Dewarping Projected Image
Ultra-short-throw projectors have a weakness – when an ordinary screen is used, even minor deflections of the screen distort the image. Ricoh has developed a technology to easily correct such distortions – the user only needs to take a photograph of the image on the screen with a handheld camera, such as a smartphone.
Why images from an ultra-short-throw projector are easily distorted
An ultra-short-throw projector tends to produce distorted images when used with an ordinary suspended or floor-standing screen. An ultra-short-throw projector emphasizes the minor deflections of the screen, which are usually not problematic with ordinary projectors.
In Figure 1, the screen is a little concaved. Thus, the rays of light reach the screen at points higher than they should. Because the viewer sees what is reflected on the screen, the user recognizes the shifting of reflection points as shifting (distortions) of the projected image.
This phenomenon occurs also with an ordinary projector, but it is barely recognizable. An ordinary projector has a long projection distance, which results in a large incident angle and small shifts of the reflection points. An ultra-short-throw projector has a short projection distance, which makes the rays of light reach the screen from below at a very acute angle, increasing the shifts and making images look distorted.
Processing distortions in advance
How can we make the projected image look straight and distortion-free? We need to shift the rays of light in the reverse direction, so that their reflections reach the viewer at appropriate angles. To do so, we need to process the image in advance. To determine how the image should be processed, we need to determine the precise conditions of the screen.
Ensuring stability of determining screen conditions
To determine precise screen conditions, a calibration pattern is projected onto the screen and captured with a camera just once before actual projection. The original calibration pattern is accurately compared with the pattern projected and captured straight from the front side, and distortion of the image is calculated based on their correlations.
Captured images, however, are affected by a variety of factors. Without compensating for these factors, the acquired information is not stable. Several measures were taken in the past: tens of types of calibration images were used; a tripod was used to prevent camera shake; and the entire process was done in a dark room to avoid influence of room lighting, for example. These measures were not practical.
Ricoh adopted to use a pattern of circles arranged in a grid, which is generally considered a stable calibration image. In maximizing convenience, a handheld smart device is a must. So Ricoh set a high goal: calibrating the image based on a single shot and allowing a certain amount of camera shake. Further, Ricoh took into account an ordinary operating environment for a projector (that is, an entirely dark room was not an option). The system we have developed compares each of the circles on the original calibration image accurately with the corresponding circle on the image captured with a camera, and analyzes how the circles are shifted.
Based on the image processing technologies nurtured for many years, Ricoh has developed a new technology to stably determine the positions of the original and captured images in sub-pixel order (in units smaller than a pixel). This technology enables the screen shape to be estimated with high precision.
Once the screen shape is determined, distortions of the projected image can be calculated. Then, the distortion can be offset by deforming the image in reverse direction beforehand. As a result, the viewer sees a distortion-free image.
Figure 6 shows an example of applying the technology. To make the effects easy to see, a significantly deformed screen is deliberately used. An image distorted that much can now be dewarped.
With the new technology, you can easily correct the distortions of an image caused by projecting on a slightly deflected screen. In your usual presentation environment, merely project the calibration image and capture it with your smart device. Our TAMAGO Pita Projection iPad/iPhone application allows you to dewarp your images during a presentation with your iPad/iPhone.
The technology was developed for use with an ultra-short-throw projector, but it can also be used with an ordinary projector. For instance, you can use the technology to correct keystone distortions, which occur when the projector does not face the screen straight.