Cosmetics filter for smoothing regenerated pictures, e.g. after signal compressing for transmission in a narrowband network

Smoothing of regenerated digital pictures (e.g. after a heavy signal compression and coding for transmission on a narrowband medium) can be effected by means of a so-called "cosmetics filter" which filter comprises a digital lowpass filter, a delay, a subtraction unit, a table and an adder. An input signal enters the lowpass filter and the delay which is arranged in parallel with the lowpass filter. The difference between the delayed signal and the lowpass-filtered signal is used to address the table. An address content from the table is passed to the adder to be added to the lowpass-filtered signal from the lowpass filter, and the sum signal from the adder is the final filtered signal to the output from the cosmetics filter.

FIELD OF THE INVENTION 
The present invention relates to a so-called cosmetics filter of the type 
stated in the introductory part of the appended patent claim 1. 
BACKGROUND OF THE INVENTION 
In connection with electronic pictures like TV pictures, noise is often a 
problem. The noise with which one is most familiar, is what can be called 
thermal noise--or random noise. It is possible to achieve clearer and 
cleaner pictures if this type of noise is reduced somewhat. 
Attempts have therefore been made to find methods for a reduction of this 
noise in the temporal domain by using various filtering techniques, which 
will often be based upon a type of recursive filter means. Examples of 
methods of such a type of noise reduction can be found in e.g. British 
patent application No. 2,020,941, U.S. Pat. No. 4,639,784 and German 
patent DE 33 11 898. All these patent publications are based upon a 
temporal picture filtering, so that the filtered picture will be less 
afflicted with noise, and hence present a more pleasant appearance, and 
simultaneously the signal time variation will not become veiled. 
Lately one has started to use digital compression techniques to reduce bit 
consumption when representing single pictures or picture sequences (live 
pictures). These are e.g. techniques used in picture telephony. The coding 
technique itself is standardized by the international telecommunication 
organization CCITT. Live pictures are encoded in such a manner that they 
do not need a higher network transmission capacity than in the case of 
telephony. In order to succeed in such a task, the live picture must be 
compressed to such a degree that also visible distortion is introduced in 
the picture. This distortion has the effect that the picture looks a 
little less pleasant. In order to remedy this effect, it is possible to 
apply a subsequent filtering of the picture to give the picture a more 
pleasant appearance. 
Thus, a situation exists where a great deal of knowledge exists regarding 
the generated noise. If one for example utilizes blocks when encoding 
pictures, one knows as a starting point that the block patterns will 
easily become visible and disfiguring. This information regarding the type 
of noise to be removed, becomes handy when the task is to construct a 
subsequent filter, i.e. a cosmetics filter. The goals to be achieved by 
means of such a filter can be summarized in this manner: 
A removal of the noise generated in the encoding process, so that the 
result becomes subjectively pleasant to the eye, 
taking care that scars from possible block magnitudes in the encoding 
process will not become visible, and 
effecting the filtering in such a manner that no veiling or further lack of 
definition are introduced in the picture. 
There exists a clear need for such cosmetics filters. One version of such a 
filter is described in "Motion video coding for visual telephony" by 
Ronald Plompen, PTT Research Neher Laboratories. This filter satisfies in 
general the requirements stated above, however the filter is rather 
demanding as to calculations. The described filter is intended used for 
block based encoding methods, and this assumes that the signal power is 
calculated for each respective block. This requires a lot of extra 
calculating power. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to provide a cosmetics filter which 
is less demanding regarding calculations, but which nevertheless gives 
approximately the same effect, i.e. in total a more cost efficient filter. 
The present invention therefore provides a cosmetics filter for use in 
smoothing pictures which are regenerated after relatively heavy signal 
compression and coding in connection with e.g. transmission of live 
pictures via a narrowband transmission medium, where picture signals are 
described by digitalized samples constituting picture signals are 
described by digitalized samples constituting picture points, said 
cosmetics filter comprising: a digital lowpass filter having an input 
coupled to the input signal X and an output providing a lowpass filtered 
signal XLOW; a means for delaying having an input coupled to the input 
signal X and an output providing a delayed signal X; a means of 
subtracting signals having a first input coupled to the lowpass filter, a 
second input coupled to the delay means and an output providing a 
difference signal (X-XLOW); a table means having an address input coupled 
to the output of the signal subtracting means and an output providing a 
corresponding address content TAB(X-XLOW); and an adder having a first 
input coupled to the table means, a second input coupled to said lowpass 
filter, and an output, whereby a total-filtered signal Y=XLOW+TAB(X-XLOW) 
is provided at the output from said adder. 
Further features and advantages of the invention appear from the attached 
dependent patent claims. 
The invention shall now be described further by means of embodiment 
examples, and referring to the enclosed drawings, where

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 is shown a sketch of the structure of the new filter. The signal 
X which is applied to the filter, is a sampled and digitalized signal, 
each respective sample representing a picture element (picture point). The 
signal X is distributed to two parallel branches, one of said branches 
containing a low pass filter and the other branch containing a delay 
means. The output signals from respectively the delay means and the 
lowpass filter as passed to a subtraction unit, and the difference between 
the signals are used as an address to a table TAB, and the content of said 
address is output as a result and returned to an adder which also receives 
the lowpass filtered signal XLOW. The sum of said lowpass filtered signal 
XLOW and the table value, constitutes the ready filtered signal Y, i.e. in 
such a manner: 
EQU Y=XLOW+TAB(X-XLOW). 
It appears from the figure that the table means contains several optional 
tables, designated TAB1, TAB2, . . . TABn, and the decision regarding 
which table should be used in a particular case, can be made on the basis 
of data from the rest of the equipment, thereby providing a possibility of 
controlling the degree of the signal filtering. This choice can be 
controlled and changed as often or as seldom as one likes, and hence the 
filtering degree can be adapted to the signal present. Thus, the filter 
can be described as follows: 
EQU Y=XLOW+TABk(X-XLOW). 
To repeat, the meaning of the various parameters is as follows: 
X: unfiltered data input, 
XLOW: lowpass filtered data, 
Y: filtered data output, 
TAB1 . . . N: a number of pre-set tables which determine the filtering 
degree, 
DELAY: a delay means which makes the signal input and the XLOW signal meet 
each other at the correct time, and 
LOWPASS FILTER is a lowpass filter which in its simplest form may operate 
by mean value calculation or average value calculation of a few picture 
points surrounding the picture point in question. 
A clear advantage of the present cosmetics filter above other previously 
known filters, e.g. the filter previously mentioned, is the very simple 
implementing thereof. I.e. the filter effect can be provided by means of 
few calculations. This is important, since the data flow in a picture 
signal is very high. The calculating operations required for this filter 
are merely: 
The lowpass filter function, which in its simplest form may consist of a 
mean value calculation of picture points, and 
TABk, which quite simply is a table, in the sense that the input signal is 
used as an address to the table, and the content of this address is the 
result of the operation. Such a table look-up is a "cheap" operation. 
In order to give a simple example of the filter function, it is referred to 
FIG. 2. Here is shown a representation of a signal which is originally an 
analogue signal, i.e. the representation is a sampled signal as a function 
of time. The values X prior to filtering are shown by means of a cross. 
The output values XLOW from the lowpass filter are shown as circles, and 
the output values Y from the cosmetics filter are shown as squares. The 
signal may for example represent a time variable light intensity along 
e.g. a horizontal line in a TV screen, where the signal was originally an 
analogue signal, but is here represented by means of samples which as a 
starting point are transmitted sequentially in a digital form. 
Interpreted as a horizontal line scan on the screen, a genuine signal 
variation which results in a picture-forming contour, will represent a 
marked "jump" in a signal shape as the one shown in FIG. 2. In order to 
preserve such jumps, but at the same time remove minor variations which 
are due to noise from the signal processing, the cosmetics filter will 
here operate in the following manner: The lowpass filter operates in 
almost the simplest manner conceivable, namely by mean value calculating 
over three values "input" to the filter, i.e. the signal value X in 
question, plus the nearest value on each respective side, divided by 
three, giving as a result the shown XLOW (circle), and consequently a 
smoothing or lowpass filtering of the curve. 
It will be clear that in most cases there will be achieved a small 
difference X-XLOW, while at the more abrupt transition a little to the 
right in the diagram, X-XLOW will exhibit a larger absolute value. 
The total filter should not make nay substantial change at the abrupt 
transition, while a smoothing effect is desirable in the other cases. The 
filtering effect must therefore depend on the magnitude of X-XLOW, i.e. 
the filter must contain a function of (X-XLOW). This function is here 
represented by a table look-up operation, i.e. TAB, and the total filter 
consequently operates in accordance with the above mentioned formulae. 
The table function TAB is constructed so as to: 
I make Y.fwdarw.XLOW when the absolute value (X-XLOW) is small, 
II make Y.fwdarw.X when the absolute value of (X-XLOW) is large. 
I In this case it is assumed that (X-XLOW) represents noise which has been 
introduced in the picture coding process. This noise is removed (or 
actually reduced) by equating Y to the filtered value XLOW. The filtering 
achieved thereby will not de-sharpen the picture since a small (X-XLOW) 
indicates that there are no sharp details. 
II When (X-XLOW) is large, it is assumed that this is due to the picture 
content, and not due to noise. One therefore lets Y assume a value close 
to X. Thus, details in the picture will not be filtered away. 
In FIG. 3 there is indicated a possible form of TAB, but in reality TAB is 
constituted, as previously mentioned, by a stored table, or preferably 
several tables among which one can be selected. As an address to a table 
is used (X-XLOW) as previously mentioned. 
However, the filter can be used in several different ways. It shall first 
be established that a single picture can be regarded as a two-dimensional 
signal, i.e. with a horizontal (H) and a vertical (V) dimension, while a 
live picture can be regarded as a three-dimensional signal with a 
horizontal, a vertical and a temporal (T) dimension. Which one of these 
dimensions the filter operates in relation to, is determined by the 
dimensions for which the lowpass filter operates. By e.g. calculating mean 
values for points in only one direction, (i.e. alternatively H, V or T 
direction), the filtering is effected only in this single dimension. 
However, the lowpass filter may very well operate in more dimensions 
simultaneously, e.g. by having the filter calculate mean values over a 
small block in the picture plane (HV combined), or e.g. over several 
blocks in the same position, but which blocks follow each other in time 
(HVT combined). 
Thus, the filter can be used in one or more dimensions. If it is used in 
several dimensions, one may also arrange the lowpass filter as two 
directly consecutive single filters, or as a combination-operating filter 
over the two same dimensions. For instance, a filter over horizontal and 
vertical direction may be realized as 
H+V, i.e. first in the horizontal direction and thereafter in the vertical 
direction, or 
HV, i.e. the filter operates by itself simultaneously in both horizontal 
and vertical direction. 
(through the operating mode of the lowpass filter). 
In the same manner, e.g. a filter which is intended to operate in all three 
dimensions, may be of the types H+V+T, or HVT.