GB2384642A

GB2384642A - Detection of colour gamut errors

Info

Publication number: GB2384642A
Application number: GB0228437A
Authority: GB
Inventors: John Robert Emmett
Original assignee: BROADCAST PROJECT RES Ltd
Current assignee: BROADCAST PROJECT RES Ltd
Priority date: 2001-12-07
Filing date: 2002-12-05
Publication date: 2003-07-30
Anticipated expiration: 2022-12-05
Also published as: GB0228437D0; GB0129385D0; GB2384642B

Abstract

R, G and B components of a digital video signal are processed to detect out of gamut (ie out of allowed component range) colour errors by detecting excursions of the signal into out of gamut regions (R1, R2) of a multi-dimensional space (R, G, B) and generating an error signal only if the excursion enters a critical region (R1) such as that near white or a skin tone for at least a predetermined period. In a related aspect, transient excursions into out-of-gamut regions are filtered out by three low-pass filters (whose time constants approximate to the sampling period in the scanning direction, the line period in the sub-scanning direction and the field or frame period) before performing any comparison, in order to avoid false error signals.

Description

VIDEO SIGNAL PROCESSING APPARATUS AND METHOD This invention relates to an apparatus and method for the indication of visible colour reproduction errors in a video signal (e. g. a digital video bitstream). The invention also relates to a method of filtering one or more components of a video signal to remove transients which might otherwise be detected erroneously as visible colour errors.

It is well known for the components of colour video signals (either analogue or digital bitstreams) to suffer from over or under excursions in excess of those allowed for, or capable of being displayed on, a typical video or television display. These over or under excursions of the colour components are not restricted to just the primary red, green and blue signal components, but may also occur in (or be caused by) the excursions of simple mathematically derivations of these signals, such as the luminance and colour difference signals. In addition, certain methods of transmission of television or video signals can limit the excursions of the derived colour components and cause a similar effect. The resulting errors may cause visible display artefacts such as a change of the colour hue of the picture areas where one or more colour components have exceeded the allowed component range or"gamut".

In the particular case where the video signal carries information that is transiently subject to exceeding the gamut range, either spatially transient in the horizontal or vertical directions on a normally scanned (i. e. raster scan) display, or transient in the time for which the error is visibly present on a display, the perceived visible annoyance factor is low and this form of error may therefore be visually acceptable. Such transient phenomena are well known in modern video or television production chains, and are often unintentionally inserted as a side effect of apparatus employing electrical signal processing. It is also known that correction of these transient errors may either be unnecessary, or may in itself cause other problems further along the television or video delivery chain.

A method exists for indicating that a colour television signal obtained from luminance and colour difference components is validly reproducible using a colour display device, and this is described in the patent EP 0 240 971 B1, wherein a system for the derivation of the red, green and blue signals is followed by comparators in order to obtain electrical logic signals to indicate that one or more of these primary colour components lies outside the allowed ranges. The resultant error indication signals may then be passed to various display or alarm systems.

One weakness of this system as applied to current video or television systems is that the alarm indication would be frequently or continuously operating because of the prevalence of the aforementioned transient excursions of one or more of the primary colour components.

Simple low-pass electrical filtering of the primary signals could be employed in order to remove the spatial transient errors corresponding to the sampling period in the horizontal (line scanned) direction, but this improvement to the system would have an insignificant effect on those spatial errors corresponding to the line period that occur in the vertical (subscanning) picture direction, and would not provide any significant allowance for the time transient picture errors corresponding to the frame period or (if the raster scan is interlaced) the field period which may be effectively invisible on a display device.

Accordingly in one aspect the invention provides a method of filtering one or more components of a video signal comprising two or more of the steps of: (a) filtering excursions corresponding to the sampling period within a line of an associated raster scan; (b) filtering excursions corresponding to the line period of said raster scan; and (c) filtering excursions corresponding to the frame period or (if the raster scan is interlaced) the field period of said raster scan.

Preferably each of filtering steps a), b) and c) is implemented.

Accordingly, in a preferred embodiment, low-pass filtering in three dimensions (horizontally, vertically and in time) is employed, followed by colour gamut comparators adjusted such that no alarm indication will occur unless a gamut error has occurred over a predetermined area of the visible picture area, and/or for a predetermined period (e. g. corresponding to a predetermined number of fields or frames).

Other methods that indicate that a gamut error has occurred over a predetermined area of the visible picture area for a predetermined period may also be employed, or an alternative estimation method for the visibility of any colour error may be used instead, or in conjunction with any of the above methods.

One alternative estimation of the visible severity of colour error that may be produced when colour components presented to a display system exceed gamut ranges may be obtained from a mapped analysis of the intended display colour, it being observed that the displayed colour error visibility may be high where colours approaching white or skin tones are predominantly involved, and of low visibility where single colour components predominate. In this approach, the filtering steps a), b) and c) noted above can be dispensed with.

Accordingly in another aspect the invention provides video signal processing apparatus for detecting gamut errors in a video signal, the apparatus comprising comparator means responsive to at least one component of the video signal to detect prohibited excursions into a multi- dimensional signal region.

This apparatus is distinguished from the apparatus of EP 0,240, 971 81 in that signal excursions into a multi-dimensional space rather than a (single- dimensional) range are detected. Preferably the space is a colour space but optionally it may have a time-related dimension.

In a related aspect the invention provides a method of assessing the visibility of a colour error resulting from at least one component of a video signal lying outside a predetermined range wherein the excursion of said

component into a predetermined multi-dimensional signal region associated with visible colour errors is detected.

Additionally to the indication of a visible picture-based gamut error obtained from the primary colour components red, green and blue, an indication of excess excursion of the luminance signal may optionally also be obtained. This error by itself may cause technical problems with signal transmission, rather than produce visible colour errors on a final display.

Preferred features of the invention are defined in the dependent claims.

Preferred embodiments of the invention are described below by way of example only with reference to Figures 1 to 4 of the accompanying drawings, wherein: Figure 1 shows a block diagram of a system for deriving R, G and B chrominance signals ; Figure 2 shows apparatus for filtering a video signal in accordance with the first-mentioned aspect of the invention; Figure 3 shows apparatus for detecting gamut errors in accordance with the other aspects of the invention; and Figure 4 is a diagrammatic illustration of the colour space utilised in the apparatus of Figure 3.

Referring to Figure 1, the primary colour components 20 may be obtained by mathematical derivation from a typical video or television signal or bitstream, 10 which may be decoded in such a way as to deliver the primary colour components. A typical luminance (Y) matrix equation used in Figure 1 might be: Y = 0.7G + 0.2R + 0. 1 B, whilst the typical delivered components might be in the form: (Y), (R-Y), and (B-Y).

All of these signals, both delivered and primary may only occupy similar ranges. Therefore, simple examination of these equations used for the typical derivation of the primary (R, G, B) signals will expose the possibilities of exceeding the ranges of R, G and/or B signals ;

Once the primary colour components have been obtained from an input signal, these components may be filtered and the resultant signal analysed in such a way as to provide an indication of the visible severity of colour error that may be produced when they are presented to a display system.

Figure 2 illustrates one method of filtering and analysis of one primary colour component in order to obtain an indication of the visible severity of colour error that may be produced when the components are presented to a display system. This method is based on digital filtering of each of the video primary colour component signals. Each video primary colour component signal is applied at the input 1 of a low pass configured finite impulse response filter F1, the delay taps of which contain components of the television picture element sample rate. The output of this filter 2 is then passed to a second and similar filter F2 wherein the delay taps relate to the line rate of the television or video signal. These two filters act in the two dimensions of the spatial display of the scanned representation of the picture, and output a signal 3 to the final filter F3 which has taps related to the picture repetition rate (frame or fields), and provides a low pass function on the time for which an impairment is present in the picture. The resultant filtered signal 4 is compared with predetermined maxima and minima, 5 and 6, in comparators I. The output alarm signals 8, may be displayed individually for each primary colour component or in a combinational manner, possibly employing the luminance signal.

Figure 3 shows diagrammatically a digital video signal processor based on a programmed microprocessor 100 provided with conventional video RAM and ROM and having three similar input branches which receive R, G and B component video signals respectively. Each of these input branches has successive delay elements 01, D2 and D3 which feed successively delayed digital samples of the incoming component video signal to video memories M1, M2 and M3 respectively. These memories are coupled to the input bus BS of microprocessor 100 which, under program

control, cycles through the three memories M1, the three memories M2 and the three memories M3 to obtain successively delayed R, G, B samples which are located in the colour space of Figure 4. If they fall outside a predetermined region of the colour space (which is representative of an allowable colour gamut) for longer than a predetermined period (e. g. if three or more samples lie outside the predetermined region) then an error signal is generated by the microprocessor 100.

Figure 4 illustrates the above method, based on a mapped analysis of the intended display colour. The solid line cube illustrates the normal range (allowable gamut), whilst the dotted line cube illustrates the out of gamut areas It is observed that the displayed colour error visibility may be high where colours approaching white or skin tones are predominantly involved, represented in Figure 4 by the typical shaded area designated R1, and of low visibility where only single colours predominate, for example in the typical areas designated R2 in the Figure. Accordingly, the apparatus of Figure 3 generates an error signal only when a predetermined number (say) three or more successive R, G, B samples lie in region R1. In a variant, an error signal may also be generated when a greater number (say) four or more successive R, G and B samples lie in the less critical regions R2.

In a variant, the signal space into which unwanted excursions are detected would have a time or a time-related (e. g. number of frames or fields) dimension and could be shaped and dimensioned to correspond not only to the colour-sensitivity but also to the time sensitivity of the human eye. In such a variant, any R, G, B, T (where T = time) points lying in the above prohibited four-dimensional signal space would be assumed to correspond to a visible artefact and would result in an error signal.

Claims

CLAIMS: 1. Video signal processing apparatus for detecting gamut errors in a video signal, the apparatus comprising comparator means responsive to at least one component of the video signal to detect prohibited excursions into a multi-dimensional signal region.
2. Apparatus according to claim1 which comprises filtering means arranged to filter out transient prohibited excursions.
3. Apparatus according to claim 2 wherein said filtering means comprises a filter arranged to eliminate excursions corresponding to the sampling period within a line of an associated raster scan and a filter arranged to eliminate excursions corresponding to the line period in said raster scan.
4. Apparatus according to claim 2 or claim 3 wherein said filtering means comprises a filter arranged to eliminate excursions corresponding to the field rate in an interlaced raster scan or the frame rate in a non-interlaced raster scan.
5. Apparatus according to any preceding claim wherein said comparator means is responsive to two or more components of the video signal to detect prohibited excursions into a multi-dimensional colour space.
6. Apparatus according to claim 5 wherein said comparator means is arranged to determine whether an excursion is prohibited by comparing an associated colour difference with the sensitivity of the human eye to such a colour difference.
7. Apparatus according to any preceding claim wherein said comparator means is arranged to determine whether an excursion is prohibited by

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reference to the spatial extent of a region of a frame or field in which the excursion occurs.
8. Apparatus according to any preceding claim wherein said comparator means is arranged to determine whether an excursion is prohibited by reference to the persistence of that excursion between successive video frames.
9. Apparatus according to any preceding claim wherein said comparator means is responsive to excursions of the luminance signal.
10. A method of assessing the visibility of a colour error resulting from at least one component of a video signal lying outside a predetermined range wherein the excursion of said component into a predetermined multidimensional signal region associated with visible colour errors is detected.
11. A method according to claim 10 wherein the duration of said excursion into said predetermined region is determined and the visibility of the colour error is assessed in dependence upon said duration.
12. A method according to claim 10 or claim 11 wherein said predetermined signal region has at least two chrominance dimensions and is so shaped as to correspond to the sensitivity of the eye to chrominance errors in different colour regions.
13. A method according to any of claims 10 to 12 wherein said colour error is assessed in dependence upon the area of a frame or field over which it extends.
14. A method according to any of claims 10 to 13, wherein said colour error is assessed in dependence upon its persistence over successive

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frames.
15. A method of filtering one or more components of a video signal comprising two or more of the steps of : (a) filtering excursions corresponding to the sampling period within a line of an associated raster scan; (b) filtering excursions corresponding to the line period of said raster scan; and (c) filtering excursions corresponding to the frame period or (if the raster scan is interlaced) the field period of said raster scan.
16. A method according to claim 15, wherein the or each filtered component is processed to detect colour gamut errors which occur over at least a predetermined area of the display and/or for at least a predetermined period.
17. Apparatus substantially as described hereinabove with reference to Figures 2 to 4 of the accompanying drawings.
18. A method of assessing the visibility of a colour error in a video signal, the method being substantially as described hereinabove with reference to Figures 3 and 4 of the accompanying drawings.
19. A method of filtering components of a video signal substantially as described hereinabove with reference to Figure 2 of the accompanying drawings.
20. A video signal component obtained from an apparatus as claimed in any of claims 1 to 9 or 17 or obtained by a method as claimed in any of claims 10 to 16,18 or 19.