1. A developer consumption determining method for determining, using a developer consumption detection mechanism, a developer consumption of an image forming apparatus which forms an image by developing a latent image formed on an image bearing member by scanning with light beams in a main scanning direction while rotating the image bearing member in a sub-scanning direction to form light beam spots on the image bearing member, which are overlapped in the main scanning direction and the sub-scanning direction and include writing pixels, comprising the steps of: dividing, using the developer consumption detection mechanism, one of the writing pixels into a plurality of scanning sub-pixels in the main scanning direction and a plurality of overlapping sub-pixels in the sub-scanning direction, the plurality of overlapping sub-pixels being sub-pixels on which the light beam spots overlap in the sub-scanning direction; determining, using the developer consumption detection mechanism, a weighting coefficient for each of a plurality of surrounding scanning sub-pixels of one of the plurality of overlapping sub-pixels, which are on a scanning line adjacent to the one of the plurality of overlapping sub-pixels, and are located within a predetermined distance from the one of the plurality of overlapping sub-pixels. on the basis of respective positional relationships of the plurality of scanning sub-pixels with the one of the plurality of overlapping sub-pixels; determining, using the developer consumption detection mechanism, respective emission coefficients for the plurality of scanning sub-pixels on the basis of one of a plurality of durations and exposures of the light beams applied to each of the plurality of scanning sub-pixels; summing products of the weighting coefficients of the plurality of scanning sub-pixels and the respective emission coefficients to determine a total exposure for the one of the plurality of overlapping sub-pixels: repeating for all of the plurality overlapping sub-pixels the steps of determining the weighting coefficient, determining respective emission coefficients, and summing products; summing the total exposures for all the overlapping sub-pixels to determine a developer consumption attributed to the overlapping of light beam spots in the sub-scanning direction; and determining using the developer consumption detection mechanism, the developer consumption of the image forming apparatus by using the developer consumption attributed to the overlapping of light beam spots in the sub-scanning direction.