Method and image-processing device for detecting a reflection of an identified object in an image frame

1 . A method, performed by an image-processing device, for detecting a reflection of an identified object in an image frame captured by a camera, the method comprising:

generating a three-dimensional model of a background scene of the image frame based on obtained three-dimensional information about the background scene;

defining the identified object in the image frame based on image information in the image frame;

defining a three-dimensional bounding box of the defined object in the three-dimensional model of the background scene;

defining surface elements of the three-dimensional bounding box, a respective surface element being defined by a centre coordinate in the three-dimensional model of the background scene and a colour value;

determining a three-dimensional coordinate of a surface in the three-dimensional model of the background scene which reflects light from a surface element of the three-dimensional bounding box of the object into the camera, wherein the determining is performed by tracing rays from the centre coordinate of the surface element of the three-dimensional bounding box of the object and based on a normal of the surface in the three-dimensional model of the background scene at said three-dimensional coordinate;

identifying a first pixel in the image frame which corresponds to the determined three-dimensional coordinate of the surface;

detecting the reflection of the object in the image frame when a mixing of a first colour value of the identified first pixel and the colour value of the surface element of the object and a ground truth colour value of the identified first pixel fulfils a mixing criteria; and

in response to detecting the reflection of the object applying a mask to a pixel area of the image frame, the pixel area comprising the identified first pixel.

2 . The method according to claim 1 , wherein the ground truth colour value is obtained from one or more background image frames or from one or more neighbouring pixels to the identified first pixel in the image frame.

3 . The method according to claim 1 , further comprising:

obtaining a second image frame of a video sequence comprising the image frame, the second image frame comprising the identified object;

defining the identified object in the second image frame based on image information in the second image frame;

determining a respective second centre coordinate of the surface element of the three-dimensional bounding box of the object in the three-dimensional model of the background scene based on an obtained second position of the object in the three-dimensional model;

determining a second three-dimensional coordinate of the surface or of a second surface in the three-dimensional model of the background scene, which reflect light from the surface element of the three-dimensional bounding box of the object into the camera and differs from the determined three-dimensional coordinate of the surface, wherein the determining is performed by tracing rays from the second centre coordinate of the surface element and based on a second normal of the surface or of the second surface in the three-dimensional model of the background scene at said second three-dimensional coordinate;

identifying a second pixel in the second image frame which corresponds to the determined second three-dimensional coordinate of the surface or of the second surface;

obtaining a second colour value of the second pixel; and

confirming detection of the reflection of the object when a mixing of the first colour value and the second colour value fulfils a second mixing criteria; or

rejecting detection of the reflection of the object when the mixing of the first colour value and the second colour value does not fulfil the second mixing criteria, a colour value of the first pixel in the second image frame equals the first colour value.

4 . The method according to claim 3 , further comprising: in response to confirming detection of the reflection of the object, applying a mask to a second pixel area of the second image frame, the second pixel area comprising the identified second pixel.

5 . The method according to claim 3 , further comprising: in response to rejecting detection of the reflection of the object, determining to not apply a mask to the second pixel area comprising the identified second pixel.

6 . The method according to claim 1 , wherein the masked pixel area comprises all pixels corresponding to a projection of the surface element of the three-dimensional bounding box onto the surface or the second surface in the three-dimensional model of the background scene, which projection is reflected into the camera.

7 . The method according to claim 1 , wherein the three-dimensional information about the background scene comprises three-dimensional coordinates of surfaces in the background scene, a respective normal vector of the surfaces, three-dimensional coordinates of the camera and an orientation of the camera.

8 . The method according to claim 1 , wherein the colour value is a hue value or a combination of Cb and Cr values in the YUV format.

9 . The method according to claim 1 , wherein extrapolating the object is performed by extrapolating bounding pixels of the object from the captured image frame.

10 . The method according to claim 1 , wherein a respective surface element of the three-dimensional bounding box corresponds to multiple pixels from the captured image frame and wherein the colour value of the respective surface element is calculated as an average of colour values of the corresponding multiple pixels.

11 . An image-processing device configured to perform the method according to claim 1 .

12 . The image-processing device according to claim 11 , wherein the image-processing device is a video camera, such as a surveillance camera, or a video-server.

13 . A non-transitory computer readable medium that has a computer program, stored therein comprising computer readable code units which when executed on an image-processing device causes the image-processing device to perform the method according to claim 1 .