Prepress gray balance knowledge review--fine correction

6, fine correction of gray balance

Gray balance is the most important factor in photo printing. Because the human eye is much more sensitive to gray than other colors. There are many factors that affect the gray balance, such as partial clogging of the nozzles of the ink head, slight variations in the paper and ink concentrations. Therefore, when changing the paper, when using 200ml ink head or changing the ink batch, it is best to check the gray balance, and if there is obvious deviation, it should be adjusted.

To facilitate and more accurately adjust the gray balance, in addition to the more accurate CMYK curve adjustment function in 2004, we also prepared 24 grayscale print test files and 24 gray scales for each color separation scheme. Corresponding ink distribution file.

How to color

In the file we provide there is a file called "HP120-600 fine gray balance data .tif", which is printed on the printer with the printing scheme you want to correct. The size of the print should be as large as possible, so that the interpretation of the gray will be more accurate.

Figure 1 CMYK data of the grayscale and grayscales printed by the finer scheme
We can see from the figure that each of the 24 gray levels has CMYK data. They are the percentage of the ink volume, which is consistent with the display in the eH2004 tone curve.

Open the print setup function in the eH2004 software and tune to the color item. At this point you can see the CMYK's color curve.

If you think that the 10th block of the 24th gray scale is green, you can find this color block in Photoshop. The CMYK ink in this program is 25%, 22%, 34%, and 0%, respectively. The relatively simple correction method is that we can use the method of adding ink, in order to correct this part of the color cast, we first select the ink channel in the adjustment curve, move the mouse to the input value of 25%, observe the "change" The change in the amount of ink in the item, for example, for a color patch with less serious color cast, can select a change amount of 5% (no sign increases, - sign decreases). After modification, generate and print HP120-600 fine gray balance data. Tif, observe and compare changes and decide the next adjustment. Until satisfied.

Basic color relations

The inks printed are yellow, blue and green. However, the color casts we see are often reddish, greenish, and blue. Under these circumstances, how can we adjust the Huangpinqing curve to correct the color cast?

Let's look at the color wheel of Figure 1, where the red hue is 0 degrees. We notice that the opposite of red is blue; the opposite of green is color; the opposite of blue is yellow. The center of the color wheel is gray. We take into account that if the gray scale is green, only increasing the color can pull the color back to gray, or the color of the color block can be returned to gray by reducing the yellow and cyan.

Other color casts are similar to this.

Some coloring techniques

Lock non-colored areas

When correcting color cast, be sure to lock the curve in the non-color cast area. Normally, a point that does not increase or decrease is added at the junction between color cast and non-color cast (change value is 0). Sometimes after adjusting the color cast area, the curve of the uncolored area outside the junction will be twisted in the opposite direction. At this time, a point lock curve should be added to this area without increasing.

dichotomy

If there is an area that is color cast, how can we correct the color cast more quickly and better? Here is a method commonly used by mathematicians called dichotomy.

Assuming that blocks 2-10 are all green, we first select the (2+10)/2=6 block to adjust. When eliminating the color cast of No. 6 color block, notice that there are no color cast areas on both sides of No. 6 color block. If so, one color patch located in the color cast area is also adjusted. This will correct the color cast area quickly and with a minimum number of adjustment points.