Symbol classification depending on cluster shapes in difference image

A computer readable medium storing a software product for classifying elements of a digital image, the software product comprising instructions operable to cause one or more data processing apparatus to perform operations comprising:

• receiving an element of the digital image and a prototype representing a class of elements;
• generating a difference image representing differences between the received element and the prototype and including a plurality of ON pixels, each ON pixel representing a local difference between the received element and the prototype, wherein the difference image includes one or more clusters of ON pixels, each cluster including one or more neighboring ON pixels;
• determining relative arrangements of ON pixels in the difference image;
• assigning one or more scores to the difference image using the determined relative arrangements of ON pixels in the difference image;
• based on the scores assigned to the difference image, determining whether the received element of the image belongs to the class represented by the prototype;
• and
• generating a representation of the digital image, the representation including a symbol map associating one or more elements of the digital image with the prototype of the class.

A system comprising:

• one or more processors;
• and one or more computer readable mediums storing a software product for classifying elements of a digital image, the software product comprising instructions operable to cause the one or more processors to perform operations comprising:
• receiving an element of the digital image and a prototype representing a class of elements;
• generating a difference image representing differences between the received element and the prototype and including a plurality of ON pixels, each ON pixel representing a local difference between the received element and the prototype, wherein the difference image includes one or more clusters of ON pixels, each cluster including one or more neighboring ON pixels;
• determining relative arrangements of ON pixels in the difference image;
• assigning one or more scores to the difference image using the determined relative arrangements of ON pixels in the difference image;
• based on the scores assigned to the difference image, determining whether the received element of the image belongs to the class represented by the prototype;
• and
• generating a representation of the digital image, the representation including a symbol map associating one or more elements of the digital image with the prototype of the class.