Hybrid phantom and a method for determining the relative position between a thermal camera and a 3D camera using a hybrid phantom

1. A method for determining the relative position between a first camera and a second camera, wherein the first camera is a thermal camera and the second camera is a 3D camera that emits light onto a hybrid phantom and analyzes reflected light from the hybrid phantom, thus generating a 3D point cloud representing points on a surface of the hybrid phantom, the hybrid phantom comprising a carrier part that carries a plurality of marker patches such that a thermal insulation resides between each of the plurality of marker patches and the carrier part, wherein at least a part of the carrier part has one of a first property and a second property, and each of the plurality of marker patches are constructed of a solid material having the other one of the first property and the second property, wherein the first property is being reflective of light emitted by the second camera thus reflecting the light emitted by the second camera back to the second camera, and wherein the second property is reflecting less of the light emitted from the second camera than the first property, the method comprising:

generating, by an associated temperature difference generating means, a temperature difference between each of the plurality of marker patches and the carrier part of the hybrid phantom

capturing a thermal image of the hybrid phantom using the first camera;

identifying an edge of each of the plurality of marker patches of the hybrid phantom in the thermal image;

identifying locations of each of the plurality of marker patches from the identified edges, thus generating thermal marker locations;

capturing a 3D point cloud of the hybrid phantom using the second camera

identifying locations of each of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations; and

determining the relative position between the first camera and the second camera from the thermal marker locations and the 3D marker locations.

2. The method of claim 1 , wherein the identifying the edge of a marker patch and identifying the location of a marker patch comprise, for each of the plurality of marker patches, finding two or more edge pixels lying on the boundary of the marker patch, calculating a direction of a gradient for each found edge pixel, and calculating the location of the marker patch from the directions of the gradients.

3. The method of claim 1 , wherein the identifying the locations of each of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations comprises:

calculating phantom position data describing the position of the hybrid phantom relative to the second camera from the 3D point cloud,

obtaining marker data representing the locations of each of the plurality of marker patches on the hybrid phantom, and

calculating the location of each of the plurality of marker patch from the phantom position data and the marker data.

4. The method of claim 1 , wherein:

the temperature difference between each of the plurality of marker patches and the carrier part is generated by emission of electromagnetic radiation onto the hybrid phantom,

the emission of the electromagnetic radiation has a varying intensity over time,

a plurality of thermal images is captured over time,

a Fourier transformation of the plurality of thermal images is performed, thus obtaining image data in the frequency domain,

a frequency filter is applied to the image data in the frequency domain, thus generating filtered image data in the frequency domain,

an inverse Fourier transformation is performed on the filtered image data in the frequency domain, thus obtaining filtered image data in the time domain, and

the edges of each of the plurality of marker patches are identified in the filtered image data in the time domain.

5. The method of claim 4 , wherein the emission of the electromagnetic radiation is caused to have the varying intensity over time with a predetermined frequency, and the frequency filter comprises a low-pass filter and the cutoff frequency of the low-pass filter is set to the predetermined frequency or higher.

6. A system, comprising:

a first camera, the first camera being a thermal camera;

a second camera, the second camera being a 3D camera;

a hybrid phantom that includes:

a carrier part that carries a plurality of marker patches, constructed of a solid material, with thermal insulation resides between each of the plurality of marker patches and the carrier part,

wherein at least a part of the carrier part has one of a first property or a second property and each of the plurality of marker patches has the other one of the first property or the second property,

the first property being reflective of light emitted by the second camera and the second property being less reflective of the light emitted by the second camera than the first property;

a temperature difference generating means configured to generate a temperature difference between each of the plurality of marker patches and the carrier part of the hybrid phantom,

a computer having at least one processor, the processor being configured to:

capture a thermal image of the hybrid phantom using the first camera;

identify an edge of each of the plurality of marker patches of the hybrid phantom in the thermal image;

identify locations of each of the plurality of marker patches from the identified edges, thus generating thermal marker locations;

capture a 3D point cloud of the hybrid phantom using the second camera;

identify locations of each of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations; and

determine the relative position between the first camera and the second camera from the thermal marker locations and the 3D marker locations.

7. The system of claim 6 , wherein, to identify the edge of a marker patch and the location of the marker patch, for each of the plurality of marker patches, the processor is further configured to:

find two or more edge pixels lying on a boundary of the marker patch;

calculate a direction of a gradient for each edge pixel found; and

calculate the location of the marker patch from the directions of the gradients.

8. The system of claim 6 , wherein to identify the locations of each of the plurality of marker patches from the 3D point cloud and generating the 3D marker locations, the processor is further configured to:

calculate phantom position data describing the position of the hybrid phantom relative to the second camera from the 3D point cloud,

obtain marker data representing the locations of each of the plurality of marker patches on the hybrid phantom, and

calculate the location of each of the plurality of marker patches from the phantom position data and the marker data.

9. The system of claim 6 , wherein the temperature difference generating means emits electromagnetic radiation to generate the temperature difference.

10. The system of claim 9 , wherein the electromagnetic radiation emitted by the temperature difference generating means varies over time with a predetermined frequency, and wherein the processor is further configured to:

capture a plurality of thermal images via the first camera;

obtain image data in a frequency domain by performing a Fourier transformation of the plurality of thermal images;

apply a frequency filter to the image data to generate filtered image data in the frequency domain;

obtain filtered image data in the time domain by performing an inverse Fourier transformation of the filtered image data; and

identify edges of each of the plurality of marker patches based on the filtered image data in the time domain.

11. The system of claim 10 , wherein the frequency filter is a low-pass filter with a cutoff frequency set to the predetermined frequency or higher.