Optics: image projectors – Distortion compensation
Reexamination Certificate
2000-08-18
2002-09-17
Dowling, William (Department: 2851)
Optics: image projectors
Distortion compensation
C353S070000
Reexamination Certificate
active
06450647
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for correcting distortion of an image caused by tilt projection of a projection-type image display apparatus.
2. Discussion of the Background
FIGS.
18
(A)-
18
(D) are explanatory diagrams showing an image projection by a conventional projection-type display apparatus. Generally, a projection-type display apparatus
20
modulates incident light from a light source by a light modulating unit such as a liquid crystal light valve (liquid crystal panel). The modulated light emitted from the liquid crystal light valve passes through a projecting optical system and is projected on a screen
30
. That is, such a projection-type display apparatus enlarges and displays the image displayed on the liquid crystal light valve onto the screen
30
.
As shown in FIG.
18
(A), if the projection-type display apparatus
20
is positioned so the optical axis
20
c
of the projection-type display apparatus
20
matches a normal line
30
n
of the center position of the screen
30
, the image
32
projected on the screen
30
is an undistorted image (hereinafter referred to as a “normal image”) similar to an image represented by image signals supplied to the liquid crystal light valve (FIG.
18
(B)). However, such an arrangement is problematic if the projection-type display apparatus is situated between the user and the screen, since the projection-type display apparatus blocks the view of the user observing the projected image.
Now, a general arrangement is to position the projection-type display apparatus
20
at a position lower than that shown in FIG.
18
(A) (a higher position will suffice as well) as shown in FIG.
18
(C), so the angle &thgr;p between an optical axis
20
c
of the projection-type display apparatus
20
and the normal line
30
n
of the screen
30
(this being hereinafter referred to as “tilt angle”) is more than 0 degrees. Image projection with a projection-type display apparatus positioned with such a certain tilt angle is generally referred to as “tilt projection.” However, if the projection-type display apparatus is situated so as to perform tilt projection, an image
34
projected on the screen
30
is enlarged in the vertical upwards direction more than an image
32
, as shown in FIG.
18
(D). In addition, a trapezoid-shaped distortion (also called “keystone distortion” or “tilt distortion”), in which the image is enlarged horizontally at the upper edge, is generated. Hereinafter, the trapezoid-shaped image displayed on the screen will be referred to as a “distorted image.” Incidentally, if the projection-type display apparatus is situated at a high position, a trapezoid-shaped distortion, in which the image is enlarged horizontally at the lower edge, is generated. The greater the tilt angle &thgr;p, the more this trapezoid distortion increases.
Such trapezoid distortion can be optically corrected by electrically processing the image. FIGS.
19
(A) and
19
(B) are explanatory diagrams showing a method for electrically processing the image to correct the trapezoid distortion. In FIG.
19
(A), a lower edge of an image
34
(distorted image) projected on the screen
30
for tilt projection, and that of an image
32
(normal image) projected on the screen
30
for no tilt projection are positioned at the bottom of the figure. To remove the distortion in the distorted image
34
and obtain an image
34
′ similar to the normal image
32
(FIG.
19
(B)), image signals should be provided to the liquid crystal light valve so the image projected on the screen
30
would be the corrected image
32
′ shown by dotted lines in FIG.
19
(B) if there is no tilt projection. That is, image signals representing the image portion
32
′
a
, which is distorted in an opposite form as the distorted image
34
, are to be applied to the liquid crystal light valve. Incidentally, hereinafter, the image portion
32
′
a
will be referred to as an “effective image portion,” and the other image portions
32
′
b
(hatched) will be referred to as a “non-effective image portion.”
The liquid crystal light valve has a plurality of pixels arrayed in a two-dimensional manner and pixel signals representing the image to be projected are input to each pixel, thus modulating the incident light. Accordingly, there is the need to provide pixel signals to all of the pixels in the liquid crystal light valve. To this end, of the corrected image
32
′, pixel signals representing the image to be displayed are provided to the pixels of the liquid crystal light valve corresponding to the effective image portion
32
′
a
. In addition, pixel signals representing black are provided to the non-effective image portion
32
′
b
so as to obtain an area where nothing is displayed. For tilt projection, providing the pixel signals representing the corrected image
32
′ to the liquid crystal light valve allows the image projected on the screen
30
to not be the distorted image
34
, but the undistorted image (normal image)
34
′ is approximately similar to the image
32
.
An example of such a correction method is described in Japanese Unexamined Patent Publication No. 8-98119. According to this correction method, the effective image portion
32
′
a
of the corrected image
32
′ and the non-effective image portion
32
′
b
thereof are obtained from the relationship between the position of an arbitrary pixel FP′ in the image
34
′ projected on the screen
30
(FIG.
19
(B)) and the position of a corresponding pixel FP (FIG.
19
(B)) in the corrected image
32
′, as a function of the tilt angle &thgr;p. Incidentally, the relationship between the position of the arbitrary pixel FP′ in the image
34
′ and the position of the corresponding pixel FP in the corrected image
32
′ is shown with the following expressions:
Yb
=
Vn
·
sin
[
π
2
+
tan
-
1
(
Vn
-
Vm
2
L
)
]
sin
[
π
2
-
θ
p
-
tan
-
1
(
Vn
-
Vm
2
L
)
]
(1a)
Xb
=
hn
·
L
+
Yb
·
sin
θ
p
L
(1b)
The parameter Yb represents a distance in the vertical direction from the lower edge of the projected image
34
′ to the position of the pixel FP′. The parameter Xb represents a distance in the horizontal direction from the left edge of the projected image
34
′ to the position of the pixel FP′. The parameter Vn represents a distance in the vertical direction from the lower edge of the corrected image
32
′ to the position of the pixel FP. The parameter hn represents a distance in the horizontal direction from the left edge of the corrected image
32
′ to the position of the pixel FP. In addition, the parameter Vm represents a distance between the lower edge and upper edge of the effective image portion
32
′
b
within the corrected image
32
′. The parameter L represents a distance from the projection lens of the projection-type display apparatus
20
to the screen
30
. The parameter &thgr;p represents the tilt angle in the vertical direction (i.e., the angle in the vertical direction between the optical axis
20
c
of the projection-type display apparatus
20
and the normal line
30
n
of the screen
30
).
From the above Expressions (1a) and (1b), a determination can be made regarding to which pixels of the liquid crystal light valve the pixel signals for each pixel in the image
34
′ should be provided in the event of projecting an image
34
′ free of trapezoid distortion on the screen
30
while performing vertical tilt projection.
The above background example illustrates an example in which the optical axis
20
c
of the projection-type display apparatus
20
and the normal line
30
n
of the screen
30
match within a horizontal plane, and the optical axis
20
c
of the projection-type display apparatus
20
and the normal line
30
n
of the screen
30
do not match within a vertical plane so there is a tilt a
Dowling William
Seiko Epson Corporation
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