linear correction onto two planar walls
Figure 3: An example of an
advanced off-axis correction
Figure 4: An example of spherical correction
To
describe the process of geometric correction in greater detail, we have
taken
the example of projecting onto a curved surface.
For cylindrical and spherical screens,
variations of barrel and pincushion correction functions are required
to
correct the image. The
exact mapping
will be dependent upon the optical characteristics of the projector,
the screen’s
size and shape as well as whether the imagery is front or rear
projected.
Geometric
correction can be performed with some special high-end projectors with
advanced
modules, but they are not common and tend to be expensive. There are also external
electronic devices
which can compensate for any geometric correction, but these also tend
to be
expensive.
Recently
with advances in computer graphics cards, software solutions can now
offer the
ability to perform geometric correction.
One such tool is Sol7.
This
method provides a user with a number of control points which can
be
increased/decreased depending upon the complexity of the screen shape
(see Figure 5). These control points can be manipulated by the
user so an image is aligned correctly to a
screen.
Figure 5: Control points for included for image warping
Typically,
when a standard projector is used on a curved (cylindrical) screen, the
image
gets a characteristic “smile” shape (see Figure 6). To overcome this we
need to use the control
points along the top and bottom of the image to pull the image back
down into
shape. Because the
pixels in the middle of the image will be bigger than the pixels at the
edges, we compensate for this by dragging the
control points near the center of the screen closer together. This tells the software
system that these
parts of the screen are further away and it automatically adjusts. Sol7 displays a
checkerboard, which helps to
create the right image warping pattern. When
all the checks in the checkerboard look the same size, the distortion
correction is correct.
There
are also other screen shapes that can be produced from complex curved and flat sections. One example of a linear based display is a wedge, composed from two flat screens. Here
a
single projector can be used with two advanced keystone correction patterns to project onto a single screen. Figure
7
shows an example of the pattern needed to rear project onto a convex
wedge
screen. To create
these shapes the distortion systems need to be told that they are dealing with flat
sections and rather than curved screens.
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