Skew Correction Technology

Trapezoid images of such items as whiteboards, documents, or schedules shot at an angle with a camera can be automatically corrected so that the images appear as if they were shot from the front. This skew correction technology is highly effective in business environments where it is not possible to shoot whiteboards, overhead projectors, schedules, signboards, etc. from the front.

Principle of Skew Correction

When an object is shot at an angle, and not from the front, as shown in Figure 1 (a), the line AC of a rectangle is longer than the line BD, which is further from the camera, as shown in Figure 1 (b). Therefore, the image of the rectangle is distorted to a trapezoid ABCD. Applying the skew correction technology, the distorted trapezoid image is automatically corrected to the rectangle A'B'C'D', which looks as if it were shot from the front, as shown in Figure 1 (c).

(a) Shot at an angle (b) Distorted imagemon? (c) Corrected image
(a) Shot at an angle (b) Distorted imagemon (c) Corrected image
Figure 1: Skew image correction

First, an automatic corner recognition algorithm detects coordinates of corners ABCD and obtains coordinates of corners A'B'C'D'. Then, parameters of the image transformation formula are calculated from the corresponding corner coordinates of ABCD and A'B'C'D'. Finally, the image transformation formula corrects the skew image to a rectangle.

Application Examples of Skew Correction Technology

The distorted trapezoid image of a rectangle can be recognized automatically and corrected in the camera, so that it appears as if it were shot from the front. Image files saved in the camera can be corrected too. The skew correction technology is highly effective in environments where whiteboards, schedules, large signboards, etc. cannot be shot from the front, making text on them easier to read. This technology is also useful for shooting paintings or landscapes that include large buildings.

Examples of images captured by a digital camera equipped with skew correction capability are shown in Figures 2 and 3.


(a)???   (b)???
(a) Before correction   (b) After correction
Figure 2:Images captured by a camera with skew correction

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(a) Before correction   (b) After correction
Figure 3:Images captured by a camera with skew correction

Practical Skew Correction Technology

The skew correction technology automatically corrects images without the user having to designate the four corners of the trapezoid. Advanced corner recognition technology is the key. With Ricoh's recognition algorithm, even if some corners do not appear in the image, corner coordinates can be estimated from the straight lines of the trapezoid. Processing time of corner detection and image transformation in a camera is suitable for practical use.

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