GB2265064A - Distortion compensation in projection systems - Google Patents

Abstract

An image is projected onto a screen by a projector which includes a spatial light modulator (24). This comprises an array of individual elements (28) which are modulated in response to a pixellated signal from an image source (36) and thereby apply light to a projection lens (30). Because projections are seldom positioned in relation to projection screens so as to project a plane undistorted image a rectangular image will usually appear to be keystone shaped when projected. In order to remove the distortion from the projected image a special effects processor (38) modifies the image before it is applied to the spatial light modulator (24). <IMAGE>

Description

PROJECTION SYSTEMS This invention relates to projection systems which use a spatial light modulator such as a deformable mirror device responsive to video data, to modulate beams of light for projection onto a screen.

For a description of the type of spacial light modulator used in the projection system of the present invention reference may be made to Hornbeck, L. J., "Deformable Mirror Spacial Light Modulators" SPIE Critical Review Series Vol. 1150, pp. 86 to 102.

A description of a projection system which uses such deformable mirror devices is given in International Patent Application No.

WO-A-9115923.

Projectors of this type comprise for each of the red, green, and blue colour signals, a spatial light modulator such as a deformable mirror device (DMD) modulated by digital signals representing one of the R, G, and B components of a video signal.

The DMD in this type of application may comprise an array of deflectable mirrors upon which a light beam impinges and is reflected through a lens for projection. There is one mirror for each picture point or pixel and each mirror is typically of the order of 20 microns square. The mirror is termed a digital mirror because it has two positions an "on" and an "off" position. In the on" position incoming light is directed through the projection lens to a display screen, and in the "off" position, light is deflected away from the projection lens, so that no light reaches the screen.

Each of the R, G, and B components for a pixel is digitally represented by an 8 bit number. Taking the example of the red component, an all O's condition for the 8 bit number for a pixel will represent a red component of 0 for that pixel. An all l's condition for the red component for the same pixel will represent the maximum intensity of red light to be applied to the screen for that pixel.

The magnitude of light from each pixel being reflected through the projection lens onto the screen is controlled by varying the "on/off" mark space ratio of the mirror for that pixel, i.e. the proportion of time in a "frame" period for which each mirror is in the "on" position. A picture displayed at 50 frames per second will have a frame period of 20ms and the maximum amount of light would be obtained if the mirror was on the full 20ms in each frame period. Thus for a red component which has a digital value between the all 0's condition and the all l's condition a mirror will be "on" for a fraction of this 20ms frame period proportional to the 8 bit digital number.

It will therefore be appreciated that using a separate DMD for the full pixel array for each of the R, G, and B components and modulating the light by varying the "on/off" mark space ratio for each pixel in each component enables an accurate full colour representation of the video data to be projected onto the display screen.

An auditorium in which such a projector is used is shown in figure 1. A projector 2 comprising DMD devices is arranged to project an image through a lens 4 onto a screen 6. The screen is positioned in front of a plurality of seats 8 arranged on a stepped rake 10. This stepped rake rises from a position 12 adjacent the bottom of the screen to a position 14 level with an upper portion of screen. The projector 2 is positioned above this position 14 and is at a level above an axis transverse and central to the screen 6. Because of the position of projector 2 the distance which light beams 16 emerging through the lens 4 and impinging on the upper half of the screen have to cover is considerably less than that covered by light beams 18 emerging through the lens 4 and impinging on the lower part of the screen.The projector is, of course, angled to point slightly downwards so that the image is projected wholly onto the screen.

Because of the greater distance light beams have to travel to reach lower parts of the screen the horizontal dispersion of light increases with distance down the screen and consequently there is an increase in the width of the picture projected onto the screen with distance down the screen. In figure 2 the picture projected on the screen 6 by the arrangement of figure 1 is shown.

The projected picture area 20 is a trapezium with its longer dimension at the base where the light beams 18 impinge on the screen. This projected shape is also referred to as a keystone shape it being equivalent to an inverted keystone. Thus there is distortion of the picture projected onto the screen B.

There have been various schemes used in the past to eliminate this keystone shape on the screen. The simplest of these involves masking off the ends of the projected image such that it appears rectangular. This does not remove distortion in the actual picture and is therefore only suitable for small amounts of distortion.

Another solution has been to tilt the screen such that the top and bottom are equai-distant from the projector and thus the projected image is rectangular. However, this tilting of the screen tilts the optimum viewing axis away from the audience. The result is that an image with reduced brightness is seen by the audience.

Other solutions have been proposed which involve introducing an optical correction either in the projector lens or on the screen. These have proved to be very expensive and have introduced some reduction in picture quality.

Preferred embodiments of the present invention seek to overcome the problem of distortion in a keystone shaped projected image.

The invention is defined in the appended claims to which reference should now be made.

A preferred embodiment of the invention will now be described in detail by way of example with reference to the accompanying figures in which: Figure 1 shows a DMD projector in use in an auditorium as discussed above; Figure 2 shows the shape of the image projected on the screen in the auditorium of figure 1; Figure 3 shows a block diagram of a DMD projector for use in an embodiment of the present invention; Figure 4 shows the image area on the DM) produced by the special effects processor of figure 3; and Figure 5 shows the DMD image area projected on a screen by the projector of figure 3.

Figure 3 illustrates the projection of one of the R, G and B components of the picture by the projector. A light source 22 provides illumination for a DMD 24 via a condenser assembly 26 which provides, in this example, a divergent light beam. The DMD includes a plurality of mirrors 28 which are movable between a position in which light from the condenser 26 is reflected on to a projection lens 30 (the position shown in bold) and a position where the light beam is reflected away from the projection lens in a beam 32 (the position shown in dotted lines).

The DMD 24 receives signals.to move the mirrors between the two positions from a DMD controller 34 in accordance with pixel data received from a picture source 36. This source is typically a source of digital video signals. A digital special effects processor 38 is provided between the picture source and the DMD controller so that special effects may be included in the picture projected onto the screen 6.

In the example of figure 3 used in the arrangement of figure 1 the whole projector is angled to point slightly downwards such that the beams 16 intercept the top of the screen 6 whilst the beams 18 intersect the bottom of the screen. This leads to distortion of the projected image as discussed above.

In order to overcome the distortion caused by the projection angle, as shown in figure 2, it is possible to pre-process the picture such that when projected on the screen is free from distortion caused by the projection angle, i.e. the active picture area is substantially undistorted.

It will be appreciated that the keystone shaped image area of figure 2 is projected with the same number of pixels across the top as there are across the bottom. In order to produce a rectangular picture on the screen 6 it is necessary to reduce the number of pixels used for image data at the bottom of the projected area and gradually increase this with distance up the screen. This is achieved using the digital special effects processor 38 which is able to put a vertical warp on a rectangular picture produced from the picture source 36.

The warp imposed on a rectangular picture emerging from picture source 36 by digital special effects processor 38 is shown in'figure 4. The picture source produces a rectangular array of pixels Y pixels wide. The processor 38, by various filtering processes, makes the rectangular picture keystone shaped by reducing the number of pixels along its base to X pixels whilst leaving Y pixels along the top and imposing a linear taper from Y to X pixels.

Pixels in the areas 40 which were originally used for image data are set to black level so that when they are applied to the DMD device 24 by the control 34 they will not produce any image on the screen, i.e. the mirrors for these areas will always deflect light away from the projection lens.

The projection of the image of figure 4 onto the screen 6 is shown in figure 5. The area 42 of figure 4 which is used for image data is projected onto the rectangular area 42 in figure 5. The effect of the projection is to undo the distortion caused by the taper put onto the picture by the processor 38. Thus a viewer will see the picture projected onto the screen in the rectangular area 42 of figure 5 and will thus see it undistorted. The process of projecting the picture onto the screen and the associated keystone shaped distortion caused by this effectively undoes the distortion put on the picture by the processor 38. The only detrimental effect is the slight loss in resolution towards the bottom of the picture caused by the fact that fewer pixels are used for image data. If a DMD of a sufficiently high resolution is used then this will be imperceptable to a viewer.

The areas 40 of figure 5 which would show any light in the areas of 40 of the image of figure 4 appear black to a viewer since no light is applied to the screen in these areas.

By use of the digital special effects processor distortions in a projected image due to the position of a projector relative to a screen can be compensated for by imposing an opposite distortion on the picture to be projected. Thus a substantially rectangular projected image can be produced for any vertical or horizontal offset of the projector relative to the screen.

The use of this technique for correcting keystone distortion is projection systems is not limited to digital spatial light modulators and can equally be appled to linear spatial light modulators.

Claims (9)

1. A projection system comprising a spatial light modulator having an array of individual elements adapted to be modulated in response to a pixellated signal from an image source thereby applying light to a projection lens for projection onto a screen, and an image processing means arranged between the spatial light modulator and the image source, whereby a distortion can be imposed on the pixellated signal to compensate for any distortion in a projected image caused by the position of the projection system relative to the screen.

2. A projection system according to claim 1 including control means for applying the pixellated signal to the spatial light modulator.

3. A projection system according to claim 1 or 2 in which the spatial light modulator is a deformable mirror device (DMD).

4. A projection system according to claim 1, 2, or 3 in which the pixellated image comprises a video signal.

5. A projection system according to any preceding claim in which the projector is positioned with an offset relative to the screen whereby a substantially rectangular image is subjected to a keystone shaped distortion and the image processing means imposes a compensatory opposite keystone shaped distortion on the pixellated image.

6. A projection system according to any preceding claim in which the image processing means causes at least some elements of the spatial light modulator to be turned off when compensating for distortion.

7. A projection system according to any preceding claim comprising three spatial light modulators, one for each colour component.

8. A projection system comprising a spatial light modulator having a plurality of separately controllable areas for modulating light applied thereto, control means responsive to a video signal to control the separate areas to selectively apply light to a projection lens and thereby to a screen, and an image processing means for distorting the video signal thereby compensating for any distortion in a projected image caused by the position of the projector relative to the screen.

9. A projection system substantially as herein described with reference to the accompanying drawings.