Image forming apparatus that corrects input gray level-output gray level characteristic data such that output gray levels gradually increase with higher input gray levels near a boundary

Claim 1

An image forming apparatus comprising: first storing means for storing input gray level-output gray level characteristic data which represent an output gray level relative to an input gray level; a density processing section which receives input gray level-output gray level characteristic data from said first storing means, said density processing section including second storing means for storing input gray level-output gray level characteristic data transferred from said first storing means, and address generating means for generating addresses corresponding to said second storing means in response to input gray level data, said second storing means and said address generating means cooperating to supply output gray level data from an address in said second storing means when said address is specified by said address generating means; inputting means for inputting characteristic correcting data, said characteristic correcting data providing input gray level-output gray level corrections corresponding to a plurality of division steps into which input gray levels are divided; correcting means for correcting input gray level-output gray level characteristic data stored in said first storing means on the basis of characteristic correcting data supplied by said inputting means; and reversal phenomenon correcting means for correcting, when there occurs at the boundary between adjacent steps such a reversal phenomenon that an output gray level in the step having relatively low input gray levels is higher than an output gray level in the step having relatively high input gray levels in input gray level-output gray level characteristic data obtained by the correction using said correcting means, said input gray level-output gray level characteristic data so that output gray levels gradually increase with higher input gray levels in the vicinity of the boundary, said reversal phenomenon correcting means including means for finding data of a point A at which a boundary line between the adjacent steps at which the reversal phenomenon occurs and a total output gray level characteristic line corresponding to the step having relatively low input gray levels which is represented by said input gray level-output gray level characteristic data obtained by the correction using the correcting means cross each other, means for finding data of a point B at which the boundary line between the adjacent steps at which the reversal phenomenon occurs and a total output gray level characteristic line corresponding to the step having relatively high input gray levels which is represented by said input gray level-output gray level characteristic data obtained by the correction using the correcting means cross each other, finding a point C having an output gray level equal to the output gray level of said point B on the total output gray level characteristic line corresponding to the step having relatively low input gray levels which is represented by said input gray level-output gray level characteristic data obtained by the correction using the correcting means, finding a point D having an output gray level equal to the output gray level of said point A on the total output gray level characteristic line corresponding to the step having relatively high input gray levels which is represented by said input gray level-output gray level characteristic data obtained by the correction using the correcting means, and means for finding a found straight line connecting said point C and said point D and changing the input gray level-output gray level characteristic data between the point C and the point D to characteristics represented by the found straight line.