Patent ID: 8126345

Claim:
An image-forming device comprising: an image acquiring unit configured to acquire image data representing an image for one page, the image having a page length in a first direction, the image including a plurality of lines arranged in the first direction, each line including a plurality of pixels arranged in a second direction orthogonal to the first direction, the image data including a plurality of sets of line data, each set of line data indicating one of the plurality of lines and including a plurality of sets of pixel data, each set of pixel data indicating one of the plurality of pixels; an image forming unit configured to form an image based on the image data, the image forming unit including: a developing roller configured to carry a developer; a supply roller configured to supply the developer to the developing roller; and a bias applying unit configured to apply a first voltage between the developing roller and the supply roller in a first mode, and to apply a second voltage between the developing roller and the supply roller in a second mode; a line data setting unit configured to set at least two sets of consecutive line data among the plurality sets of line data as determined line data, the determined line data corresponding to an area having at least two consecutive lines, the area having a prescribed length in the first direction, the prescribed length being smaller than the page length; a determining unit configured to determine whether a ratio of pixels to be printed to total pixels in the area is higher than or equal to a prescribed ratio based on the determined line data; a controller configured to control the bias applying unit to apply the first voltage when the determining unit determines that the ratio is lower than the prescribed value, the controller controlling the bias applying unit to apply the second voltage when the determining unit determines that the ratio is higher than or equal to the prescribed ratio; and an updating unit configured to repeatedly update the determined line data by one line, and wherein, when the updating unit updates the determined line data, the determining unit determines whether the ratio is higher than or equal to the prescribed ratio based on the updated determined line data.