Abstract:
A method for detecting dynamic video pixels by using adaptive counter threshold values according to field difference value of the frame in the video, thereby to determine whether the frame is an interlaced frame or a progressive frame and to eliminate incorrect judgements resulting from field difference and to improve accuracy of frame determination.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for detecting video frame types with adaptive thresholds that determines whether a video frame is an interlaced frame or a progressive frame by using adaptive thresholds to avoid incorrect judgement of the frame type resulting from excessive field difference and improve detection accuracy. 
     2. Description of the Prior Art 
     The video picture that is commonly adopted at present is transmitted at thirty frames per second to produce continuous and dynamic images. Each frame consists of a plurality of scan lines (for instance, National Television System Committee—NTSC, standard is 525 scan lines). That is, 525 scan lines are transmitted in 1/30 second for one frame. 
     Said video frame can be classified into two categories: interlaced frame and progressive frame. Scanning of the interlaced frame is divided in a top-field and a bottom-field. The top-field has an odd number of scan lines while the bottom-field has an even number of scan lines. They are scanned separately at two times. For instance, the top-field is scanned first, then the bottom-field is scanned to generate a complete frame. On the other hand, all of the scanning lines of the progressive frame are transmitted at the same time, then are scanned individually one by one. 
     Refer to  FIGS. 1(   a ) and  1 ( b ) for an interlaced frame  10  and a progressive frame  11 . As the interlaced frame  10  is scanned at different times and is transmitted respectively for the top-field (odd number of scan lines) and the bottom-field (even number of scan lines), a comb-shaped pattern is generated on the peripheral edges of the square picture  100  and the triangle picture  101  (generally called comb factor). The resolution is less desirable and video quality is poorer. On the other hand, the whole progressive frame  11  is transmitted at the same time, therefore the comb factor does not occur as shown in  FIG. 1(   a ). 
     Based on previous discussion, it is obvious that the comb factor has great impact on resolution. Hence prior to video output, a detection process is performed to determine whether the input frame is an interlaced frame or a progressive frame. If the interlaced frame is detected, then a deinterlacing process is executed to remove the comb factor to obtain a higher quality video. If the progressive frame is detected, the video may be directly output without deinterlacing. 
     Refer to  FIG. 2  for a conventional detection process. First, at step  201 , set the comb factor counter to zero and input a frame F (n) at time instance “n” for calculating the comb factor of pixels; next, at step  202 , transfer all of the pixels into the frame function F (n); at step  203 , scan pixels in the raster order and calculate the comb factor of the pixels (the equation for calculating the comb factor will be discussed later); at step  204 , determine whether the comb factor at each pixel is greater than the threshold value, if “yes”, proceed step  205  and increase the value of comb factor counter by one; if “no”, proceed step  206  to determine if it is the last pixel in the frame; if it is the last pixel, proceed step  207 ; if “no”, repeat step  203  through step  206 ; at step  207 , determine whether the value in the comb factor counter is smaller than a threshold value, if yes, the frame is judged as a progressive frame at step  208 ; if no (i.e. the counter value is greater than the threshold value), proceed step  209  and judge the frame as an interlaced frame; finally end the detection process for frame F(n). 
       FIG. 3  illustrates the calculation of a pixel comb factor at step  203  shown in  FIG. 2 . The pixel  30  to be calculated is located at the coordinate (x,y) and denoted as X in the picture. O- 31  indicates a first adjacent pixel “b” and O- 32  indicates a second adjacent pixel “e”, then the equation for calculating the comb factor of the pixel  30  is as follows:
 Comb Factor ( x,y )=( b−x )×( e−x )−( b−e ) 2   
     Although the equation set forth above may be used to calculate the value of the comb factor for every pixel, and the comb factor value may be compared with a set constant threshold value to determine whether the pixel is a comb factor, then the frame type may be determined based on the total number of the comb factor pixels (i.e. the counter number). Such a method still may result in incorrect judgement. It is because the number of the comb factor is proportional to the difference of the top-field and the bottom-field. The proportional relationship of the difference is illustrated in  FIG. 4 . It is substantially a linear direct proportion. In other words, if an interlaced frame contains objects of a slow motion or a lower frequency, the field difference would also be small and the comb factor number also would be small. However, if video content is more complex, such as including complicated objects like leaves or stones, even if there is no time difference between the top-field and the bottom-field (i.e. progressive frame), the calculated comb factor number could be relatively large. Therefore to determine the frame type based on merely one constant threshold value could mistakenly treat an interlaced frame of a slow motion or a low frequency as a progressive frame; or mistakenly treat a progressive frame of a fast motion or a high frequency as an interlaced frame. As a result, the interlaced frame could be output without undergoing the deinterlacing process and video output quality may suffer. 
     SUMMARY OF THE INVENTION 
     The primary object of the invention is to provide a method for detecting video frame types by using adaptive thresholds to accurately determine whether a video frame is an interlaced frame or a progressive frame thereby to overcome the problem of incorrect judgement of the frame type that occurs to conventional methods that use a constant threshold value. 
     The method of the invention performs the initial determination based on video field difference, then dynamically adjusts the threshold value in the counter based on the field difference thereby to determine whether the frame in the video is an interlaced frame or a progressive frame. Therefore incorrect judgements resulting from the field difference may be eliminated, and accuracy of determining the frame type may be improved. 
     In order to achieve the foregoing object, the method of the invention includes: first, setting the counter to zero and inputting a frame F (n) at time field “n” at time instant “n”, then calculating the field difference value of the frame at time field n; next, comparing with a set field difference upper bound value (Fi-th); when the field difference value is smaller than the upper bound value (Fi-th), dynamically setting the threshold value in the counter based on the field difference value; then calculating the comb factor number of the frame that is greater than the comb factor threshold value, and comparing with the set threshold value in the counter; determining whether the frame is an interlaced frame or a progressive frame; and determining whether to proceed deinterlacing process. 
     The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(   a ) is a schematic view of video outputs of an interlaced frame. 
         FIG. 1(   b ) is a schematic view of video outputs of a progressive frame. 
         FIG. 2  is a process flow for determining frame type adopted a conventional detection technique. 
         FIG. 3  is a schematic view for calculating comb factor value of a pixel of a frame. 
         FIG. 4  is a chart showing the relationship between the comb factor number and the field difference value. 
         FIG. 5  is a process flow for determining frame type according to an embodiment of the invention. 
         FIG. 6  is a schematic chart showing the field difference value being divided in four threshold values according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Refer to  FIG. 4  for an embodiment of the method for detecting video frame types by using adaptive thresholds according to the invention. The method mainly includes the following steps: 
     first, proceeding step  401 , setting the counter to zero, and inputting a frame function F (n). These processes are substantially same as the ones of the conventional method set forth above, thus details are omitted. 
     next, proceeding step  402 , calculating and obtaining field difference value of the frame at time field n; the calculating equation is: 
         (   FieldDiff   )     =       ∑     y   =   0       M   /   2       ⁢           ⁢       ∑     x   =   0     N     ⁢           ⁢            F   ⁡     (     x   ,     2   ⁢   y     ,   n     )       -     F   ⁡     (     x   ,       2   ⁢   y     +   1     ,   n     )                      
         where M and N are respectively the height and width of the frame, and F (x,2y,n)andF (x,2y+1,n) are respectively the pixel values in top-field and bottom-field.       

     proceeding step  403 , comparing the field difference value with a set upper-bound value (Fi-th) of the field difference; 
     proceeding step  412  if the field difference value is greater than or equal to the set upper-bound value (Fi-th), and the frame is judged as an interlaced frame; 
     proceeding step  404  if the field difference value is smaller than the set upper-bound value (Fi-th), and dynamically adjusting the threshold value for the comb factor counter (the method of dynamical adjusting will be explained later); 
     proceeding step  405 , transferring all pixels of the frame in the frame function F (n); 
     calculating comb factor values of all the pixels (step  406 ); 
     determining whether the comb factor value is greater than or equal to the comb factor threshold value (comb TH) (step  407 ); 
     increasing the counter by one if the comb factor threshold value (comb TH) is greater than or equal to the comb factor value (step  408 ); 
     proceeding step  409  if the comb factor threshold value (comb TH) is smaller than the comb factor value and determining whether the pixel is the last one; 
     proceeding step  410  if the pixel is the last one; 
     repeating step  406  if the pixel is not the last one and calculating the next comb factor value; 
     determining whether the counter value is small than or equal to the adjusted counter threshold value (step  410 ); 
     determining the frame is a progressive frame if the counter value is smaller than the adjusted counter threshold value (step  411 ); 
     determining the frame is an interlaced frame if the counter value is not smaller than the adjusted counter threshold value (step  412 ); 
     stopping the process. 
     Refer to  FIGS. 5 and 6  for an embodiment of dynamically adjusting the counter threshold value at step  404 . As shown in  FIG. 5 , the field difference value is substantially in a linear proportional relationship with the comb factor number. Hence the set upper-bound field difference value (Fi-th) may be divided into several numbers to form a plurality of ranges. In the embodiment, four ranges are formed as shown in  FIG. 6 . They are a first field difference value d 1 , which is equal to the set upper-bound field difference value (Fi-th), a second field difference value d 2 , a third field difference value d 3 , and a fourth field difference value d 4 . When the calculated field difference value is within the range of the first field difference value d 1  and the second field difference value d 2 , the threshold value in the counter is dynamically adjusted to a first counter value (Count th 1 ). When the calculated field difference value is within the range of the second field difference value d 2  and the third field difference value d 3 , the threshold value in the counter is dynamically adjusted to a second counter value (Count th 2 ). When the calculated field difference value is within the range of the third field difference value d 3  and the fourth field difference value d 4 , the threshold value in the counter is dynamically adjusted to a third counter value (Count th 3 ). When the calculated field difference value is within the range of the fourth field difference value d 4  and zero, the threshold value in the counter is dynamically adjusted to a fourth counter value (Count th 4 ). 
     By means of the method of the invention set forth above, first, the field difference value of a frame at time n is calculated and obtained, next compare the field difference value with an upper bound value of the field difference for determination, then the threshold value in the counter is dynamically adjusted to improve the judgement whether the frame is an interlaced frame or a progressive frame. As a result, accuracy of determining the frame type can be increased. 
     Furthermore comparing the field difference value with an upper-bound value gives a fast determination of the frame type, as for a progressive video frame, the field difference value will not exceed this upper-bond value Therefore, if the field difference value exceeds this upper-bond value, the video frame can be straightly classified as an interlaced frame without going through said step  404  to step  410  in  FIG. 4 . 
     While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiment thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.