the apparatus of fig1 comprises an arbitrary number of cameras 1 , 2 connected to a video mixer 3 . each camera comprises an image sensor 4 which transmits an input video image stream formed of consecutive video images to mixer 3 via a cable 5 , a control unit 6 having a plurality of keys for setting an operating mode of the camera 1 , 2 and a viewfinder 7 in which the image stream from the image sensor 4 of the camera is displayed . the control units 6 and the viewfinders 7 of the cameras 1 , 2 are connected to the video mixer 3 by said cables 5 , too , thus enabling the control units 6 to transmit control instructions to the mixer 3 or receive control instructions from it and enabling the viewfinder 7 to play back image information it receives from the mixer 3 . primarily , the mixer 3 is a switch for selecting one of the input video image streams from the cameras 1 , 2 and outputting it as an output video image stream , e . g . to a recording device or to an antenna , not shown , for being broadcast . the mixer 3 is adapted to switch over the output video image stream continuously from a first one of the input streams originating e . g . from camera 1 to a second one of the input streams originating e . g . from camera 2 by dividing the image area into first and second regions and extracting each pixel of the first region of an output image from an image of the first input stream and each pixel belonging to the second region from an image of the second input stream . by continuously increasing the size of the second region at the expense of the first one , the output image stream , which initially was identical to the first input stream , gradually becomes identical to the second one . this kind of switchover is also known in the art as a “ wipe ”. control means for triggering a wipe and for setting its parameters such as duration , the starting point of the second region and its way of growing may be provided at the video mixer 3 , where they can be handled by a mixer operator . according to the invention , such control means are provided in the control units 6 of the video cameras 1 , 2 , in addition to or instead of those at the mixer 3 . by transmitting wipe parameters and , eventually , a wipe command from the control unit 6 to the video mixer 3 , a mixer operator can be dispensed with . fig2 is an outline of the communication between the cameras 1 , 2 and the video mixer 3 during a wipe . it is assumed that initially , the video image stream output by mixer 3 originates from camera 1 , symbolized in fig2 by a broad arrow 8 extending from a line representing camera 1 to a line representing mixer 3 , the width of which arrow 8 is representative of the time interval in which the output of mixer 3 is derived from camera 1 . at an arbitrary instant of this time interval , the video mixer 3 receives a wipe test request 9 from one of the cameras connected to it . it will be assumed in the following that the request originates from camera 1 , but it is understood that any other camera might submit the request , too . the request defines the target camera of the wipe , i . e . the camera whose image stream will be output from mixer 3 once the wipe has been carried out , the duration of the wipe , the initial location of the second region and its type of growth . the mixer 3 communicates the request to the cameras concerned by messages 10 , which also specify the parameters of the wipe . based on these parameters , each camera 1 , 2 calculates how the border between first and second image regions will move if the wipe is actually carried out and displays the moving border in its viewfinder 7 , superimposed upon the view which is currently being taken by the camera . thus , a cameraperson who handles camera 1 and caused it to transmit the wipe test request 9 can estimate the effects the wipe would have on the view which is currently being taken by camera 1 and may eventually select a view which is better suited . similarly , the cameraperson handling target camera 2 can tell from the border displayed in his viewfinder that his camera is a target of a switch request and that the view he is shooting will eventually soon be on the air . when the cameraperson of camera 1 is satisfied with the wipe test , he / she may cause the camera 1 to send a wipe command 11 to the mixer 3 . the mixer 3 forwards the wipe command 11 to camera 2 . by sending and receiving the wipe command 11 , the cameras 1 , 2 are enabled to receive the output image stream 12 from the video mixer 3 , which will be displayed in the viewfinders 7 of the cameras 1 , 2 during the switchover . when the switchover is complete , only the video image stream from camera 2 , represented by broad arrow 13 , is output by video mixer 3 . fig3 is a series of schematic representations of what is seen in the viewfinders 7 of cameras 1 , 2 during the procedure of fig2 , according to a first embodiment of the invention . in each section of the fig ., the left image corresponds to camera 1 , and the right image corresponds to camera 2 . a thick outline drawn around an image frame indicates that the corresponding camera provides or contributes to the output image stream at the instant concerned . the squares of different sizes shown in the image frames of fig3 must not be construed as objects seen by the cameras , but merely as patterns which indicate the origin of the particular image region shown : if the region is filled by small squares , it originates from camera 1 , if it is filled with large squares , it originates from camera 2 . section a ) of fig3 corresponds to an instant in time before a wipe test request is transmitted . as can be recognized based on the above explanations , camera 1 is the only source of the output image stream , and each viewfinder 7 displays the scene which is being shot by the camera to which it belongs . section b ) corresponds to an instant in time shortly after a wipe test request has been notified to the cameras 1 , 2 by the message 10 . the request specifies that the second region 16 will be a square which starts to grow from the upper right corner of the viewfinder screen . in each viewfinder , the border 14 between first and second regions 15 , 16 is shown superimposed onto the scene which is observed by the camera to which the viewfinder belongs , the scene remaining visible in first and second image regions 15 , 16 . in stage c ) the second image regions 16 have grown larger in both viewfinders . by watching the second region 16 grow , the camerapeople at cameras 1 , 2 can tell how the wipe will affect the views they are shooting and whether it is appropriate to change the view in order to make the wipe look good in the output image stream . when the border 14 has moved across the entire viewfinder screen in stage d , the viewfinders look the same as in stage a again . when a cameraperson decides to carry out a wipe from camera 1 to camera 2 and sends the wipe command 11 , the output image stream begins to be downloaded to cameras 1 , 2 , so that in a first instant of the wipe , shown in section e ), the viewfinder of camera 2 displays the scene taken by camera 1 . the viewfinder of camera 1 also displays the output image stream , but since in stage e ), the output image stream is provided by camera 1 alone , the scene which is shown in the viewfinder is the same as before . subsequently , the border 14 between a first image region 15 displaying the view of camera 1 and the second image region 16 displaying the view of camera 2 propagates across the screens of both viewfinders , as shown in sections f ), g ). when the wipe is finished , in stage h ), the mixer 3 ceases to transmit the output image stream to the two viewfinders 7 , and these display again the scenes viewed by their respective cameras . fig4 shows a series of viewfinder screenshots from cameras 1 , 2 according to an alternative embodiment of the method . the initial situation , shown in section a ) of fig4 , is the same as in section a ) of fig3 . when the video mixer 3 receives a wipe test request from camera 1 which is currently providing the output video stream of the mixer 3 or from camera 2 which would provide the output video stream when the wipe has been carried out , the image shown in the viewfinder of camera 2 is modified , as indicated by hatching in section b ). the modification can be of any type that affects the entire area of the image , e . g . the image may become darker , it may turn from colour to black and white , or the like . subsequently , as shown in stages c ) and d ), the image area is split into first and second regions 15 , 16 , the second region 16 expanding continuously all over the viewfinder until it occupies the entire image area in stage e ). in the viewfinder of camera 1 , the second image region is modified in the same way as was the entire viewfinder image of camera 2 in stage b , whereas in the viewfinder of camera 2 , the second image region 16 is displayed normally again . in this way , it is evident for the camerapersons at any time which one of the two regions 15 , 16 that they can see in their viewfinder will be present in the output image stream during the wipe . nevertheless , they can still see the entire field of view of their camera during the wipe test . when a wipe is actually started in stage f ) of fig4 , the viewfinders 7 of the cameras 1 , 2 begin to receive the output image stream from mixer 3 . camera 1 , initially being the only source of the output image stream , displays it normally , whereas camera 2 applies to it the same modification as during the wipe test . again , the image area in the viewfinders is divided into first and second regions , and the second region , now carrying image information from camera 2 , expands continuously , as shown in stages g ) and h ), until it fills the entire viewfinder screen . when this happens , the wipe is finished , camera 2 has become the only source of the output image stream , mixer 3 ceases to feed back the output image stream to the viewfinders 7 , and these display the field of view of their respective cameras again , as shown in stage i ). fig5 illustrates a third embodiment of the switchover method . the initial situation , show in section a ), is the same as in fig3 and 4 . when a wipe test is carried out , in the viewfinder of camera 1 , a border 14 between first and second image regions 15 , 16 , propagates from the upper right corner across the entire viewfinder screen , as shown in sections b ) and c ). in the viewfinder of camera 2 , however , the border 14 propagates in the opposite direction , starting from the lower left corner . in this way , the camera person who operates camera 1 can tell that if a wipe is carried out with the parameters set for the wipe test , the view taken by camera 1 will gradually be superseded by the view from camera 2 in the output image stream , starting at the upper right corner , whereas the cameraperson operating camera 2 can see that the lower left corner of the view taken by camera 2 will be the first to appear in the output image stream during the wipe . after the wipe test , as shown in section d ), the screens of both viewfinders appear the same as at the beginning , in section a ). as in the case of fig3 , when the wipe is started , the output image stream is fed back to both viewfinders , as shown in section e ). as can be seen in sections f ), g ), the view taken by camera 2 gradually moves across the screen , i . e . objects viewed by camera 2 move across the screen , whereas those viewed by camera 1 remain stationary . when the images from camera 1 have been superseded completely by those from camera 2 in the output image stream , the wipe is finished , and the view finders revert to normal operation , as shown in section h ).