As shown in FIG. 30, output information (RGB image gray gradation image, CMYK image data) from a personal computer (PC) is supplied to the image processor provided in or connected with the image forming apparatus. In the image forming station, the output information from the PC is processed by a gamma correction portion for making correction in accordance with the gamma condition set by the printer driver depending on the user preferable, a converting portion to L*a*b* (only L* for gray), a converter to), C′M′Y′K′.
A color space conversion from a many dimension to a many dimension, for example, from RGB to L*a*b* or the like, is effected by the image processor using a multi-dimensional conversion table information of an ICC profile.
Referring to FIG. 31, there is shown a color setting portion of a printer driver which is a user interface (UI) provided for the user set a desired tone gradation. On the basis of the set point information, the gamma correction portion effects a gamma conversion using a unidimensional LUT.
Referring to FIG. 30, the image data converted to the C′M′Y′K′ information is subjected further to a unidimensional LUT process by the engine tone gradation corrector. The engine tone gradation corrector is effective to keep the constant tone gradient of the printer engine. The state of the engine is detected using a patch image outputted on the recording material, that is, the sheet, and a unidimensional LUT is produced so as to provide a predetermined tone gradation curve. A stabilization control in which a patch outputted on the sheet thereafter is called automatic tone gradation correction.
A pseudo-half-tone processing portion functions to reproduce a half-tone using a dither method or the like. Since the engine tone gradation property is different depending on the kind (line number and/or dot growth method) of the half-tone, the tone gradation corrector is provided with a LUT for each of half-tone processes.
A technique assuring the stabilization property of color using the engine tone gradation corrector has been proposed.
Japanese Laid-open Patent Application Hei 1-309082, for example, a patch pattern for toner content detection formed on the photosensitive member is read by a density sensor, and the read information is fed-back to the toner content controller in the developing device to provide a proper density.
Generally, such a toner patch is easy to produce and erase, but what is obtained thereby is only density information prior to image fixing, and therefore, when the control based on the toner patch is carried out, the influence of the secondary transfer or the fixing step are not reflected.
Japanese Laid-open Patent Application Hei 1-309082 proposes that an image is read by a reader portion of a copying machine main assembly, and on the basis of the result of the reading, the image formation is controlled. With this method, the output print provided by fixing the toner on the sheet is read by the reader portion.
As a result of printing on standard, non-standard paper A (smooth thin paper), non-standard paper B (rough thick paper) under constant image forming conditions (charging, latent image and developing conditions), the density values detected by the reader portion are as shown in FIG. 32. On the basis of this information, a LUT shown in FIG. 33 is produced to provide a density.
The read density value is different depending on the paper surface of property, whiteness degree, thickness or the like, even if the toner amount thereon is the same, and therefore, the LUT varies. The LUT prepared on the basis of the non-standard paper B is lower in the maximum density than the target, and therefore, after input value of 240, data of 255 is used, for all. Therefore, over 240th level, the signal value exhibits no tone gradation. This is critical defect for the user who takes the tone gradient in the shadow portion in such a range important.
In addition, when the LUT is prepared on the basis of the non-standard paper A, the signal of 255 is outputted around the level of 240. Therefore, in the case of a signal indicative of solid image (halftone dot area percentage is 100%), a half-tone process pattern appears with the result that quality deterioration, and/or jaggy patterns (screen patterns) appear at edge portions. Thus, the quality of letters and/or lines deteriorates.
In order to solve such a problem, Japanese Laid-open Patent Application 2006-165752 proposes that when the input signal is 255 (halftone dot area percentage is 100), the OUT side of the LUT is made 255, thus providing a certain degree of tone gradation.
FIG. 34 shows a LUT prepared on the basis of the non-standard paper B, and a LUT prepared by the method disclosed in Japanese Laid-open Patent Application 2006-165752. It will be understood that the LUT exhibits a smooth tone gradation. FIG. 35 shows a reproducibility of the density using the LUT. In the shadow portion, the density is different from the target density, but the tone gradient is slightly improved.
However, with such technique, the tone gradation change of the shadow portion is so intense that an inflection point appears at (a) in FIG. 35, and therefore, smooth tone gradation property is not provided. Because of this, some professional users are not satisfied.
As another technique relating to the automatic tone gradation correction, Japanese Laid-open Patent Application 2004-289200 and Japanese Laid-open Patent Application 2006-222804 discloses that a patch image is printed on the non-standard paper using tone gradation correction data prepared on the basis of the standard paper, and the result of reading the image is used for a correction target. The automatic tone gradation correcting method is proposed as being free of the difference in the density or surface property of the paper per se.
In many cases, the density in the foregoing automatic tone gradation corrections is not an absolute density but is a paper based density (Null Density, relative density). In the following, the paper based density is called relative density. Relative density=absolute density of image−absolute density.
The density management method in the relative density is stipulated in ISO13656 or the like, and is an ordinary reference in the printing field Using the relative density, it is unnecessary to change the target density even when the printing is carried out on various kinds of paper, and therefore, the relative density has been used for management of the printing machines in printing companies
Also in the automatic tone gradation correction for an image forming apparatus, the automatic tone gradation correction has been carried out on the basis of the paper based density in view of variations of the reading apparatus and change with time thereof, differences of the paper density due to the differences of the paper lot difference, or the like
However, in the case of using the automatic tone gradation correction disclosed in Japanese Laid-open Patent Application 2004-289200 and Japanese Laid-open Patent Application 2006-222804, the user has to prepare standard paper beforehand. In addition, the user has to be aware of the type of the non-standard paper and has to always carry out the tone gradation correction for the non-standard paper this means that what results is mere replacement of the standard paper with the non-standard paper. There is a liability that the non-standard paper registered on the basis of the standard paper may become out of stock. In such a case, the problem arises, because the user encounters the difference in the surface property, the whiteness degree, thickness, and/or grain of the paper due to lot difference, in addition, the difference in the whiteness degree, the paper per se due to the atmospheric corrosion resistance.
When a plurality of kinds of paper are registered against the inventory depletion of the non-standard paper, the user has to make choice the kind of paper to use for the automatic tone gradation correction. With increase of the number of the prepared kinds, the operation becomes more difficult, and the probability of mistake increases. In addition, the memory capacity for the registration has to be large correspondingly with the result of the increase in cost
Furthermore, the user is required to carry out twice the reading operations each including producing tone gradation correction data on the basis of reading of the printed patch on the standard paper, printing a patch on non-standard paper using the produced tone gradation correction data, reading the patch by the image reading station, and making correction. Therefore, the usability is not sufficient.
Moreover, the coloring material of electrophotographic image is low in the transmission factor, and therefore, a solid image density is substantially immune to paper density. The conventional automatic tone gradation correction utilizing the density detection method in the printing field as it is, when the same amounts of the coloring material are on different sheets of paper, the relative densities are different. For this reason, when the automatic tone gradation correction is effected with different paper density, the result of the density after correction is different, and therefore, the automatic tone gradation correction with the standard paper is required.