a configuration of a first embodiment according to the present invention is shown in fig1 . this is a video processing apparatus which temporarily writes an input video signal into a frame buffer on a ram 11 , reads it , and generates and outputs a video image . a double buffer ( first and second storage means ) is formed on the ram 11 . the video signal is represented in a predetermined digital signal . pixel value information in a video frame is sent from the top left to the bottom right in coordinates , with a pixel clock together , as shown in fig1 . each blank portion bl 1 is filled with a dummy pixel value . first , an analyzer 12 analyzes the input video signal , and finds coordinates of the current pixel value information on the video frame . on the basis of the coordinates , a downscaler 13 performs downscaling processing on the video frame . for example , the downscaler performs ½ scaling by deleting one pixel at intervals of one pixel in each of the vertical and horizontal directions . thereupon , a writer 14 sends a frame buffer writing request to an arbitrator 15 . on the other hand , a reader 16 sends a reading request to the arbitrator 15 . on the basis of pixel value information read out , a generator 17 suitably adds a blank and a synchronization signal , and generates and outputs a video signal . the arbitrator 15 arbitrates requests sent from the writer 14 and the reader 16 , actually accesses the ram 11 in a time division form , and performs data transfer . a read buffer decider 18 obtains decision timing on the basis of line information which is being currently processed , sent from the generator 17 . on the basis of line information ( coordinate in the vertical direction ) which is being processed , sent from the analyzer 12 and information of a buffer into which writing is being currently performed in the double buffer , sent from the writer 14 , the read buffer decider 18 decides from which buffer to read , and delivers a result thereof to the reader 16 . components shown on the left side of the arbitrator 15 are referred to as input system . components shown on the right side of the arbitrator 15 are referred to as output system . the read buffer decider 18 makes a decision as described below . fig4 shows states of an input video image and an output image . bl 2 and bl 3 denote blank portions of video images . the input video image is downscaled to ½ as described above , and disposed on a top left portion of the output video image . line information obtained from the analyzer 12 when the generator 17 is processing a location of line 0 indicated by a solid line arrow y 1 shown in a right - hand drawing of fig4 is denoted by cl . since downscaling processing is incorporated , the line movement rate on a video image becomes faster on the output side than on the input side . at time when the output system completes reading , i . e ., at time when the arrow y 1 in the right - hand drawing moves to a dotted line arrow y 2 , the processing proceeds from the current time by blank_height +( frame_height − blank_height )/ 2 . therefore , a line which is being processed by the input system at that time is presumed to be at cl + blank_height +( frame_height − blank_height )/ 2 . if reading is performed from a buffer into which writing is being performed in the double buffer when the above line { cl + blank_height +( frame_height − blank_height )/ 2 } remains in the current frame , outrunning occurs . at this time , therefore , a buffer different from the buffer into which writing is being performed should be used as the read buffer . its condition is represented as follows : it is possible on the reading side to always prevent ruining of video image due to outrunning , by using buffer information the input system is currently writing , the location on the video frame where the input system is currently processing ( cl in the case of the present example ), and the disposition condition of the output downscaled video image ( blank_height +( frame_height − blank_height )/ 2 in the case of the present example ) as heretofore described . by the way , there may be a plurality of input systems and a plurality of output systems . in the case where a plurality of input video images is arranged on the output video image , for example , the above - described decision is made for each of the input systems . in the foregoing description , the processing location on the frame is determined by taking a line as a unit . alternatively , the processing location on the frame may be determined by taking , for example , a pixel instead of a line as the unit . a configuration of a video processing apparatus in a second embodiment of the present invention is shown in fig2 . in the same way as the first embodiment , the video processing apparatus temporarily writes an input video signal into a frame buffer on a ram 11 , reads the input video signal , and generates and outputs a video image . the second embodiment differs from the first embodiment in that the layout can be changed freely . a reader 16 receives information as to which part of the input video image should be cut . a downscaler 13 receives a downscaling factor to be applied to the part of the input video image . a generator 17 receives where to dispose the downscaled video image . each of the reader 16 , the downscaler 13 and the generator 17 performs processing according to an order of layout information . in addition , in the present embodiment , a read buffer decider 18 also watches the layout information and performs decision processing . except them , other function units function in the same way as the foregoing description . in the same way as fig4 , fig5 shows states of an input video image and an output video image . this time , a part of the input video image is cut , downscaled to , for example , ½ , and pasted on the output video image . at that time , a coordinate of a bottom end of a cutting range on the input video image is supposed to be oel ( in the display region , the same shall apply hereinafter ), and a coordinate of a bottom end of a cutting range on the output video image is supposed to be el . the el corresponds to , for example , an end location . the oel corresponds to , for example , a location on the input video frame which is associated with the end location . if the input system is performing processing on a line cl when the output system is performing processing on a line 0 indicated by a solid line arrow y 3 in a right - hand drawing , then the read buffer decider 18 makes a decision as follows . when the location where the output system is processing arrives at the bottom end of the pasting range as indicated by a dotted line arrow y 4 , the location where the input system is processing becomes cl + el + blank_height . if this location is above the bottom end of the cutting range on the input video image , then outrunning occurs or an immediately preceding frame becomes later ( new ) one , and consequently reading from it should be performed . therefore , a condition under which a frame buffer different from a frame buffer into which writing is being currently performed is used as a read buffer is as follows : as appreciated from the fact that the right side of the preceding expression becomes minus , the condition always becomes false . instead , when the location where the output system is processing arrives at the bottom end of the pasting range , the input system moves to the next frame buffer in the double buffer and in addition the bottom end of the cutting range is reached in the next frame buffer in some cases . at that time , reading from the next frame buffer should be performed in order to obtain a later video image . therefore , a condition under which a frame buffer different from a frame buffer into which writing is being currently performed is used as the read buffer is as follows : this is the decision condition . the case where the decision is made at the top end ( see a solid line arrow y 5 ) of the pasting range on the output video image as shown in fig6 will now be considered . the top end of the pasting range on the output video image is denoted by sl . when the location where the output system is processing has moved to a bottom end of the pasting range on the output video image as indicated by a dotted line arrow y 6 in fig6 , the location where the input system is processing moves to cl +( el − sl ). if it does not exceed the bottom end of the cutting range on the input video image , then a different frame buffer is used as the read buffer in the same way as the foregoing description . in other words , it follows that : if the decision condition is thus set by suitably using the layout information according to decision timing , it is possible to prevent the output video image from being ruined according to the present invention and decrease the delay for the input video image . the decision timing may be at any time as long as it precedes the execution of reading . according to the above expression , sl is set correspondingly . the case where the video signal is an interlaced signal as prescribed in , for example , itu - r ( international telecommunication union - radio communication ) bt . 656 will now be considered . as for the states of the input video image and the output video image in this case , one frame includes two fields : a field a and a field b as shown in fig7 . in this case , a decision is made every field . more details will now be described . in the case where the output system makes a decision at the head of the frame ( line 0 , head of the field a ) as the foregoing description , a similar result is obtained and consequently its description will be omitted . this time , the case where the output system makes a decision at the head of the field b as indicated by a dotted line arrow y 7 in a right - hand drawing will be considered . the way of thinking is the same . when the location where the output system is processing arrives at an end of the pasting range as indicated by a dotted line arrow y 8 , the location where the input system is processing becomes cl + el + blank_height . if this location is above the end of the cutting range of the field b , then outrunning occurs or an immediately preceding frame becomes later one , and consequently reading from it should be performed . therefore , a condition under which a frame buffer different from a frame buffer into which writing is being currently performed is used as a read buffer is as follows : if the input video signal and the output video signal are transmitted at definite timing , the interlaced video image may also be used in this way . the decision condition should be suitably set according to it . a third embodiment of the present invention is shown in fig3 . in the present embodiment as well , a video processing apparatus temporarily writes an input video signal into a frame buffer on a ram 11 , reads out the input video signal , and generates and outputs a video image in the same way as the first and second embodiments . the third embodiment differs from the first and second embodiments in that the downscaler preceding the writer 14 is eliminated and instead an upscaler 19 is provided after the reader 16 . this time , a part of a cut input video image is upscaled with an ordered upscale factor and pasted on the output video image . it is a point that the moving rate of a location on an input video image which is being processed by the input side becomes faster than the moving rate of a location on an input video image which is being processed by the output side to the contrary to the foregoing description because of the difference between the downscaling and upscaling . except it , operation is the same as that in the first and second embodiments . fig8 shows states of the input video image and the output video image in the same way as fig4 and 5 . this time , a part of the input video image is cut as shown in fig8 , and it is upscaled to twice and pasted on the output video image . at that time , a coordinate of a top end of a cutting range is denoted by osl and a coordinate of a top end of a pasting range on the output video image is denoted by sl . the coordinate sl corresponds to , for example , the start location . the coordinate osl corresponds to , for example , a location on the input video frame associated with the start location . if the input system is performing processing on a line cl when the output system is processing a line 0 indicated by a dotted line arrow y 9 in a right - hand drawing , then a read buffer decider 18 makes a decision as follows : when the location where the output system is processing arrives at the top end of the pasting range as indicated by a dotted line arrow y 10 , the location where the input system is processing becomes cl + sl + blank_height . if this location is above the bottom end of the cutting range on the input video image , then outrunning occurs or an immediately preceding frame becomes later one , and consequently reading from it should be performed . therefore , a condition under which a frame buffer different from a frame buffer into which writing is being currently performed is used as a read buffer is as follows : as appreciated from the fact that the right side of the preceding expression becomes minus , the condition always becomes false . instead , when the location where the output system is processing arrives at the top end of the pasting range , the input system moves to the next frame buffer and in addition the top end of the cutting range is reached in the next frame buffer in some cases . if reading from the next frame buffer is performed at that time , a later video image is obtained . therefore , reading from the next frame buffer should be performed . accordingly , a condition under which a frame buffer different from a frame buffer into which writing is being currently performed is used as the read buffer is as follows : this is the decision condition . the case where the decision is made at the top end ( see a solid line arrow y 11 ) of the pasting range on the output video image as shown in fig9 will now be considered . if cl does not exceed the top end of the cutting range on the input video image at this time point in this case , then a different frame buffer is used as the read buffer . in other words , it follows that : if the decision condition is thus set in the same way even when upscaling is performed , effects of the present invention are obtained in the same way . in the case of interlacing as well , the condition should be set in the same way . by the way , the downscaler and the upscaler described in the first to third embodiments are nothing but an example . they do not restrict the present invention . in the first to third embodiments , the case where the input side performs writing into the frame buffers alternately and the output side decides a buffer to read from has been described . however , the present invention is also effective to the case where the input side decides a buffer to write in when performing writing into a frame buffer and the output side reads alternately from the frame buffers . hereafter , an example of this case will be described . however , the present invention is not restricted to the example described hereafter , but an embodiment corresponding to the first to third embodiments may also be incorporated . a configuration of a fourth embodiment according to the present invention is shown in fig1 . fig1 differs from fig2 in that the input side decides a buffer to write in when writing into a frame buffer and the output side reads alternately from the frame buffers . the frame buffers in this embodiment are buffers for output video frame . the reader 16 reads the output video frame from the whole area of one of frame buffers . the writer 16 writes into a part of or whole area of one of frame buffers . except this , operation is the same as that in the foregoing description . for the sake of convenience , reference numerals are reassigned to elements . except the above - described point , operation of each element is the same as that of an element having the like name . fig1 shows states of an input video image and an output video image . fig1 is the same as fig5 except that cl is present on the output side . if the output system is performing processing on a line cl when the input side is performing processing on a line 0 indicated by a solid line arrow y 12 in a left - hand drawing , then a read buffer decider 28 makes a decision as follows . when the location where the input system is processing arrives at a bottom end of a cutting range , the location where the output system is processing becomes cl + oel + blank_height . if this location is below a bottom end of a pasting range on the output video image , then outrunning on the reader side occurs or the next frame is read earlier , and consequently writing into it should be performed . therefore , a condition under which a frame buffer different from a frame buffer from which reading is being currently performed is used as a write buffer is as follows : the preceding expression always becomes true . instead , when the location where the input system is processing arrives at the bottom end of the cutting range , the output system moves to the next frame buffer and in addition the bottom end of the pasting range is reached in that frame buffer in some cases . at that time , writing into the current buffer which is a buffer subsequent to the next buffer should be performed in order to cause the output side to read a later video image . except this case , therefore , a condition under which a frame buffer different from a frame buffer into which writing is being currently performed is used as the write buffer becomes as follows : as described above , in this embodiment , the read buffer decider 28 makes an opposite decision of the second embodiment . in the third embodiment , in which an upscaling situation that a shift of a writing position of the writer 14 is faster than a shift of a reading position of the reader 16 is described , the read buffer decider 28 makes an opposite decision of the second embodiment , too . that is , the write buffer is set as the buffer from which the reader 16 reads then if the reading position of the reader 16 at time when the writer 14 writes into the write start location exceeds the write end location . in the fourth embodiment heretofore described , the decision is made at the head ( cl in fig1 ) of the output frame . alternatively , the decision may be made at another location ( time point ) in the same way as the second and third embodiments described above . as an example of the first to fourth embodiments heretofore described , the case where the upscaling or downscaling processing is performed has been mentioned . however , the present invention is not restricted to this . it is essence that the writer side is different from the reader side in movement rate on the frame buffer . therefore , the present invention may be applied to the case where simply the rate of the input video image is different from the rate of the output video image . in that case as well , a similar decision is made according to the present invention . each of the elements in the configurations shown in fig1 , 3 and 11 may represent a function obtained by causing a computer to execute a program generated using an ordinary programming technique , may represent hardware , or may represent a combination of them . according to the embodiments of the present invention , it becomes possible to always make the buffer selecting decision suitably in double buffering even when the input video image and the output video image are asynchronous and in addition the cutting location of the input video image and the pasting location on the output video image are arbitrary , as heretofore described . as a result , it becomes possible to prevent the input image from being ruined and reduce the delay of the output video image from the input video image .