Object detection method and image search system

An object detection method to be performed by an image analysis device configured with a computer including a processor and a memory, the object detection method comprising:

• a first step of receiving, by the processor, input of an image and storing the image to the memory;
• a second step of extracting, by the processor, a partial area which is a part of the input image and storing the partial area to the memory;
• a third step of evaluating, by the processor, symmetry of the partial area using features extracted from the partial area and reflected features obtained by applying one or more transformation matrices to corresponding one or more image feature vectors obtained from the features;
• and
• a fourth step of determining, by the processor, that the partial area is an object area including an object to be detected in a case where the symmetry of the partial area is evaluated as higher than a predetermined threshold,
• wherein the third step includes:
• a step of distributing the orientations of intensity gradient vectors extracted as the features to a range from 0 to 180 degrees to process in a case where the input image is a binary image;
• and
• a step of distributing the orientations of intensity gradient vectors extracted as the features to a range from 0 to 360 degrees to process in a case where the input image is not a binary image.