Method of concealing scratch errors in a video signal derived from a film scanner

Film scratches that produce errors in video signals produced by a film scanner generally run vertically over one or more moving picture frames and are particularly troublesome where they are intersected by picture contours, i.e. oblique boundaries between light and dark picture content. For concealing the disturbance produced by film scratch in such a case the course of a picture contour across the scratch is determined. First the pixel values before and after a television line crosses the scratch are compared line by line, with simple interpolation inbetween if the difference found is slight. In other cases the direction of rise of an oblique picture contour crossing the scratch is determined and the pixel coordinates of the entrance and exit of the contrast edge into and out of the scratch are generated and stored, while at the same time the pixel values at the edge opposite the entry and exit are also stored. When the pixel values at opposite sides of the scratch are again equal the other end of the crossing of the scratch by the picture contour has been reached, the position of the picture contour across the scratch is determined when the contour transition point in each line is calculated for controlling the concealment of the scratch disturbance.

This inventions concerns a method of concealing errors in a video signal 
resulting from television film scanning which are caused by film scratches 
extending in an essentially vertical direction in the neighborhood of an 
oblique edge intersecting the film scratch which defines the boundary 
between mutually contrasting areas of the picture content. 
For the concealment of error areas in a video signal an error signal is 
conventionally generated by a suitable recognition circuit for delivering 
information of the position and size of the error location, as disclosed, 
for example, in British published patent application 2,140,245A. Such a 
signal makes it possible to replace the disturbed areas of the picture 
with information derived from previously transmitted portions of the video 
signal. If the video signal is from a film scanner, the preceding lines 
and frames may be disturbed at corresponding locations by vertically 
running scratches, making the reference to earlier lines or picture fields 
of little value. Furthermore, the previously known procedure is unsuitable 
when the picture content of the previous picture frame is not identical 
with the current picture frame at the particular location, for example in 
the case of movement in the picture or a change of scene. 
For avoiding the above-described disadvantages, it is known from European 
Patent 0 101 180 to utilize the signal derived from the error location to 
initiate an interpolation between two undisturbed pixels at boundaries of 
the error location and thereby to conceal the error location. Especially 
in the case of broad scratches in a film, however, this interpolation 
method has the disadvantage that the interpolated zone is emphasized amid 
the remaining picture content, because it occupies a fixed position in the 
picture for a perceptible time interval. Furthermore, in such cases 
oblique edge features of the picture content are interrupted over the 
width of the scratch and perpendicular edges of picture content are 
horizontally smeared. If the error location signal is slightly narrower 
than the error location itself, that leads to erroneous interpolation that 
can be just as disturbing as the disturbance it is intended to conceal. 
In U.S. Pat. No. 4,941,186 a method of concealing errors in a video signal 
is described by which the above-mentioned effects of a simple 
interpolation can be greatly reduced by a partial expansion and filtering. 
When there are movements in the picture content affecting large areas, 
however, these partially expanded and filtered regions can still be 
visible if they are produced by scratches on a scanned film, because they 
remain fixed in location while the surrounding picture content moves. 
SUMMARY OF THE INVENTION. 
It is an object of the present invention to provide a method of error 
concealment by which disturbances produced by film scratches in the 
neighborhood of oblique picture content lineaments or boundaries 
intersecting the scratch are made no longer visible in a video signal. 
Briefly, when an error area is found in a line, the respective pixel values 
preceding the first edge and following the last edge are compared line by 
line to produce pixel value differences. When these pixel value 
differences (pvd) are small, simply interpolated pixel values are 
substituted for pixels in the disturbed area. When the pixel value 
difference (pvd) is large the polarity (dexter or sinister slope) of an 
oblique edge pictorial feature is determined from the sign of the 
difference and the change from a preceding small pvd line. A polarity 
signal is then produced for use in determining a transition point between 
pre-scratch and post-scratch pixel values. The pixel coordinates are then 
generated for designating the beginning and the end of a scratch region in 
the line in which the investigated oblique edge feature enters into the 
scratch, as follows. The pixel coordinates and the pixel value at the 
entrance of that edge feature into the scratch are compared with the same 
data of contemporary pixel in the same line at the opposite side of the 
scratch. The value comparison is done for following lines. In the event 
that last comparison shows equality of pixel value, an edge transition 
line is recorded as terminating in one of these pixels of equal value. The 
pixel coordinates of the scratch edge both at the beginning and the end of 
the error location, as seen in horizontal direction, are then stored. 
Then, with the help of the oblique edge transition line that has thus been 
located, interpolation (actually extrapolation to edge transition) is 
performed on both sides of that oblique line in the horizontal direction 
of all the television lines affected. 
The method of the invention has the advantage that contours that are 
missing in a scratched area are reconstructed so that the darkness of the 
picture is essentially maintained in the region of a scratch.

DESCRIPTION OF THE ILLUSTRATED METHOD. 
In the overall equipment group shown in FIG. 1 for concealing errors in a 
video signal there are generated error signals produced by the presence of 
dust or scratches by means of an infra-red sensitive optoelectrical 
converter, in the illustrated case a line-shaped semiconductor sensor 1, 
during television scanning of a motion picture film. This error location 
signal SDF furnished by the sensor 1 designates a signal classification 
that includes both error signals SF generated by scratches and error 
signals DF produced by dust specks. In a following separating circuit 2, 
the output of sensor 1 is split into a scratch detection signal SF and a 
dust detection signal DF. The separating circuit 2 is shown in more detail 
in FIG. 2 and is further explained below. The dust signal DF is supplied 
to a control signal generating circuit 3 in which control signals are 
derived for the following video signal processing circuit 4. The manner of 
operation of the circuit 3 and 4 are already described in U.S. Pat. No. 
4,941,186, so that these circuits 3 and 4 of the present description do 
not need to be further discussed here. The scratch signal SF separated by 
the separation circuit 2 is supplied to a signal reconstruction circuit 5 
which will be further explained with reference to FIG. 3. At the input 6 
of the circuit 5 a video signal is also supplied which consists of a 
luminance component Y and a chrominance component C. The video signal is 
first freed from scratch-originated disturbances in the circuit 5 and then 
freed of dust-generated disturbances in the circuit 4. At the output 7 of 
the circuit 4 a video signal freed of both these kinds of disturbances is 
then made available. 
As already mentioned, FIG. 2 shows the nature of the separation circuit 2 
of FIG. 1. The SDF signal is supplied to an input 11 and is supplied 
thereto through a number of line memories of which only five line memories 
12-16 are shown in the figure. The outputs of the line memories are 
supplied to an AND gate 17, in which the pixel-specific signals are 
correlated to produce a single output signal. At the output of the AND 
gate 17 a pulse stretching circuit 18 is connected in which the input 
signal is widened, in an essentially symmetrical manner, in order to cover 
possible situations in the position of a scratch. By means of the 
resulting control pulse signal the SDF input signal is split into a DF 
signal and an SF signal by means of two AND gates 19 and 21 to which the 
control pulse signal is applied in opposite polarity, being inverted by 
the inverter stage 22 for supply to the AND gate 19 for generating the 
dust error signal DF. As already mentioned that dust signal DF available 
at the output 23 is usable in a known way for carrying out error 
concealment by interpolation of expansion in accordance with the 
disclosure of U.S. Pat. No. 4,941,186. The SF signal made available at the 
output 24 is used for reconstruction of oblique picture content edges in 
the picture signal in accordance with FIG. 3. 
A contour-discerning circuit 26 is provided in the circuit shown in FIG. 3, 
at the first input of which the scratch error signal SF is supplied from 
the terminal 24, while at the second input of the circuit 26 the digital 
video signal Y, C is provided from the terminal 27. The circuit 26 has 
timing inputs to which the synchronizing signal S consisting of H and V 
signals and also a clock signal SCK are delivered from the terminal 28. In 
the contour-discernment circuit 26 the pixels of the video signal before 
the beginning of the scratch signal SF and after its end are compared line 
by line with each other. If the boundary pixel values differ only slightly 
from each other, a signal is supplied over the connection 29 which is 
supplied to one input of a data mixing circuit 32 equipped with an 
interpolator after passing through a propagation time equalizing circuit 
for delay line 31. In such a case only an interpolation between the 
particular edge pixel values is carried out. If on the other hand a large 
level difference between pixels before and after the scratch is found by 
the circuit 26, a signal designating the slope polarity P of the oblique 
edge is produced and delivered to the connection 33. 
A circuit 34 for generating SF coordinates is provided in which the 
coordinates of the scratch signal SF1, which differs from the signal SF 
merely by a difference in timing resulting from propagation time are 
determined in the X and Y directions. These coordinates are determined by 
the momentary condition of two counters in the circuit 34 controlled by 
the S signal and the pixel rate signal SCK, as a result of interrogating 
the counters. The pixel coordinates of the scratch signal SF1 are then 
stored, together with the corresponding pixel value or level of the video 
signal V1, in a first support value memory 36. In this memory 36 there is 
also stored the signal of slope polarity P obtained from the connection 33 
and an intermediate propagation time equalizing component 37. According to 
the particular slope polarity, either the left or right scratch edge value 
is used as the support value. The slope in question relates to the slope 
of the boundary (dexter or sinister in the Heraldry sense) rather than to 
the direction of change of luminance across the boundary. The heraldry 
term dexter means a slope which as viewed tips downward to the right, 
while sinister means downward to the left. The output of the support value 
memory 36 is connected with one input of the comparator 38, to the second 
input of which the contemporary (real time) video signal V1 is supplied. 
In the comparator 38 the stored pixel value from one side of the scratch is 
compared in every line with the pixel value on the other side of the 
scratch. In the case of equality of these values, i.e. in the case of the 
lower passage of the oblique edge of picture content of the video signal 
through the scratch edge, the complete set of coordinates for calculating 
the transition point is now available and is stored in the computing 
circuit 39 which receives for its control and operation the timing signals 
S and SCK and the output of a line counter 41. The scratch signal SF2 
(propagation time again adjusted) is also supplied to the computing 
circuit 39. The output of the computing circuit 39 is supplied through 
still another propagation time equalizing component 43 to one input of the 
data contour fading circuit 32. Contour fading means providing continuity 
through a transition without appreciable loss of contour sharpness. The 
pixel values of the scratch edge which were taken from the delayed video 
signal V2 by the comparator 38 are supplied to a second support value 
memory 44 and there stored. These are now available in parallel to the 
contour fading circuit 32. The support memory 44 delays a video signal V2 
by a fixed interval of time. This time determines the maximum recognizable 
edge steepness. At other inputs of the contour fading circuit 32 there are 
provided the video signals V3 and V4 supplied by the support value memory 
44, of which V3 always is the video signal of the left-hand scratch edge 
and V4 the signal of the right-hand scratch edge. If there is no scratch, 
V4 always corresponds to the normal video signal V2. When there are 
scratch disturbances, contour fading is provided between the signals V3 
and V4. 
The support value memory 44 must be supplied with the timing signals S and 
SCK as well as with the scratch signal SF2. These signals are also 
available to the contour fading circuit 32, with regard to which the 
scratch signal SF2 is led through still another propagation time 
compensation component 45. 
In connection with the state of the line counter 41, the contour transition 
point that is different from line to line can now be determined with 
reference to the undisturbed picture data to the left and right of the 
scratch. The contour fading is provided by at least two pixels. In the 
case of relatively steep or almost perpendicular contours the transition 
point is located for many or all lines of a frame in or near the middle of 
the scratch. 
FIG. 4 shows the so-called pulling stripe type of scratch having the 
boundaries x.sub.0 and x.sub.1 and intersected by an oblique edge k in one 
part of a film picture, here shown on a magnified scale. Pulling stripes 
extend generally in a vertical direction mostly over a complete film 
picture, move substantially parallel to the edge of the film and vary only 
slightly in width. The oblique edge k is for example the boundary between 
a dark picture portion at the left and a bright picture portion at the 
right of k. This edge k enters into the scratch at a height of y.sub.0 
through the boundary x.sub.0 and exits from the scratch at the height of 
y.sub.1 through the boundary x.sub.1 of the scratch. Consequently the 
passage coordinates of the edge k through the scratch are x.sub.0, y.sub.0 
and x.sub.1, y.sub.1. By means of the method of the present invention, the 
portion of the edge k between these coordinates x.sub.0, y.sub.0 and 
x.sub.1, y.sub.1, therefore the missing portion of the edge, will now be 
reconstructed by means of the method of the invention. This can take place 
for example with the circuit according to FIG. 3 for carrying out the 
method of the invention. 
For that purpose it is first necessary to determine the direction of the 
rise of the edge in the picture, i.e. the steepness polarity P. Then there 
will be carried out for example a comparison between the line signals 
previous to entry and with the entry of the edge k into the scratch. Then 
the coordinates x.sub.0, y.sub.0 of the entry as well as the corresponding 
pixel value, i.e. the signal level, are determined. Finally, the exit 
coordinates x.sub.1, y.sub.1 and likewise the corresponding pixel value 
are to be determined. On the basis of these data there can now be 
calculated in every line the corresponding contour fading transition point 
U.sub.1, U.sub.2 . . . U.sub.n. 
Outside of the scratch, of course, the contemporary video signal will be 
transmitted as usual. 
Although the invention has been described with reference to a particular 
illustrative example, it will be understood that variations and 
modifications are possible within the inventive concept.