1. Technical Field of the Invention
The present invention relates to an image formation method and in particular to an image formation method effective for providing a record image of high quality.
2. Description of Related Art
In recent years, full-color laser beam printers and thermal transfer printers have been rapidly finding spreading use as output terminals of personal computers, workstations, etc. In this case, required performance includes the capability of outputting text data, line drawing data, and image data, namely, continuous-tone output is required.
As this kind of system, for example, a system disclosed in Unexamined Japanese Patent No. Hei 9-1866 is well known.
The system divides an image into a plurality of blocks, changes the input/output characteristics corresponding to the positions of pixels therein for each block, converts image data density level into density level to recorded dots, and changes and disperses the positions where dots of the same size are recorded for each block, thereby preventing appearance of structural pattern in the image such as moire.
Basically, the conventional system is a kind of a pseudo area gradation technique using a multilevel dither method having only less discrete density level. When, the configuration as described above is adopted, characteristics such as, size and, placement regularity of recorded dots is weakened as much as possible, thereby preventing appearance of structural pattern in the image such as moire.
In more detail, image data is converted from R(red), G(green), and B(blue) of brightness data into density data of three primary colors of C(cyan), M(magenta), and Y(yellow) for printing, then an under color is removed for preparing a black plate based on the density data, and C, M, Y, and BK (black) data is prepared. Further, correction processing such as masking is applied, then gradation processing is executed in such a manner that the pixels at the positions in the block described above are divided into odd and even lines and different gradation conversion table characteristics are applied to each of the lines.
However, in the system, the gradation conversion table characteristics are monotonous, thus occurrence of roughness is inevitable when low-density recorded pixels are formed on a white background.
The system assumes that a printed matter has stable characteristics. In fact, it is necessary to consider printing on a printed matter for which is hard to say that its characteristics are always stable. That is, with photosensitive material for thermal developing or photo-thermal-sensitive recording material, even if energy at the minimum level required for recording is given, the threshold as to whether or not color is developed varies due to variation in sensitivity caused by humidity, etc., and the print result becomes delicately different.