Electrophotographic printing with column-dependent tonescale adjustment

A method of printing an image on a receiver using an electrophotographic printer having an optical writer adapted to expose a photoreceptor one row at a time in a cross-track direction and to deposit toner in a plurality of columns on the receiver, each column corresponding to a different cross-track position, the method comprising:

• printing a test target having a plurality of input pixel values in each of the plurality of columns on a first receiver;
• receiving measured densities for the printed test target to provide a respective column tonescale for each column, each column tonescale describing a relationship between the input pixel values and the measured densities for the respective column;
• receiving an aim tonescale describing a relationship between the input pixel values and aim densities;
• computing a respective column-tonescale gain for each column so that a scaled column tonescale formed by applying the column-tonescale gain to its respective column tonescale intersects the aim tonescale at a crossover point within a specified crossover range of pixel values;
• computing residual errors for each column representing a difference between the aim tonescale and the scaled column tonescale;
• determining an adjustment tonescale using the respective residual errors, wherein the adjustment tonescale relates each input pixel value to an exposure adjustment needed to more closely reproduce the aim density at that input pixel value, the adjustment tonescale changing sign within the crossover range;
• determining a respective adjustment-tonescale gain for each column using the respective scaled column tonescale, the adjustment tonescale, and the aim tonescale;
• receiving a plurality of input pixel values, each input pixel value having an associated column;
• and
• for each input pixel value:
• determining an aim exposure for the input pixel value using the aim tonescale;
• determining a first adjusted exposure by multiplying the aim exposure by the column-tonescale gain for the associated column;
• determining a second adjusted exposure by multiplying the value of the adjustment tonescale at the input pixel value by the adjustment-tonescale gain for the associated column and adding the result to the first adjusted exposure;
• using the optical writer to expose the photoreceptor in a respective pixel area according to the second adjusted exposure;
• applying toner to the photoreceptor to form thereon a visible image corresponding to the exposure of the photoreceptor;
• and
• transferring the visible image to a second receiver so that a print corrected for nonuniformity is produced.