1. Field of the Invention
The present invention relates to a recording apparatus and a recording method for recording data by using a recording head having a plurality of recording elements.
2. Related Background Art
An ink jet system has been known as an example of a type of recording system. In this recording apparatus or method, one line recorded by the scan of a recording head is formed by a plurality of ink droplets discharged from one discharge port (or ejection orifice). As a result, if there is a variance in the direction of discharge of the ink droplet and the amount of discharge from discharge port to discharge port, a stripe (or banding) may run or irregularity of density may occur in the recorded image, which should be uniform. Namely, if there is a scatter in the ejecting direction of the ink droplets, the positions of dots formed on the record sheet are shifted and the stripe runs on the image. Further, when there is a variation in the amount of discharge of the ink droplets, the sizes and the densities of the dots formed on the record sheet scatter and irregularity of density occurs in the image.
As one of the approaches to solve the above problems, it has been proposed to manufacture a recording head with a high precision to minimize the scatter of the direction of discharge and the amount of discharge of the ink droplets from discharge port to discharge port, but it involves problems of high manufacturing cost and low manufacturing yield.
As an approach to solve the problem of the irregularity of density by software, it is effective to change the number of implanted ink droplets so as to compensate the irregularity in the amount of discharge from discharge port to discharge port, but the build-in of such means leads to a rise of system cost. In addition, even with such an approach, it is not very effective to reduce stripe and when the scatter of the amount of discharge of the ink droplets from discharge port to discharge port changes by aging, it is necessary to readjust the number of implanted ink droplets and the maintenance ability of the apparatus is lowered.
As another approach to solve the problems of the stripe and the irregular density, it has been proposed to form one line by ink droplets discharged from a plurality of discharge ports to reduce the scatter in the direction of discharge and the amount of discharge of the ink droplets, as disclosed in U.S. Pat. No. 4,967,203.
FIG. 12 illustrates that method. In a first scan, pixels (one pixel consists of four dots in the illustrated example) which are not adjacent in vertical and horizontal directions are recorded. Then, the record sheet is fed in a sub-scan direction (vertically in the illustrated example) by four discharge port lengths so that the remaining pixels not recorded in the first scan are recorded in the second scan.
In this method, since one line (lateral array of dots) is formed by using two discharge ports, the scatter in the direction of discharge is averaged so that the stripe is less visible. Where the scatter of the amount of discharge of the ink droplets from discharge port to discharge port has a normal distribution with a standard deviation .sigma., the scatter of the amount of implanted ink decreases to .sigma./.sqroot.2. Since the scatter of the amount of ink between lines is recognized as a scatter of the image density, an image of smaller scatter of density is attained by this method.
In this method, however, since only two discharge ports are used to form one line along the direction of movement of the carriage, the effect to reduce the stripe and the irregularity is not sufficient. Further, depending on the image, such as one in which pixels to be recorded are arranged alternately, the stripe and the irregularity are not reduced at all. In order to enhance the effect of reducing the stripe and the irregularity, more discharge ports may be used to record one line but this significantly reduces the recording speed because one line must be recorded by a number of times of scans.
The above method is applicable to a binary recording method (in FIG. 12, one dot is recorded by one ink droplet). A recording method which suppresses the stripe and the irregularity in a multi-level recording method (in which one dot is recorded by two or more ink droplets) is disclosed in U.S. patent application Ser. No. 894,555 assigned to the assignee of the present invention.
FIG. 13 illustrates that method. It shows a method for recording 0 to 3 ink droplets per dot by using a recording head having 12 discharge ports.
As shown in FIG. 13, when the discharge ports #9-#12 are used in a first scan, a record sheet is transported in a sub-scan direction (vertically in FIG. 13) by four discharge port lengths, and in a second scan, data is recorded by using the discharge ports #5-#12. In this scan, a portion of the dots recorded by the discharge ports #5-#8 is overlapped with the dots recorded in the first scan.
Similarly, in a third scan, the data is recorded by using the discharge ports #1-#12. As a result, the record of a first region is completed, and second and third regions are sequentially completed in subsequent scans.
In this method, like the method disclosed in U.S. Pat. No. 4,967,203, since one line is formed by the ink droplets discharged from a plurality of discharge ports, the scatter in the direction of discharge and the amount of discharge of the ink droplets from discharge port to discharge port is reduced and the stripe and the irregularity are harder to occur. In this method, the pixels in one line are sequentially allotted to scans for recording. For example, FIG. 13 shows the scan number at which the ink is discharged to each pixel of the first region. As seen therefrom, the number of times of ink discharge is assigned to each pixel. In this method, even if the pixels to be recorded are arranged alternately, the stripe and the irregularity are improved because those pixels are sequentially recorded by different discharge ports, and it is superior to the recording method disclosed in U.S. Pat. No. 4,967,203.
However, in this method, since the number of different discharge ports used to record one line in the direction of movement of the carriage is three, the effect to reduce the stripe and the irregularity is not sufficient. Further, if the recording of one line is made by using more discharge ports to enhance the reduction effect, the recording speed is significantly lowered.