Image acquisition device and method for determining a focus position based on sharpness
View Patent ↗A method for acquiring an image comprises acquiring a first image frame including a region containing a subject at a first focus position; determining a first sharpness of the subject within the first image frame; identifying an imaged subject size within the first image frame; determining a second focus position based on the imaged subject size; acquiring a second image frame at the second focus position; and determining a second sharpness of the subject within the second image frame. A sharpness threshold is determined as a function of image acquisition parameters for the first and/or second image frame. Responsive to the second sharpness not exceeding the first sharpness and the sharpness threshold, camera motion parameters and/or subject motion parameters for the second image frame are determined before performing a focus sweep to determine an optimal focus position for the subject.
1. A method for acquiring an image comprising:
acquiring a sequence of image frames of a given scene;
detecting within each image frame any region containing a subject with a reference subject size;
recording an identifier of any region containing such a subject on which auto-focus might be based;
determining a number of variations of subject identifiers for a given number of frames preceding a given frame in said sequence;
responsive to said number of variations exceeding a threshold, disabling subject based auto-focus for subsequent frames until at least a condition for a given scene is met;
responsive to said number of variations not exceeding a threshold in a given frame containing at least one subject, utilising subject based auto-focus for acquisition of at least a subsequent frame in said sequence; and
otherwise using non-subject based auto-focus for at least a subsequent frame in said sequence.
2. A method according to claim 1 wherein said number of variations is defined as a sum of changes of identifiers of subjects on which auto-focus might be based in previous pairs of successive frames of a given scene.
3. A method according to claim 1 , wherein determining a change of identifiers between a successive pair of frames is modified by at least one tuning parameter and a maximum number of detected subjects in any image frame of a scene.
4. A method according to claim 1 wherein a face detection based auto-focus comprises:
checking if a lens actuator setting for acquiring a given image frame provides maximum sharpness for a region of said image frame including a detected subject on which auto-focus might be based; and
responsive to said sharpness not being maximal:
determining a lens actuator setting providing a maximum sharpness for the region of said image frame including said detected subject;
determining a lens displacement corresponding to said lens actuator setting;
calculating a distance to said subject based on said lens displacement;
determining a dimension of said subject as a function of said distance to said subject, said imaged subject size and a focal length of a lens assembly with which said image frame was acquired; and
tracking said subject through at least one subsequent image frame of said scene including auto-focussing on said subject using said determined dimension of said subject; and
responsive to said sharpness being maximal, tracking said subject through at least one subsequent image frame of said scene including auto-focussing on said subject using said reference subject size.
5. A method according to claim 1 wherein said subject comprises a face and wherein said reference subject size is a distance between eye regions of said face.
6. A method according to claim 1 wherein said condition for a given scene comprises detecting a change of scene.
7. A method according to claim 6 wherein a change of scene is based on an assessment of one or more of: camera motion and subject motion.
8. An image acquisition device comprising an auto-focus module arranged to acquire image acquisition, subject motion and camera motion parameters for an acquired image frame; and to perform the method of claim 1 .
9. A computer program product comprising a non-transitory computer readable medium on which computer executable instructions are stored which when executed on an image acquisition device are arranged to perform the method of claim 1 .