DE102016212331A1 - Lighting dynamics invariant pattern - Google Patents

Abstract

The invention relates to a means having a pattern (201); wherein the pattern (201) comprises a first part (203, 303) and a second part (205, 305); wherein the first part (203, 303) consists of areas of a first brightness and areas of a second brightness; wherein the second part (205, 305) consists of areas of a third brightness and areas of a fourth brightness; and wherein the areas of the first brightness are brighter than the areas of the second brightness and the areas of the third brightness are brighter than the areas of the fourth brightness. The areas of the first brightness are brighter than the areas of the third brightness.

Description

The invention relates to an agent according to the preamble of claim 1 and a method according to the independent method claim.

Calibration targets are samples for calibrating optical cameras. As a rule, these are black and white patterns, such as checkerboard patterns. Within the patterns, certain points whose arrangement is known exactly as measuring points are defined. The points are detected in the camera image. By a priori knowledge about the geometric arrangement of the detected points, aberrations of the camera can be determined. In this way extrinsic and intrinsic camera parameters can be determined.

Extrinsic parameters characterize the spatial position of the camera in relation to its surroundings. They are used to map three-dimensional environment coordinates to three-dimensional camera coordinates. By means of intrinsic parameters, the camera coordinates can be mapped onto two-dimensional image coordinates.

In well-defined environments, the lighting conditions are determinable so that a calibration target is optimally illuminated and no crossfades or underexposures occur. However, the calibration of surroundview camera systems of large vehicles, such as buses, must be done for lack of sufficiently large roofs or dwellings available in the open air.

For calibrations performed outside a well-defined environment, the white areas of the calibration target may be over-illuminated to overpaint the pixels and, as a result, lighten the less exposed neighboring pixels. This is done by skipping effects on the image sensor. An accurate detection of the measuring points is affected. Whether sufficient measurement accuracy can be achieved in such cases is uncertain.

• 1. Das Kalibriertarget ist nicht hinreichend groß und prominent im Vordergrund positioniert. Dies hat zur Folge, dass es einen zu kleinen Teil der Gesamtbildfläche einnimmt. Somit ist es statistisch nicht hinreichend relevant für das Ziel eines ausgewogenen Histogramms über das gesamte Bild.
• 2. Die Gesamtdynamik im Bild so groß, dass es nicht mehr möglich ist, für den Bereich des Kalibriertargets hinreichend Dynamik bereitzustellen. Dies ist der Fall, wenn sehr dunkle Bereiche, zum Beispiel Schatten, und sehr helle Flächen im Bild sind, die zusätzlich noch direktem Licht ausgesetzt sind.

The problems described result from the fact that with an automatic shutter, the camera is adjusted so that it represents a balanced histogram in the overall picture. In individual cases, it is accepted that smaller areas of the image are over- or undersaturated. Particularly problematic are situations in which the following two conditions occur simultaneously:

• 1. The calibration target is not sufficiently large and prominently positioned in the foreground. As a result, it occupies too small a part of the overall picture area. Thus, statistically, it is not sufficiently relevant to the goal of a well-balanced histogram over the entire image.
• 2. The overall dynamics in the image are so great that it is no longer possible to provide sufficient dynamics for the range of the calibration target. This is the case when there are very dark areas, for example shadows, and very bright areas in the image, which are additionally exposed to direct light.

In the worst case, the sun directly illuminates the bright areas of the calibration field, which, however, occupy only a smaller part of the overall picture. Their transition is then very likely.

The only variable parameters offered by cameras are shutter speed and aperture. One solution would be to manually adjust the shutter to optimize for the target's position. However, this is not always possible and also associated with an increased workload. A positioning of the camera, so that the target occupies the largest proportion of area in the image and thus is primarily relevant to the histogram, can not always be realized.

The invention has for its object to calibrate a camera bypassing the known from the prior art solutions inherent disadvantages. In particular, it should be possible to calibrate the camera outside a predefined environment, especially in daylight outdoors.

This object is achieved by an agent according to claim 1 and a method according to the independent method claim. Preferred developments are contained in the subclaims.

To solve the task, it would be helpful if you could adjust the brightness dynamics of a calibration target. However, as these are generally not digital screens with variable content, but printed surfaces, they are inevitably static. Nevertheless, it is possible to display different levels of brightness by using multiple shades of gray instead of the conventional black and white pattern. The calibration algorithm can now use the local dynamics of the respective subtargets as source of information when choosing the area of the target to be used in order to decide which area offers the highest reliability.

This idea is implemented by a means with a pattern comprising a first part and a second part. The agent is an item that is one with the pattern having provided surface. In particular, it may be a card, a blackboard, a sheet, a sheet or a plate. Such an agent is called a calibration target.

A pattern is generally an arrangement of objects, especially surfaces that have different optical properties. In the present case, the first part of the calibration target consists of areas of a first brightness and areas of a second brightness. Accordingly, the second part consists of areas of a third brightness and areas of a fourth brightness.

The brightness of a surface indicates an average luminance of the surface. Different brightnesses of surfaces come about through differently bright colors and / or gray values. Instead of a homogeneous color or a homogeneous gray value, the surface may also be provided with a texture.

The two parts of the pattern of the calibration target according to the invention are of different brightness. Thus, the areas of the first brightness are brighter than the areas of the second brightness and the areas of the third brightness are lighter than the areas of the fourth brightness.

In addition, according to the invention, the areas of the first brightness are brighter than the areas of the third brightness. This is especially true when the pattern is exposed to homogeneous illumination.

If the areas of the first brightness are so bright during a recording to calibrate a camera that it is to be feared that they will be irradiated into the areas of the second brightness, the camera can be calibrated without sacrificing reliability using the second part of the pattern. Since the areas of the third brightness are darker than the areas of the first brightness, it is to be expected that in the second part no blooming takes place.

In a preferred embodiment, the means is a passive calibration target. This results in differences in brightness due to a different reflectivity of the individual surfaces to light, d. H. electromagnetic radiation from the visible spectrum. The higher the proportion of incident light that reflects an area, the greater its brightness. Accordingly, the areas of the first brightness have a higher reflectivity to light than the areas of the second brightness, and the areas of the third brightness have a higher reflectivity to light than the areas of the fourth brightness.

In a preferred embodiment, the average brightness of the first part of the pattern is greater than the average brightness of the second part. Accordingly, the areas of the second brightness are brighter than the areas of the fourth brightness. If the pattern is overexposed, the darker part - the second part for further development - is selected for calibration. Conversely, if the pattern is underexposed, the brighter part - the first part according to the training - is selected for calibration.

The pattern further developed with differently bright parts can be realized as a passive calibration target in that the areas of the second brightness have a higher reflectivity with respect to light than the areas of the fourth brightness.

As an alternative to a pattern with different bright parts, a pattern with parts whose contrast is different, suitable for solving the problem underlying the invention. A corresponding development provides areas of the fourth brightness, which are brighter than the areas of the second brightness. As a result, the contrast of the first part of the pattern is greater than the contrast of the second part of the pattern. Due to the differences in contrast, the second part of the pattern is then used for calibration if the first part is either over- or under-exposed.

A first part and a second part with different contrast can be realized in a passive calibration target with surfaces of the fourth brightness, which have a higher reflectivity to light than the surfaces of the second brightness.

The pattern is printed or painted in a preferred embodiment. In this case, the calibration target comprises a support and color printed on the support or with which the support is painted.

In particular, the calibration target can form at least one planar surface that includes the pattern.

Preferably, the pattern is also developed as a checkerboard pattern. The first part of the pattern forms a first checkerboard pattern and the second part forms a second checkerboard pattern.

A checkerboard pattern is a pattern consisting of square areas of different brightness. Each edge of these surfaces either forms an edge of the pattern or is congruent with the edge of another surface, the brightness of which differs from the brightness of the former surface different. The individual surfaces are aligned in rows and columns.

The calibration target is part of an arrangement in a preferred development, which comprises a camera in addition to the calibration target. In particular, it may be an optical camera. In the recording area or detection or viewing area of the camera, the means is arranged. This means that the images taken by the camera include the calibration target.

In addition, in a preferred development, the arrangement comprises at least one vehicle to which the camera is fixed. The vehicle may be a motor vehicle, in particular a commercial vehicle.

In a further preferred development, the arrangement also comprises an evaluation unit. This is designed to carry out the method described below.

The above procedure is used to calibrate the camera. The individual process steps are preferably carried out in the order given. Unless otherwise stated in the description, any different sequences of execution of the individual steps are possible.

• – Aufnahme mindestens eines Bildes mit der Kamera. Im Rahmen dieses Schrittes erzeugt die Kamera ein internes zweidimensionales Bild der Gegenstände, die sich im Aufnahmebereich der Kamera befinden. Da unter anderem das Kalibriertarget im Aufnahmebereich der Kamera angeordnet ist, enthält das aufgenommene Bild eine Abbildung des Kalibriertargets. Weiterhin enthält ein erster Bereich des Bildes eine Abbildung des ersten Teils des Musters, ein zweiter Bereich des Bildes eine Abbildung des zweiten Teils des Muster.
• – Auswählen mindestens eines Bereiches des Bildes. Bei diesem Bereich handelt es sich um den ersten Bereich oder den zweiten Bereich des Bildes. Zum Auswählen des Bereichs wird das aufgenommene Bild zu der Auswerteeinheit übertragen. Diese identifiziert in dem Bild das Muster und wählt den ersten oder zweiten Bereich entsprechend der jeweiligen Helligkeits- und Kontrastverhältnisse geeignet aus.
• – Kalibrieren der Kamera unter Verwendung des ausgewählten Bereichs. Sobald der erste oder zweite Bereich des Bildes ausgewählt wurde, lässt sich die Kamera unter Verwendung des in dem ausgewählten Bereich enthaltenen Abbilds des ersten oder zweiten Teils des Musters mittels eines herkömmlichen, aus dem Stand der Technik bekannten Verfahrens kalibrieren.

The method comprises the following steps:

• - Take at least one picture with the camera. As part of this step, the camera creates an internal two-dimensional image of the objects that are in the camera's exposure area. Since, inter alia, the calibration target is arranged in the receiving area of the camera, the captured image contains an image of the calibration target. Furthermore, a first area of the image contains an image of the first part of the pattern, a second area of the image contains an image of the second part of the pattern.
• - Select at least one area of the image. This area is the first area or the second area of the image. To select the area, the captured image is transmitted to the evaluation unit. This identifies the pattern in the image and appropriately selects the first or second area according to the respective brightness and contrast ratios.
• - Calibrate the camera using the selected area. Once the first or second area of the image has been selected, the camera can be calibrated using the image of the first or second part of the pattern contained in the selected area by a conventional method known in the art.

The area of the image is selected in a preferred development so that the area contains no area whose brightness corresponds to a maximum brightness that can be displayed by the camera or minimum brightness that can be displayed by the camera. For example, if it is a digital camera with a n-bit dynamic range, the camera will be able to display 2 ⁿ shades. Thus, a sensor of the camera provides values between zero and 2 ⁿ - 1. A region with dark areas, to which the value zero is assigned, is not selected according to further development. Likewise, an area with bright areas, which is assigned the value 2 ⁿ -1, is discarded.

The method according to the invention is preferably implemented in a computer program product that can be executed on the evaluation unit of the arrangement described above.

1 includes a calibration target known in the art. Preferred embodiments of the invention are in the 2 to 4 contain. In this case, matching reference numbers designate the same or functionally identical characteristics. In detail shows:

1 a checkerboard pattern;

2 Checkerboard pattern of different brightness;

3 Checkerboard pattern of different dynamics; and

4 a combination of checkerboard patterns of different brightness and different dynamics.

This in 1 illustrated checkerboard pattern 101 consists of black and white squares. The points where four adjacent squares meet at their tips are suitable measurement points. They can be determined exactly under favorable lighting conditions both vertically and horizontally. In unfavorable lighting conditions, however, the white squares can become overexposed to the black squares. As a result, the white squares appear larger and the black squares smaller in the picture. In addition, the white squares merge. Such a pattern makes the determination of measuring points difficult.

To avoid this problem, this includes in 2 illustrated pattern 201 a dark checkerboard pattern 203 and a bright checkerboard pattern 205 , The dark checkerboard pattern 203 consists of black and gray squares, the bright checkerboard pattern 205 of gray and white squares. It should be noted that the gray tone of the gray squares of the dark checkerboard pattern 203 not with the gray tone of the gray squares of the bright checkerboard pattern 205 must match.

Depending on the lighting situation, the most suitable of the two checkerboard patterns will be 203 . 205 to be selected. This is how the bright checkerboard pattern becomes 205 selected on a dark picture. In a bright picture, the dark checkerboard pattern becomes corresponding 203 selected.

3 includes a pattern 301 with a checkerboard pattern 303 with high dynamics and a checkerboard pattern 305 with low dynamics. The checkerboard pattern 303 exists, like that in 1 illustrated checkerboard pattern 101 , made of black and white squares. The checkerboard pattern 305 with low dynamics consists exclusively of gray squares of different brightness.

Due to its high dynamics and its high contrast range is the checkerboard pattern 303 neither robust against overexposure nor underexposure. Therefore, in case of overexposure or underexposure, instead of the checkerboard pattern 303 to calibrate the camera the checkerboard pattern 305 selected.

This in 4 illustrated pattern 401 combines that in 2 illustrated pattern 201 with the in 3 illustrated pattern 301 , The pattern 401 consists of the dark checkerboard pattern 203 , the bright checkerboard pattern 205 , the checkerboard pattern with high dynamics 303 and the checkerboard pattern with low dynamics 305 , By combining the two patterns 201 . 301 can the robustness of the pattern 401 continue to improve over varying lighting conditions.

LIST OF REFERENCE NUMBERS

101

checkerboard

201

template

203

dark checkerboard pattern

205

bright checkerboard pattern

301

template

303

Checkerboard pattern with high dynamics

305

Checkerboard pattern with low dynamics

401

template

Claims (15)

Means with a pattern ( 201 ); where the pattern ( 201 ) a first part ( 203 . 303 ) and a second part ( 205 . 305 ); the first part ( 203 . 303 ) consists of areas of a first brightness and areas of a second brightness; the second part ( 205 . 305 ) consists of areas of a third brightness and areas of a fourth brightness; and wherein the areas of the first brightness are lighter than the areas of the second brightness and the areas of the third brightness are brighter than the areas of the fourth brightness; characterized in that the areas of the first brightness are brighter than the areas of the third brightness. Means according to claim 1; characterized in that the areas of the first brightness have a higher reflectivity to light than the areas of the second brightness; wherein the areas of the third brightness have a higher reflectivity to light than the areas of the fourth brightness; in which Means according to one of the preceding claims; characterized in that the areas of the second brightness are brighter than the areas of the fourth brightness. Means according to the preceding claim; characterized in that the areas of the second brightness have a higher reflectivity to light than the areas of the fourth brightness. Composition according to claim 1 or claim 2; characterized in that the areas of the fourth brightness are brighter than the areas of the second brightness. Means according to the preceding claim; characterized in that the areas of the fourth brightness have a higher reflectivity to light than the areas of the second brightness. Means according to one of the preceding claims; characterized in that the pattern ( 201 ) is printed or painted. Means according to one of the preceding claims; characterized in that the means forms at least one planar surface; where the area is the pattern ( 201 ) includes. Means according to one of the preceding claims; characterized in that a first checkerboard pattern comprises the first part ( 203 . 303 ) and a second checkerboard pattern the second part ( 205 . 305 ). Arrangement with an agent according to one of the preceding claims; characterized by at least one camera; wherein the means is arranged in the receiving area of the camera. Arrangement according to the preceding claim; characterized by at least one vehicle; the camera being fixed to the vehicle. Arrangement according to one of the preceding two claims; marked by at least one evaluation unit; in which the arrangement is adapted to carry out a method according to one of the following two claims. Method for calibrating a camera; the camera being part of an arrangement according to one of the preceding three claims; with the following process steps: - taking at least one image; Selecting at least one region of the image, the selected region being an image of the first part ( 203 . 303 ) or the second part ( 205 . 305 ) of the pattern ( 201 ); and - calibrating the camera using the selected area. Method according to the preceding claim; characterized in that the region of the image is selected such that the region does not contain a partial region whose brightness corresponds to a maximum brightness that can be displayed by the camera or to a minimum brightness that can be displayed by the camera. Computer program product executable on the evaluation unit of an arrangement according to one of Claims 10 to 12 and implementing the method steps of selecting and calibrating a method according to one of Claims 13 or 14.

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