CH709013A2 - Colour filter for digital sensor and method for adjusting a photo-finish camera with such a filter. - Google Patents

Abstract

The subject of the invention is a color filter for a digital sensor formed of a two-dimensional matrix of pixels, each pixel corresponding to a given color. The pixels are arranged according to at least one repeated basic pattern on a given surface, which has at least three pixels of three different colors. The color filter is characterized in that it comprises a plurality of distinct basic patterns, each of these patterns having optical properties of sensitivity (S), color quality (Q) and resolution (N) predefined, and that it is subdivided into a plurality of distinct areas (601-607), each corresponding to a two-dimensional sub-array of pixels. Each of the basic patterns is repeated respectively over an entire area (601-607).

Description

Technical area

The present invention relates to an adaptive color filter for digital matrix sensor, particularly suitable for a photofinish camera, and a particular adjustment method associated with such a camera.

State of the art

[0002] In the context of digital photography, photosensitive sensors formed of a mosaic of pixels have long been known; the sensors most used for digital cameras use, for example, CCD technology (charge-coupled device) or CMOS (complementary metal oxide semiconductor). In order to produce color images, color sensors are applied to these digital sensors, which are also in the form of a matrix of different colors, each of the colors being dedicated to a pixel of the sensor to which it is superimposed. The color that must be associated with each pixel on the resulting image is then obtained through image processing algorithms.

[0003] One of the best known and most used color filters is the Bayer filter, which is the most classic form of RGB filter (R for red, ie red, G for green, it is that is, green, and finally B for blue, that is, blue, these three colors being the basic colors allowing to obtain any other by combination of additive synthesis). Such a filter uses a basic pattern covering 4 pixels, which is repeated over the entire surface of the sensor, with two green pixels in opposite corners complemented by a blue pixel and a red pixel.

A disadvantage of this type of color filter is the considerable reduction in the sensitivity of the associated digital sensor due to significant attenuation of the amount of light that reaches him. Indeed, a good part of the wavelengths being absorbed by the different filters. For these reasons, different variants of the Bayer filter have since been proposed, in particular by replacing one of the two green pixels with a white pixel in order to improve the sensitivity of the sensor. In this case, the modified basic pattern is also repeated on the entire filter.

In the field of sports competitions, there are also known auxiliary timing devices based on image recognition, better known as photo-finish. Such systems allow an operator to decide between competitors crossing a finish line by viewing images taken successively by a high definition camera centered very precisely at the finish line. The analysis of the sequence of images, taken at successive given instants and consequently corresponding to different timed times, makes it possible to determine delayed, but nevertheless very quickly after the arrival, more precise times of the crossing of the line of each competitor, for example by the thousandth of a second and classify competitors reliably.

Cameras used by photo-finish type devices often include Linescan type CCD sensors (better known by the acronym LS-CCD) whose image capture rate is even higher than that of CCD cameras. customary, allowing a temporal resolution up to ten thousandths of a second. The first sensors used for such cameras have presented a very particular bar-shaped matrix structure, that is to say a single column of pixels aligned very precisely on the finish line. Nowadays we tend to use two-dimensional standard matrix sensors, so the selection of the column aligned with the finish line can be done by software.

For such photofinish cameras equipped with LS-CCD sensors, filters of the aforementioned RGB type can of course be used to obtain color images. Nevertheless, these filters are not suitable to provide adequate quality in any type of race conditions. Indeed, depending on for example the time of the course of the competition or weather conditions, the ambient brightness can vary greatly, and thus greatly alter the quality of the images obtained. The same goes for race type considerations, which condition the speed of competitors according to the events and consequently the shooting parameters, including the exposure time.

[0008] There is therefore a need for color filters for digital sensors free of these known limitations.

Brief summary of the invention

An object of the present invention is to provide a color filter for digital sensor having better optical properties.

Another object of the present invention is to provide an effective control method for a photo-finish camera using such a sensor.

These goals are achieved in particular by a color filter for digital sensor formed of a two-dimensional array of pixels, each pixel corresponding to a given color. The pixels are arranged according to at least one repeated basic pattern on a given surface, which has at least three pixels of three different colors. The color filter is characterized by comprising a plurality of distinct basic patterns, each of which patterns has optical properties of predefined sensitivity, color quality, and resolution, and is subdivided into a plurality of distinct areas. , each corresponding to a two-dimensional sub-matrix of pixels, each of the basic patterns being repeated respectively over the entirety of a corresponding zone.

These goals are also achieved by a setting method for a photo-finish camera according to using such a color filter, characterized in that it comprises the following steps:

Particular embodiments of the invention are defined in the dependent claims.

An advantage of the present invention is to continuously optimize the optical properties of color photos obtained through such a filter, regardless of the conditions of use.

Another advantage of the proposed solution is to allow a simple implementation of the adjustment of the optical parameters chosen for a shooting.

Brief description of the figures

Examples of implementation of the advantageous invention are indicated in the description and illustrated by the appended figures in which: <tb> fig. 1 <SEP> respectively illustrates a top view of a photofinish camera used in the context of the invention, aligned with a finish line; <tb> fig. 2 <SEP> illustrates a scheme of a filter according to a preferred embodiment of the invention; <tb> fig. 3 <SEP> illustrates a filter and its different basic patterns replicated on different areas according to a preferred embodiment of the invention; <tb> fig. 4 <SEP> illustrates a diagram of the various steps of an adjustment method for a photofinish camera used in the context of the present invention.

Example (s) of Embodiments of the Invention

FIG. 1 shows a block diagram of a photo-finish camera used in the context of the present invention. The photo-finish camera 3, preferably LS-CDD, is aligned on a finish line 2 of a track 1 preferably comprising several lanes 10 in which the various competitors operate. Note that the example of an athletics track 1 is totally indicative and not limiting, photo-finish cameras 3 can also be used in cycling races, track racing, or for horse racing.

The photo-finish camera 3 preferably comprises 4 degrees of freedom, including three in rotation and one in translation. In fig. 1, only the degree of freedom in translation T and a first degree of freedom in rotation R1 about a vertical axis are illustrated; nevertheless, the camera is preferably rotatably mounted around a ball joint 8 which simultaneously possesses additional second and third degrees of freedom in rotation R2.R3 in order to manage the alignment of the optical axis 4 of the camera 3 on the line d 2. The photo-finish camera 3 comprises a digital sensor 6 formed of a matrix of pixels, as well as an image processing device 7 supplying a digital signal 9 at the output corresponding to the image detected by the sensor. . In order to form color images, the photo-finish camera 3 furthermore comprises a particular color filter 60 whose optical properties can be adjusted according to various predefined parameters, discussed below. According to a preferred embodiment, this color filter 60 is affixed directly to the pixels of the digital sensor 6, so that by language abuse we speak of pixels also for the filter in order to denominate the different colors respectively covering each pixel of the sensor digital 6.

Such a color filter 60 according to a preferred embodiment of the invention is illustrated schematically in FIG. 2, on which can be distinguished a plurality of zones 600 covering the entirety of the columns and rows of pixels, respectively referenced 61 and 62, and which correspond to the width and the total height of the color filter 60. As will be seen from FIG. the light of the description of FIG. 3, each of these different zones 600 is intended to present a basic pattern having distinct specific optical properties, thus making it possible to adapt the shooting parameters as a function of the area 600, and therefore of the corresponding basic pattern, who is chosen. According to preferred embodiment illustrated, each of the areas extends over all the rows of pixels 62, that is to say the entire height of the sensor, typically 2048 pixels. Such a configuration of a color filter 60 is particularly advantageous for a photo-finish camera that uses at most a few columns of pixels aligned on the finish line, but columns of maximum height, since this sets the height of the image that was taken. However, it is conceivable, in the context of the present invention, to subdivide into zones 600 formed of two-dimensional matrices whose height may be chosen by means of software and which may therefore not extend over the entire the height of the filter, that is to say the entirety of its lines of pixels 62. According to this preferred embodiment, the color filter 60 also preferably comprises between 1024 and 2048 columns of pixels.

FIG. 3 represents a filter according to a preferred embodiment of the invention comprising a plurality of different zones, here 7 in number, with which are associated respective specific basic patterns. Thus, a first pattern M1, which corresponds to the first conventional RGB pattern with 2 green pixels G complemented by a red pixel R and a blue pixel B, is repeated over the whole of the first area 601, a second pattern M2 is repeated on the first area. whole second zone 602 etc. to the seventh ground on the seventh zone 607.

As can be seen in FIG. 3, each of the areas 601 to 607 extends over all of the pixel lines 62, i.e. the height of the entire filter. The width of each of these zones is preferably between 100 and 200 pixels, and the total width of the color filter 60, corresponding to the entire number of columns 61, is preferably between 1024 and 2048 pixels. These widths of zones are referenced respectively 611 for that of the first zone 601, 612 for that of the second zone 602, 613 for that of the third zone 603, 614 for that of the fourth zone 604, 615 for that of the fifth zone 605, 616 for that of the sixth zone 606, and finally 617 for that of the seventh zone 607. According to the preferred embodiment illustrated in FIGS. 2 and 3, each of the zones 600 has the same width, so that the widths 611 to 617 are identical. However, according to another preferred embodiment, it would be possible to adjust the width of each of the different filter areas according to their probability of use, determined empirically or statistically. Thus, it will be possible to grant a larger width to an area on which the repeated basic pattern has optical properties optimized for normal use conditions. For a photo-finish camera 3 using such a color filter 60 in the context of athletic competitions, conditioning a scroll of about 1000 lines per second, it will be possible, for example, to define a pattern associated with the average diurnal and nocturnal luminosities, and to increase the widths of the zones on which these patterns are repeated, in order to minimize the physical movements of the camera once the finish line 2 is aligned with one of these zones.

By performing a detailed analysis of each of the patterns used by the filter 60 of FIG. 3, it is found that each of the basic patterns systematically contains at least one red pixel R, green G, and blue B, and that:

The optical properties obtained using each basic pattern are conditioned, inter alia, by the following 3 parameters:

To evaluate overall the optical performance of each pattern, it will be possible to multiply the scores obtained for each of the parameters S, Q, and N above; however, the choice of the pattern can be made taking into account any weighting, especially on the sensitivity parameter S.

It can be seen that, of the seven patterns illustrated, at least two basic patterns comprise a different number of white pixels W, for example the fourth pattern M4 and the sixth pattern M6, which makes it possible to adjust the sensitivity to at least three distinct levels (strong-medium-low) by counting the basic patterns that do not include any white pixel W is that are therefore not suitable for low brightness. Moreover, the fact that the number of pixels P1-P7 of the basic patterns M1-M7 are included, according to the preferred embodiment illustrated in FIG. 3, between 3 and 6, a very small number, facilitates the image processing process whose complexity is comparable to that of a standard 4-pixel Bayer RGB filter. Finally, the fact that each of the basic patterns M1-M7 extends over at most 4 columns or 4 lines makes it possible to choose a good compromise between the color quality Q and the resolution N of the image obtained, while leaving a a large choice of basic pattern numbers, preferably greater than 6, in order to adapt to as many types of races and weather conditions as possible, with for each type of race at least 3 basic scenarios in terms of brightness , typically favorable for a clear day, average for a cloudy day and unfavorable for a night run.

A particularly advantageous characteristic of the various basic patterns M1-M7 chosen according to the preferred embodiment of the invention and which are illustrated in FIG. 3, is that they allow a random selection of adjacent column sets for optical properties of sensitivity S, of color quality Q, and of identical resolution N, and thus do not require software adjustment on a set of specific columns. of an area. It can indeed be verified that for any of the patterns M1-M7, only the respective widths N1-N7 in number of pixels condition the sensitivity properties S of all the selected columns, the other parameters being at that moment. there already frozen:

This property is particularly interesting for the use of a color filter 60 which would be integrated in a photo-finish camera 3, which could then dispense with a fine adjustment step after determining the pattern to use and therefore the corresponding area.

Fig. 4 illustrates a sequence of adjustment steps of a photofinish camera 3 according to a preferred embodiment of the invention.

Following a first step E1 of choosing a basic pattern having optical properties of sensitivity S, color quality Q and resolution N predefined according to race parameters such as weather conditions and the type of race (the speed of scrolling the number of lines of pixels per second can vary from 1000 lines per second for athletics to more than 10000 lines per second for car races, the speed for cycling and horse races being of order of 3000 lines per second), the photo-finish camera 3 is physically moved during a second step E2 so that the finish line 2 of the timed race is positioned opposite an area corresponding to said pattern of selected base. This problem does not have a single solution in view of the degrees of freedom involved, that is to say the degree of freedom in translation T and the three degrees of freedom in rotation R1, R2, R3, we can for this purpose adopt inverse kinematics algorithms such as those used in robotics in order to find the solution requiring the least possible manipulations, possibly with weights on some of the degrees of freedom that we wish to freeze as much as possible.

Finally, a third software adjustment step E3 of a set of adjacent columns, depending on the basic pattern of the selected area, may be performed for example using a reticle, as in the solution proposed in patent EP0898249. However, it will simply be necessary to choose a number of columns corresponding to the width N of said base pattern selected from the M1-M7 patterns, but not to select particular columns from the columns of the corresponding zone. To align the set of adjacent columns on the finish line 2, it would theoretically be necessary to align the center column of this set on the finish line 2 for odd numbers of columns, and the finish line 2 between However, this last alignment step is very simple in practice when using a reticle as mentioned above and the column traversed by the reticle is selected by default. It is then sufficient to complete this column crossed by the reticle randomly by adjacent columns until the desired number corresponding to the width of the selected pattern.

However, it will be understood that the M1-M7 patterns have been given by way of example to illustrate a particularly preferred embodiment. However, other basic patterns, such as a 2 * 2 RGB pattern with interposed white W pixel columns, could also be envisioned to match extremely adverse light conditions. The basic pattern would then consist of two lines, like a classic RGB pattern, but not two columns, but four (one of two white pixels W, one of red-green pixels R-G, then one of two white pixels W, followed by one of two green-blue pixels G-B). Such a pattern nevertheless has resolution problems because of the number of high pixel columns that must be employed. Similarly still a pattern of 16 pixels (4 * 4) of a RGB filter whose dimensions would be doubled, that is to say a matrix of 2 * 2 pixels of red R, blue B, and two such matrices of 2 * 2 pixels of green in the opposite corners of the 4 * 4 matrix. Such a pattern would have advantages in terms of "binning", that is to say the reduction of the effects of noise due to the grouping of adjacent pixels of the same color before amplification and digitization, but would result in parallel in a decrease in the resolution , and no longer allow the random selection of a number of adjacent columns, as in the third step E3 above.

Furthermore, another color triptych could also be envisaged to perform the color synthesis of each pixel, such as cyan, magenta and yellow instead of red, green and blue.

Claims (10)

A color filter (60) for a digital sensor (6), said color filter (60) being formed of a two-dimensional matrix of pixels, each pixel corresponding to a given color, said pixels being arranged according to at least one repeated basic pattern on a given surface, said base pattern having at least three pixels of three different colors, said color filter (60) being characterized by comprising a plurality of distinct basic patterns, each of said patterns having optical sensitivity properties ( S), of predefined color quality (Q) and resolution (N), and that it is subdivided into a plurality of distinct areas (600), each corresponding to a two-dimensional sub-array of pixels, each of said basic patterns being repeated respectively over the entirety of a said corresponding zone (600). 2. Color filter (60) according to claim 1, characterized in that each of said zones (600) extends over all the rows of pixels (62) of said filter. 3. Color filter (60) according to one of the preceding claims, characterized in that said zones (600) have a variable width. 4. Color filter (60) according to one of the preceding claims, characterized in that all the basic patterns comprise at least one blue pixel (B), one green pixel (G) and one red pixel (R), and that at least some basic patterns further comprise at least one white pixel (W). 5. Color filter (60) according to claim 4, characterized in that it comprises a plurality of basic patterns comprising a different number of white pixels (W). 6. Color filter (60) according to one of the preceding claims, characterized in that the number of pixels of said basic patterns is between 3 and 6. 7. Color filter (60) according to one of the preceding claims, characterized in that each basic pattern extends at most on 4 rows of pixels and 4 columns of pixels. 8. Photo-finish camera (3) comprising a color filter (60) according to one of the preceding claims and a digital sensor (6) matrix, said photo-finish camera (3) being rotatably mounted around a ball joint (8) having three degrees of freedom in rotation (R1, R2, R3), and also has a degree of freedom in translation (T) in the direction of the course of the race. 9. A method of adjustment for a photo-finish camera (3) according to claim 8, characterized in that it comprises the following steps: A first step (E1) of choice of a basic pattern having optical properties of sensitivity (S), color quality (Q) and resolution (N) predefined as a function of stroke parameters; A second step (E2) for moving said photofinish camera (3) so that the finish line (2) of the timed race is positioned opposite an area corresponding to said selected base pattern; A third software adjustment step (E3) of a set of adjacent columns, whose number corresponds to the width of said selected base pattern, on said finish line (2). 10. A method of setting a photo-finish camera (3) according to claim 9, the choice of said set of adjacent columns depending solely on the number of pixels determining the width of said selected base pattern.