Printhead Control

Apparatus for printing a two-dimensional bit-mapped image having a number of pixels per row, said apparatus having a plurality of overlapping printheads (300) or a printhead or printheads indexed through overlapping positions, the or each printhead having a row of ejection channels (301), each ejection channel having associated ejection electrodes to which a voltage is applied in use sufficient to cause concentration of particles in the printing fluid at the ejection channels, and wherein, in order to cause volumes of printing fluid to be ejected from selected ejection channels of the overlapping printheads thereby forming a pixel of a predetermined optical density and/or greylevel, voltage pulses of respective predetermined amplitude and/or duration, as determined by respective image pixel bit values, are applied to the electrodes of the selected ejection channels, characterised in that

• for each row of the image, the values of the voltage pulses to be applied to the overlapping printheads (300) to form pixels printed by overlapped ejection channels (301) are adjusted in dependence on the position of the pixel within an overlapped region of the printheads and in dependence on the predetermined optical density and/or greylevel of the pixel,
• wherein, for at least one pixel in the overlapped region, the total volume of ink ejected by the overlapped channels is greater than that required if that pixel were formed by a single channel ejection.