Important Adjustments Before Printing Color Prints
The original print needs to be separated into four colors for plate making. The originals separated by Apple computers include reflective originals (such as color pictures, paintings, watercolors, oil paintings, traditional Chinese paintings, New Year pictures, gouache, etc.) and transmissive originals (such as color reversal films (natural color positives), color negatives, etc.). The brightness and darkness on the color negative original are opposite to the original scene, and the colors are complementary to the original scene. For example, the complementary color of magenta on the negative is green; the complementary color of yellow is blue-purple; the complementary color of red is cyan. The brightness and darkness on the positive original are the same as the original scene, and the colors are exactly the same as the original scene.
The reproduction of transmissive originals is a visual image formed under the action of transmitted light. Reflective originals are visual images formed by the reflection of the original under light source illumination.
The tone of a print is the classification of colors according to brightness and darkness. In print reproduction work, color reproduction, tone reproduction, texture reproduction, and sharpness reproduction must be achieved. Color reproduction means accurate hue, and the reproduced print perfectly reproduces the colors of the original without color cast or stray colors.
Ideal tone reproduction refers to the "faithful" reproduction of the tones on the original in the printed product, meeting the reproduction requirements of the original. Since the maximum recording density of the print is 1.8, after reaching 1.8, it becomes a straight line parallel to Dt (original density). Therefore, whether it is a reflective original or a transmissive original, adjustments must be made to compress contrast, correct color differences, and achieve a tone range suitable for print reproduction.
For example, the highest density on the color positive original is Dt = 3.0, while the maximum recording density of the print is Dr = 1.8.
From the curve, it can be seen that the reproduction of the print cannot achieve the highest density reproduction of the original because the materials used in the print reproduction process, including ink, paper, photosensitive materials (film, PS plate), color sequence, and printing machinery, limit the maximum recording density to Dr = 1.8. However, when Dt = 3.0, the highest density of the original is large, and the contrast must be compressed, otherwise the actual effect cannot be printed.
Tone adjustment is often required for tone reproduction in prints. When the density contrast of the original is greater than the maximum recording density of the print, in order to reproduce all tones of the original on the print, appropriate adjustments must be made to the tone reproduction. Secondly, to meet the reproduction requirements of certain special originals (such as photos with bright tones or dark tones), certain tones of the original must be appropriately emphasized, while other tones are compressed accordingly. The main characteristic of linear compression is the proportional adjustment of each tone segment. Linear compression does not represent a true visual proportional compression because the D-Y curve is also nonlinear. After this compression, the human eye cannot easily distinguish the bright tones, resulting in a "flat gray" phenomenon. [next]
Tone reproduction refers to the relationship between the original density and the reproduced dot percentage.
To ensure that all tones and colors on the original are well represented on the print, the correct gradation curve must be selected first. Gradation is the basis of tone and color changes. If the gradation on the image is not rich, the color representation will inevitably be monotonous. Only with realistic gradation can there be vivid color changes.
Absolute faithful reproduction of the original is impossible because the density contrast of most originals is greater than that of the print. Therefore, print reproduction must compress the density, which will result in the loss of some subtle details. In practice, a reasonable gradation curve should be adjusted according to the contrast and tone content of the original. This means compressing the gradation of the secondary parts of the original, whether it is a reflective or transmissive original, and emphasizing the main parts with a focus on their characteristics.
Under the premise of normal contrast of the color original, the gradation curve should be correctly adjusted according to different types of originals. The gradation control of the electronic color scanner and the Apple computer used for color separation is both convenient and flexible. It can locally strengthen or compress, but attention must be paid to the balance of the gradation curve of the separation film, and the neutral gray balance must be appropriate. Incorrect gradation curves will distort the reproduction. Therefore, special attention must be paid to the correct use of gradation when operating the electronic color scanner and Apple computer. However, for the overall color cast of the original, it can be corrected by adjusting the gradation curve.