Edge blending, is a technique used to describe the process of visually
combining several
projected images to make a single seamless image. Typically,
this is done to increase the size
of a projected image to make a very wide image, or by combining a
number of
lower resolution devices together to increase the total resolution of a
display. In both
cases, a number of projectors are
needed which are firstly overlapped and then visually joined together
using an edge blending technique.
Figure
1: A wide image that we want to project using three projectors
At
first glance it might appear that the easiest way to produce this image
across
three projectors would be to line up the projectors next to each other.
This method is
technically referred to as
image butting because the projected images are butted up next to one
another. To try and
accomplish this, the projectors are
carefully arranged so that the right-hand edge of the left-hand
projector sits
perfectly next to the left-hand edge of the right-hand projector.
Unfortunately,
achieving a seamless image using this technique is near impossible. In principle it sounds easy,
but in practice
it is very hard to do. There
are a
couple of reasons for this. Firstly,
different
projectors have different characteristics, even if you are using the
same brand
and model. For
example, an image may
drift as they warm up and the brightness and colors will also differ
slightly even
if the same projector brand and model are used.
Secondly, it is a difficult task to manually adjust a
projector to
exactly the right position so that they match up at the edges. Some areas will overlap
whilst others will have
a gap. This results
in a titled
appearance, rather than a seamless one as shown in Figure 2.
The
answer lies with the use of image blending. When
image blending, we overlap projectors to create
what is referred to as an overlap region. For
a detailed description about overlapping
regions, please read our explanation here.
In this overlapping region the same
proportion of the image is projected from both projectors to provide a
duplicate image region. This
means we
can no longer simply cut the projected image into exact parts to
match the
exact resolution of the total number of pixels used in all projectors,
but we
now have to compensate for overlapping regions.
Figure 3: Wide image with overlapping regions
Figure 3 shows an image with overlapping regions which are overlapped in the
final
projection. A
larger overlapping region
provides better control of the overall image, but this has to be
balanced
against the reduction in image size and the loss of overall number of
pixel used
in the display.
Now
that we have an overlap region, it is a little easier to match up the
projectors
without having obvious discontinuities in the image. But
now we have a new problem: the overlap
region is twice as bright as the rest of the image, as this area now
has the brightness of two projectors as shown in Figure 4.
Figure 4: Double brightness in the overlap region
To overcome the double brightness in the overlapping regions, we can add a brightness (luminance) control function in each of these areas. On first appearances, we could halve the luminance of the overlap region, so that when the two projectors combine, it comes back to the full brightness on a single projector. Unfortunately that makes alignment almost as hard to line up as image butting. The solution is to blend the images together. This is where the blending part of the image blending name comes in: we gradually drop off the luminance from one side as we increase the luminance from the other as shown in Figure 5.
As
one projector gradually takes over from the other, this overcomes the
problems
we identified above: Firstly,
because one
projector slowly gives way to the other, changes in color and
brightness are
less apparent. Secondly,
it is also
quite error tolerant. If
the projectors
don't perfectly line up, the gradual blending makes it hard to notice.
The
amount of edge blending performed in each region is controlled by a
gamma
function. A gamma
ramp (as often used in
computer graphics to deal with luminance), can be used where the
brightness
drops off in a non-linear curve.
A
gamma ramp has a single parameter, called the gamma value. If the gamma value is 1,
then the gamma ramp
is linear. If the
gamma value is less
than 1, then the blend region quickly becomes bright and slowly
approaches full
brightness. If the
gamma value is
greater than 1, then the ramp stays darker and rapidly approaches full
bright.
Figure 6: Examples of gamma ramps: red is γ =
0.3,
blue is γ =
1,
and green is γ =
3.
The
mathematical relationship is o = iγ,
where o and
i are
the output and
input values in the range [0..1] and
γ is
the gamma
value. Typically, a
larger overlap
region, blended with a lower gamma value will produce the best blends. This is because it will
give a slow change
over from one projector to the other.
Figure 7: Image once blended
For
further information please contact us.