1. Field of the Invention
The present invention relates to a printing system that enables adjustment of positional misalignment of dot creation, an equivalent method of adjustment, and a recording medium.
2. Discussion of the Background
Ink jet printers that cause ink to be ejected from a print head and thereby implement printing have been widely used as an output device of the computer. The ink jet printer moves the print head back and forth relative to a printing medium during a main scan and ejects multiple color inks to create dots. Variable dot printers that are capable of expressing multiple densities with regard to each pixel, instead of just two stages, dot-on and dot-off, have recently been proposed. The multiple densities are attained, for example, by creating dots of different ink quantities.
In some of these ink jet printers, dots are created during both forward and backward movements in the main scanning direction for the purpose of enhancing the recording speed (hereinafter this recording process is referred to as bidirectional recording). In this case, it is required to make the positions of dots created in the forward pass coincident with the positions of dots created in the backward pass in the main scanning direction. A relative positional misalignment between dots created in the forward pass and dots created in the backward pass is attributable to a rough touch to a resulting printed image and deteriorates the picture quality of the resulting printed image. One proposed method to reduce such a positional misalignment utilizes a predetermined test pattern for the adjustment. The background art technique adjusts the creation timings of, for example, black dots in the forward pass and in the backward pass to reduce the positional misalignment of dots in the course of bidirectional recording.
This background art adjustment technique can not, however, sufficiently reduce the positional misalignment of dots in the variable dot printer that creates dots of different ink quantities. The flight characteristics of an ink droplet ejected from the print head depend upon the ink quantity. In some cases, even when the dot creation timing is adjusted for one of the variable dots, the adjustment of the dot creation timing may be insufficient for the other dots. The background art adjustment technique leaves some dots having an insufficiently corrected positional misalignment in the course of bidirectional recording. This causes deterioration of the picture quality in the case of bidirectional recording.
In the case of bidirectional recording, even a slight positional misalignment of dot creation often significantly affects the picture quality. For example, it is assumed that a print head moves left and right during a main scan and creates dots in the forward pass at positions deviated leftward from the expected positions. Because of the characteristic of the print head, dots are created in the backward pass at positions deviated rightward from the expected positions. The relative positional misalignment between the dots created in the forward pass and the dots created in the backward pass in the course of bidirectional recording is approximately twice the positional misalignment of dots created in only one of the forward pass and the backward pass. Namely, the presence of dots having insufficiently adjusted creating positions significantly deteriorates the picture quality in the case of bidirectional recording.
The printer is generally required to perform high-quality and high-speed printing. Bidirectional recording with a printing speed almost double that of unidirectional recording is desirable for the purpose of the high-speed printing. Due to the positional misalignment of dots discussed above, bidirectional recording produces a reduced picture quality and is thus used in a print mode that gives preference to printing speed over picture quality. With the recent advance of the printer for the higher resolution and a higher picture quality, an image tends to be printed by a greater number of passes of the main scan. Since unidirectional recording has a low printing speed, the enhanced printing speed of the bidirectional recording is highly demanded. The recent trend simultaneously requires an extremely high picture quality, even in the case of bidirectional recording. In the variable dot printer that enables a multi-value expression in each pixel for the improved picture quality, the deterioration of the picture quality due to the positional misalignment of dots in the course of bidirectional recording is of great significance.
The variable dot printer varies the quantity of ink ejected from an identical nozzle and enables dots of different ink quantities to be created in respective pixels. No technique has been proposed to adjust the positional misalignment of variable dots created by the identical nozzle. Further, different ink quantities generally result in different flight velocities of the ink droplets. There is accordingly a positional misalignment of dots having different ink quantities even when they are created by the identical nozzle.
The ink ejection timing is set in advance to prevent the positional misalignment by taking into account the differences in flight velocity. However, it is extremely difficult to completely cancel such a positional misalignment, since there is a variation in flight velocity of the ink droplet due to the variation in manufacturing error of the print head. The positional misalignment of dots is also ascribed to a variation in thickness of printing paper. In the case of thick printing paper, there is a smaller distance between the print head and the printing paper. This shortens the flight time of the ink droplet. In the case of thin printing paper, on the contrary, the ink droplet has a longer flight time. The ejection timings to form the ink droplets of different flight velocities at an identical position are set, based on the relation to the flight time. When the thickness of the printing paper is changed from the initial setting, there is a positional misalignment of ink droplets. Such a positional misalignment significantly damages the picture quality even in the case of creating dots only in one direction of the main scan.