Image processing apparatus and image processing method

An image processing method employed in an image processing apparatus having a storage configured to store image data representing a color image, the image data being constituted by multiple pixels, each of the multiple pixels having a gradation value,

• the method comprising:
• generating a histogram index values corresponding to quantities of the brightness values of the multiple pixels constituting the image data stored in the storage;
• setting an original threshold value of the index value based on the histogram which is referred to for binarization;
• detecting a mound-shaped part, in the histogram, satisfying a particular condition;
• setting an adjusting direction in which the original threshold value is to be adjusted;
• setting the index value at a base on a particular direction side of a particular mound-shaped part which is one of mound-shaped parts existing on the adjusting direction side with respect to the original threshold value in the histogram as an adjusted threshold value;
• and
• applying a binarizing process to the image data using the adjusted threshold value.