GB2215935A - The use of interlace in fractal scanning - Google Patents

Abstract

An image is scanned along a trace formed by a co-ordinate sequence defined by one order of a hierarchical set of fractal curves, for example, a Peano curve. From the output of the scan generated q sets of samples, each set of samples corresponds to points on the scan at intervals of q sampling points. Each set of samples represents a different set of sampling points and the sets of samples are transmitted successively for display. When q=2, alternate samples are transmitted in one picture period and the intervening samples in the next. The image may be sampled at pixel rate, alternate samples being discarded, or the scan may dwell at the selected points for transmission, moving quickly between such points. Analogue and digital transmission are envisaged as are both discrete and continuous displays. In the latter case the alternate transmitted samples are interleaved with black level for display. <IMAGE>

Description

THE USE OF INTERLACE IN FRACTAL SCANNING The present invention relates to video scanning systems, such as those used in television. In particular, the invention relates to scanning systems using fractal scanning paths.

Our published British patent application no. 2193411 describes a method of scanning an image based on a Peano curve (sometimes called a Hilbert curve). A nth order curve joins points in a 2n x 2n array and the set of curves, in increasing order, form a hierarchical family, each member of which is derived from the previous member by application of a simple algorithm. Figure 1 of the drawings shows a fifth order curved connecting an array of 32 x 32 points. Such curves belong to the set of structures known as FRACTALS.

The significance of these curves, when used for image scanning and display, is that, when smoothed, an nth order curve becomes an (n-l)th order curve, the transition being continuous with progressive amounts of smoothing. Thus, it is possible to display an image with an order of scan which is different from that used to originate the video signal. This enables an evolution of scanning acuity to occur compatibly.

A fundamental disadvantage of this method of scanning, compared with conventional raster-based scanning, lies in the bandwidth needed for transmission of the video signal obtained. As with conventional scanning, a field may be defined as one traverse of the image. To avoid perceptible flicker, the field frequency must be higher than a limit set by the brightness of the display. This limit is generally agreed to be in the region of 50-60 Hz. To conserve bandwidth, conventional raster scanning transmits the brightness of image points on only alternate lines of the image in a single field, the brightness of the points on the remaining lines being transmitted during the following field.This is termed 'interlaced scanning' and halves the bandwidth needed as compared with a non-interlaced scan whilst preventing flicker by maintaining the refresh frequency of a region in the display. It is effective so long as the vertical detail in the image is not enough to cause the brightness content of spatially adjacent lines to differ markedly. If it does so, then a disturbing flicker at half the field frequency, known as 'inter-line twitter' is produced. Given that this technique is generally effective and used universally for image broadcasting, it can be seen that, unless a similar technique is applied to the proposed method of scanning, it will be at a disadvantage as, otherwise, all the brightness values must be transmitted in one field period, giving rise to a video signal of twice the bandwidth.

Following the analogy with conventional scanning, it may be thought that the concept of interlaced scanning might be extended to the Peano scan by interlacing scans of lower order. For example, two second order scans might be interlaced to form a third order scan as shown in Figure 2 of the drawings. In practice this fails to work because, as can be seen, some points are not visited whereas others are visited twice. This results from the fact that the curve of the next lowest order connects only a quarter of the points on any given curve. Moreover, even if this difficulty were overcome by modifying the interlaced curves, the hierarchical advantage would be lost. This is because the spacing of the interlaced curves in the next lowest order would be doubled whereas the information in the video signal would correspond to the original spacing and thus be inappropriate.

We have recognised that only a single curve is needed, the interlace being obtained by sample selection along the curve.

In accordance with the invention there is provided a method of generating from an image a signal for display, comprising scanning the image along a trace formed by a co-ordinate sequence defined by one order of the hierarchical set of fractal curves; generating q sets of samples from the scanning output signal, each set of samples corresponding to points of the scan at intervals of q sampling points and each set of samples representing a different set of sampling points; and transmitting the q sets of samples obtained from the scanning output signal successively for display.

Preferably, q is 2 and each set of samples represents alternate pixels. Preferably, the fractal curve is a Peano curve. Thus, at the source, the image is scanned with the Peano curve but only, say, the odd samples are taken in the first field with the even samples being taken in the second field. Using this approach, the hierarchical property is preserved and all the information needed can be conveyed.

A method in accordance with the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 shows a fifth order Peano curve; Figure 2 shows two interlaced lower order Peano curves; Figure 3 shows a sampling pattern in accordance with the method of the invention; and Figure 4 is a schematic block diagram of apparatus for use in the method of the invention.

As outlined above, it is proposed that the image field be scanned along a trace formed by a co-ordinate sequence defined by one order of a hierarchical set of fractal curves, in this case, a Peano curve. Sets of samples are then generated from the output of the scan, each set consisting of samples taken at intervals corresponding to q sampling points. For example, if q is 2 the sampling points are as shown in Figure 3 of the drawings.

If the source is discrete then the sample selection is straightforwardly done through the addressing sequence. If the source is continuous, for example, through the mechanism of a scanning beam, then there are two ways in which this may be done.

In the first method which is preferred, the scan traces the full Peano curve and the signal so obtained is sampled at the pixel rate.

Alternate samples are then discarded, the alternation being such as to select even samples on even fields and odd samples on odd fields.

The samples may be sent as a digital signal or interpolated to provide an analogue signal. In the second method the scan traces only the appropriate points, the signal being obtained directly. In this case, the scan should be such as to spend a reasonable proportion of each sample period at the sample points and a relatively small proportion of the period moving between the points.

The signal may be sampled for digital transmission if required.

At the display, the incoming signal is arranged to illuminate only those points corresponding to the sample values that are sent.

Again if the display is discrete then this can be done straightforwardly through the addressing sequence. If the display is formed by a continuously moving beam then, again, there are two ways this may be done. In the first method the signal, if analogue, is sampled at the transmission sample rate (which is half the original pixel rate) and augmented with samples corresponding to black-level at the odd positions in one field and the even positions in the other field. This operation doubles the bandwidth of the signal but only locally, in the display circuitry, not in the transmission path. Meanwhile the display scan traces the full Peano curve as in the preferred method at the source. The insertion of the black-level samples in the signal then creates the spaces into which the interleaved samples on the other field fit.In the second method the scan traces only the appropriate points and the signal may be used directly without black sample insertion. As in the second method at the source the same qualification applies concerning the movement of the trace.

In the second method, at either source or display, a small amount of black level insertion may be used to 'cover' the period of scan transit between alternate sample sites.

Just as with conventional scanning, this interlacing will be effective only so long as the image is not fine enough to create a significant difference in the content of the two sets of sample values. With conventional scanning such differences occur on sharp horizontal edges or fine horizontal gratings. With the proposed method, it occurs on diagonal edges or gratings. Inasmuch as these occur less often and the spacing of the diagonal rows in finer, the situation is less likely to occur. Thus, the method of the invention has in this respect a significant advantage over conventional scanning.

It can be shown that the hierarchical properties of such a system are preserved in that any source Peano scanning order may be combined with any display Peano scanning order. Moreover, the preferred method of scanning at the display may be used with either interlaced or non-interlaced scanning at the source so enabling an evolution to occur of the source from interlace to sequential scanning.

The method described can be generalised to arbitrary interlace factors wherein only every qth point is transmitted on each field, different sets being transmitted successively, taking q fields to transmit a full picture. In such an qth order interlace there are (q-l) factorial ways of selecting the sequence of sets. In such a situation it is best to define the sequences, where possible, by the expression p*i modulo q where i is any integer, q is the interlace order, p is about one half of q and p and q are coprime, i.e. share no common factors.

For example, if q is 12 and p is 5 then the sequence 0, 5, 10, 3, 8, 1, 6, 11, 4, 9, 2, 7, 0, is obtained. An advantage of using such a high order interlace is that the field frequency may be increased considerably without increasing the bandwidth of the transmitted signal.

Figure 4 shows a practical arrangement in which both the source and display are discrete.

A generator 1 produces Peano sequences which are used as the z and y addresses of a source sensor 2. The generator 1 is synchronised by clock pulses from a clock oscillator 3 and field pulses applied over a line 4, the clock oscillator 3 being locked to the field pulses by suitable circuitry 5. The frequency of the clock oscillator 3 is n times the field frequency where n is the total number of image points in both fields (assumed to be even). The output of the generator 1 is then selectively sampled by a sampler 6 which acts so as to select alternate outputs during one field and the other outputs during the other field. The frequency of the sampler 6 is therefore half the clock frequency but it must be reset after each field as there are an even number of points in the image.

The video signal obtained from the sensor 2 will consist of a sequence of values at these points, interpolated to an extent determined by the sensor circuitry. At one extreme this may consist of 'boxcar' values and at the other extreme it may be interpolated by an infinite-order filter. The video signal may be subjected to, further filtering imposed by the channel if an analogue or resampled if the channel is digital. Synchronisation information, which must contain an identification of field parity is added to the video d signal by circuit 13.

At the receiver a further generator 7 produces Peano sequences which are used as the x and y addresses of the display device 8. This generator 7 is synchronised by clock pulses from an oscillator 9 and field pulses from a circuit 10 which separates field synchronisation and parity information from the incoming video signal. The clock oscillator 10 is locked to the field pulses by suitable circuitry 11 and runs at a frequency of n' times the field frequency where n' is the total number of points in the display. The output of the generator 7 is selectively sampled by means of a sampler 12 which acts so as to select alternate outputs during one field and the other outputs during the other field. The sampler 12 is reset at the end of each field to select the interleaving generator values.

The video signal, if analogue, is fed directly to the display which acts as a gating device, that is, it displays at each pixel the signal as it varies during the sample. Thus, if n' were equal to n and the video consisted of boxcar values, there would be an exact correspondence between displayed values and source values. Where the video is interpolated, there is an approximate correspondence due to the integrating effect of the display which introduces a slight attenuation of fine detail. This is a form of aperture loss and can be corrected for.

Claims (8)

1. A method of generating from an image a signal for display, comprising scanning the image along a trace formed by a co-ordinate sequence defined by one order of a hierarchical set of fractal curves; generating q sets of samples from the scanning output signal, each set of samples corresponding to points on the scan at intervals of q sampling points and each set of samples representing a different set of sampling points; and transmitting the q sets of samples obtained from the scanning output signal successively for display.

2. A method according to claim 1, in which q is 2, each set of samples representing alternate pixels.

3. A method according to claim 1 or 2, in which each scan traces all points defined by the scanning curve, the q sets of samples being generated during a single scan of the image field.

4. A method according to claim 3, in which sets of samples are transmitted with (q-l) black level samples interposed between each pair of adjacent samples derived from the scan.

5. A method according to claim 1 or 2, in which each scan only traces those points on the scanning curve necessary to generate one set of samples, the q sets of samples being generated during q scans of the image field.

6. A method according to any preceding claim in which the fractal curve is a Peano curve.

7. A method of generating from an image a signal for display, the method being substantially as hereinbefore described.

8. A method of generating from an image a signal for display, the method being substantially as hereinbefore described with reference to Figure 3 of the drawings.