Method, system and non-transitory computer-readable recording medium for measuring ball spin

1. A method for measuring ball spin, comprising the steps of:

detecting at least one mark appearing in a region corresponding to a ball in each of a plurality of images in which the ball is photographed, wherein a physical quantity of the ball is to be measured;

recognizing a mark commonly detected over a first image and a second image that are temporally adjacent, with reference to information on properties of the at least one detected mark in each of the plurality of images, and generating a mark sequence including information on the properties of the recognized mark in the first and second images; and

measuring a physical quantity related to spin of the ball with reference to the generated mark sequence,

wherein the detecting step comprises:

compensating, for each of the plurality of images, the properties of the at least one mark in each of the plurality of images, with reference to a line of sight in which a camera photographing the ball views the ball, and

wherein the compensating step comprises:

calculating, for each of the plurality of images, an angle between a real line-of-sight vector, which specifies the line of sight in which the camera views the ball, and a virtual line-of-sight vector, which is parallel to a reference axis defining a direction of gravity and specifies a line of sight in which a virtual camera located on a virtual straight line passing through the ball views the ball;

calculating, for each of the plurality of images, a compensation reference axis vector with reference to the real line-of-sight vector and the virtual line-of-sight vector; and

rotating, for each of the plurality of images, coordinates of the at least one mark by the calculated angle around the compensation reference axis vector, on a spherical coordinate system defining a surface of the ball.

2. The method of claim 1 , wherein the properties include at least one of a location, size, direction, and shape.

3. The method of claim 2 , wherein the properties are specified with respect to a spherical coordinate system defining a surface of the ball.

4. The method of claim 1 , wherein in the recognizing step, a first mark appearing in the first image and a second mark appearing in the second image are recognized as a mark commonly detected over the first and second images, when a relationship between the properties of the first mark in the first image and the properties of the second mark in the second image satisfies a predetermined condition.

5. The method of claim 4 , wherein in the recognizing step, whether the relationship satisfies the predetermined condition is determined with reference to information on at least one of a time interval between points of time at which the first and second images are respectively photographed, a normal spin rate or spin direction of the ball, and a previously measured spin rate or spin direction of the ball.

6. The method of claim 4 , wherein in the recognizing step, the first and second marks are recognized as the mark commonly detected over the first and second images, when a location and a shape of the first mark in the first image and a location and a shape of the second mark in the second image are similar at or above a predetermined level.

7. The method of claim 1 , wherein the physical quantity related to spin of the ball includes a spin rate and a spin direction of the ball.

8. The method of claim 1 , wherein in the measuring step, a spin rate of the ball is determined with reference to a distance between a location of the recognized mark in the first image and a location of the recognized mark in the second image, and a time interval between points of time at which the first and second images are respectively photographed, and a spin direction of the ball is determined with reference to a moving path between the location of the recognized mark in the first image and the location of the recognized mark in the second image.

9. A system for measuring ball spin, comprising:

a non-transitory computer-readable recording medium; and

program modules stored in the non-transitory computer-readable medium, the program modules comprising:

a mark sequence management unit configured to:

detect at least one mark appearing in a region corresponding to a ball in each of a plurality of images in which the ball is photographed, wherein a physical quantity of the ball is to be measured, and

recognize a mark commonly detected over a first image and a second image that are temporally adjacent, with reference to information on properties of the at least one detected mark in each of the plurality of images, and generate a mark sequence including information on the properties of the recognized mark in the first and second images;

a spin measurement unit configured to measure a physical quantity related to spin of the ball with reference to the generated mark sequence; and

an angle-of-view compensation unit configured to compensate, for each of the plurality of images, the properties of the at least one mark in each of the plurality of images, with reference to a line of sight in which a camera photographing the ball views the ball,

wherein the angle-of-view compensation unit is configured to:

calculate, for each of the plurality of images, an angle between a real line-of-sight vector, which specifies the line of sight in which the camera views the ball, and a virtual line-of-sight vector, which is parallel to a reference axis defining a direction of gravity and specifies a line of sight in which a virtual camera located on a virtual straight line passing through the ball views the ball;

calculate, for each of the plurality of images, a compensation reference axis vector with reference to the real line-of-sight vector and the virtual line-of-sight vector; and

rotate, for each of the plurality of images, coordinates of the at least one mark by the calculated angle around the compensation reference axis vector, on a spherical coordinate system defining a surface of the ball.

10. The system of claim 9 , wherein the properties include at least one of a location, size, direction, and shape.

11. The system of claim 10 , wherein the properties are specified with respect to a spherical coordinate system defining a surface of the ball.

12. The system of claim 9 , wherein the mark sequence management unit is configured to recognize a first mark appearing in the first image and a second mark appearing in the second image as a mark commonly detected over the first and second images, when a relationship between the properties of the first mark in the first image and the properties of the second mark in the second image satisfies a predetermined condition.

13. The system of claim 12 , wherein the mark sequence management unit is configured to determine whether the relationship satisfies the predetermined condition, with reference to information on at least one of a time interval between points of time at which the first and second images are respectively photographed, a normal spin rate or spin direction of the ball, and a previously measured spin rate or spin direction of the ball.

14. The system of claim 12 , wherein the mark sequence management unit is configured to recognize the first and second marks as the mark commonly detected over the first and second images, when a location and a shape of the first mark in the first image and a location and a shape of the second mark in the second image are similar at or above a predetermined level.

15. The system of claim 9 , wherein the physical quantity related to spin of the ball includes a spin rate and a spin direction of the ball.

16. The system of claim 9 , wherein the spin measurement unit is configured to determine a spin rate of the ball with reference to a distance between a location of the recognized mark in the first image and a location of the recognized mark in the second image, and a time interval between points of time at which the first and second images are respectively photographed, and determine a spin direction of the ball with reference to a moving path between the location of the recognized mark in the first image and the location of the recognized mark in the second image.