BACKGROUND OF THE INVENTION Computer vision systems are based on informa- tion obtained from various measuring devices. Informa- tion can be measured using e. g. a laser device, a measuring head or via recognition from an image. The information obtained can be utilized e. g. in quality control systems, where, on the basis of this informa- tion, it is possible to determine e. g. the correctness of shape of an object, coloring errors or the number of knots in sawn timber.

A computer vision system is generally com- posed of cameras. Traditional computer vision systems comprised only one camera, which took a picture of the object. By processing the picture, various conclusions could be drawn from it. By using different algorithms, it is possible to distinguish different levels in im- ages on the basis of their borderlines. The border- lines are identified on the basis of intensity changes. Another method of recognizing shapes in an image is to connect it to masks and filters so that only certain types of points will be distinguished from the image. The patterns formed by the points in the image can be compared to models in a database and thus recognized.

In a three-dimensional computer vision sys- tem, several cameras are needed. To determine a three- dimensional coordinate, an image of the same point is needed from at least two cameras. Most three- dimensional computer vision systems therefore comprise several cameras to allow an object to be imaged from different directions without having to move the ob- ject. The points are formed on the surface of the ob- ject via illumination. The illumination is typically implemented using a laser. The point is imaged by cam- eras calibrated in the same coordinate system. When an image of the point can be produced by at least two cameras, it is possible to determine three-dimensional coordinates for the point. For the same position, a number of points are measured. The set of points thus formed is called a point cloud.

The object to be measured can be placed on a movable support, e. g. a rotating table. Rotating ta- ble'means a support that rotates about its axis. If the object can be rotated, then the camera system need not be able to measure the entire object from one po- sition and normally fewer cameras are needed than when measurements are carried out with the object on a fixed support. The movable support may also be a car- rier moving on rails.

With a computer vision system, it is also possible scan the object and produce from it a model that can be processed. The methods of three- dimensional scanning and discrimination are considera- bly more complicated than corresponding two- dimensional methods.

To measure the overall shape of an object, the point clouds scanned in different positions have to be connected to the same coordinate system. In prior art, point clouds scanned in different positions were combined using various mathematical methods in which different overlapping areas are measured. The overlapping areas must represent the same surface and they must therefore be congruent. The method works poorly if the object has relatively flat shapes.

In an embodiment, known shapes are disposed near the object, e. g. balls, which are repeatedly measured from every position. The overlapping measure- ments and the repeated measurements of the balls in each position take measuring time. Measuring time is also spent on matching the mathematical models meas- ured from different positions. One solution to this problem is to place the object to be measured onto a support and measure the movements of the support by means of various angle or motion detectors. However, the accuracy of these wear-prone components limits the overall measuring accuracy.

OBJECT OF THE INVENTION The object of the invention is to eliminate the above-mentioned drawbacks or at least to signifi- cantly alleviate them. A specific object of the inven- tion is to disclose a new type of method for connect- ing point clouds measured by a computer vision system.

A further object of the invention is to simplify and accelerate the process of measuring the three- dimensional shape of an object.

BRIEF DESCRIPTION OF THE INVENTION The invention describes a method for connect- ing point clouds measured from an object in its dif- ferent positions to the same coordinate system. The system of the invention comprises a movable support, an illuminating device and a sufficient number of cam- eras. In addition, the system comprises means for storing the measured information and calculating coor- dinate transformations.

The object to be measured is fastened to a movable support. The support is provided with a mecha- nism which is used to move it so that the cameras could see it from different directions. The support plate typically has a circular shape and it is rotated about a central shaft, but the support plate may also have some other shape in it may be moved in several directions. The circular support plate movable about a shaft is called a rotating table. Several reference marks are fastened onto the rotating table. The marks may be individually designed to allow them to be iden- tified by the cameras, but they may also be of identi- cal design. The reference marks need not be three- dimensional bodies; instead, two-dimensional marking is sufficient. At the start of a measurement, the cam- eras measure the reference marks on the support plate.

Reference marks may also be attached to the object it- self e. g. by means of a magnet, and it is even possi- ble to place all the reference marks on the object. In this case the procedure involves the drawback that the reference marks may happen to be placed on the area to be measured, thus changing the object shape perceived by the measuring device.

The measurement is carried out by illuminat- ing points on the surface the object to be measured.

The illumination is typically carried out by producing luminous points. Normally, a number of luminous points forming e. g. a matrix are created, thus illuminating a plurality of points simultaneously. The points are im- aged by cameras, and the system can be provided with as many cameras as required. The position of each in- dividual point can be measured accurately when at least two cameras can see it. The point matrix can be moved over the object by deflecting the beams produc- ing the luminous points using e. g. two mirrors. The set of points measured from the same position is called a point cloud. After the object has been meas- ured completely in one position, its orientational po- sition must be changed to allow any blind areas to be scanned.

When the object is turned, the coordinate system comprising it will also have to be turned to allow the new measured points to be placed in the original coordinate system to fill the blind areas.

After the turning, measurement is started by measuring the reference marks. The positions of the reference marks are compared to the positions measured at the beginning of the measurement. The marks are of indi- vidual shape, they can be identified by a camera, but the marks may also be mutually identical. In this case, each mark is identified on the basis of its po- sition in relation to the others, for it does not change although the set of points is moved as an array from one place to another. From the change in the po- sitions of the reference marks, it is possible to cal- culate a transformation of the coordinates of the mov- able support, and the point cloud measured from the new position can be placed mathematically in the original coordinate system on the basis of the coordi- nate transformation.

By utilizing the system and method of the in- vention, the requirements regarding accuracy of motion of the movable support are eliminated, because the change in the coordinates is verified subsequently by measuring the reference points. The elimination of the accuracy requirement makes it possible to move the support plate with a simpler and more advantageous mechanism, by means of which a sufficient number of points can be measured quickly and advantageously. In addition, the elimination of mechanical sensors also improves the measuring accuracy. In the methods used, the required calculation power is low, so the connec- tion of large numbers of points is a fast operation and requires no more than normal calculation capacity.

LIST OF ILLUSTRATIONS In the following, the invention will be de- scribed in detail with reference to drawings, wherein Fig. 1 presents a function diagram represent- ing the method of the invention, Fig. 2 presents an embodiment of the system of the invention.

DETAILED DESCRIPTION OF THE INVENTION In the method represented by Fig. 1, measure- ment is started by placing the measuring object and the reference points onto the movable support 10. The reference points can also be fixedly mounted on the support, in which case they need not be placed again for each measurement but are only moved when neces- sary. If a sufficiently large number of reference points are mounted on the support, they will not nec- essarily have to be moved at all, because even if some of the points should be hidden behind the object, there will still be a sufficient number of them avail- able.

After the placement, the positions of the reference points and object points are measured 11.

The reference points are measured by cameras and their positions are stored in memory. Next, the object is illuminated by means of a laser or other radiation source to produce points on the surface of the measur- ing object that are visible to the cameras. The points are measured by the cameras and stored in memory. Af- ter the object has been measured completely from one position, the support is moved 12 to a new position.

After the support has been moved, measurement is started by measuring the reference points and per- forming a coordinate transformation 13. Based on the measured and the original reference point positions, a transformation of the coordinates of the movable sup- port can be calculated. Next, the object is illumi- nated again and new coordinates are calculated for the points produced. These new coordinates are connected to the earlier ones, transforming them into the origi- nal coordinate system 14.

If any blind areas still exist, the rotating table can be turned again 15. The extent of blind ar- eas can be estimated by qualitative criteria or e. g. by using a predetermined number of movements and an approximate change. If the object is moved again, then the procedure is resumed at step 13, otherwise the measurement is ended 16.

Fig. 2 represents a system according to the invention. The system comprises a movable support, which in the example embodiment is a rotating table 20, cameras CAM1 and CAM2, a laser pointer LASER, ref- erence points 22 and a data system DTE for the storage and transformation of results. The rotating table 20 is provided with a mechanism that allows it to be turned and locked in place. The magnitude of the turn- ing angle need not be accurately predetermined, so it is not necessary to provide the rotating mechanism with any special measuring devices or mechanical pre- cision components. Reference points 22 are mounted on the rotating table. The reference points, or some of them, may also be fixedly placed on the table.

The object 21 attached to the rotating table is illuminated with laser beams. The illuminating de- vice LASER may consist of several lasers, which are typically mounted in the form of a matrix. The laser beams illuminate points on the surface of the measur- ing object mounted on the rotating table and these are measured by a camera system, which in the example em- bodiment comprises cameras CAM1 and CAM2. The system is provided with as many cameras as needed, usually four to eight cameras being used. The measured points are stored into the data system DTE. After the object 21 has been measured completely, it can be rotated to measure blind areas. After the rotation, the change in its position is measured on the basis of the reference points. The object is rotated again until all blind areas have been measured.

The invention is not limited to the embodi- ment examples described above; instead, many varia- tions are possible within the scope of the inventive concept defined in the claims.