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.
When compressing the tone contrast of the original, the following methods are generally used to compress certain parts.
① Under normal circumstances, for a normal original with appropriate contrast, the low-key gradation should be compressed flatter, especially when emphasizing the mid-tone and highlight gradation, it is more appropriate.
② If the original is dull and the highlight tone is flat, emphasizing the dark and mid-tone gradation will relatively flatten the mid-tone gradation.
③ If the original contrast is dull and the low-key area is large, the low-key gradation should be expanded, but the mid-tone gradation will appear relatively flat.
④ If the mid-tone gradation of the original is flat and the contrast is small, the highlight gradation can be compressed to make the mid-tone gradation contrast larger.
⑤ If the original contrast is basically suitable for print reproduction and the photo is mainly mid-tone with a small dark area, the dark tone gradation can be compressed to emphasize the mid-tone gradation.
⑥ Color photos are often encountered, and these originals are thin with small contrast. For these originals, the highlight and dark tone gradation can be compressed to make the mid-tone gradation contrast larger.
⑦ If the highlight gradation of the original is flat and the contrast is small, the highlight gradation can be expanded, but the mid-tone gradation will be relatively flat. <next>
The contrast of transmissive originals, such as natural color positives, is relatively large, usually around 3.0, so contrast compression is usually needed to meet the requirements of print reproduction. Reflective originals have smaller contrast and are dull, so bright reproduction can be performed according to the characteristics of different originals. For example, the key to reproducing portrait paintings is the skin tone, with y and m being the key colors; the focus of reproducing landscape paintings is the natural scenery, with y and c being the key colors. Flexibly using the gradation adjustment to adjust a reasonable gradation curve, correcting the shortcomings of the original, maintaining the main parts, and sacrificing the secondary parts, bold choices should be made to achieve the best reproduction effect. However, if the gradation curve is not adjusted reasonably, it may result in a reversed main and secondary focus, unclear gradation, and inaccurate color.
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The main drawbacks of color reproduction in prints are insufficient primary colors and excessive complementary colors. To reproduce high-quality prints, the primary colors should be deeper without losing gradation. The primary colors should reach 100%, medium primary colors should be full, light primary colors should be appropriate, and highlight primary colors should not be lost.
Complementary colors should be appropriate. Due to the insufficient purity, hue, and transparency of the ink on the market, and the poor whiteness and smoothness of the paper, the complementary colors of brighter colors should be smaller, the complementary colors of darker colors should be used, and the complementary colors of small areas in the brightest parts can be omitted, thus playing a role in color change and gradation contrast. Proper use can make the primary colors more vivid, solid, and rich in color.
Color reproduction in prints is actually the subtractive color reproduction of the three primary colors y, m, and c with additive color light as the primary color light. The three primary colors y, m, and c are the basis of all colorful changes. The black plate does not cause any color change. The role of the black plate is only to stabilize the gray balance, acting as a skeleton in the color image, making the dark tones darker, emphasizing the low-key gradation and outline, and enhancing the thickness and three-dimensional sense of the image. The three primary colors y, m, and c transition to the black field, that is, y + m + c = approximate black. Part of the neutral background color (three primary colors) formed by removing part of the three primary colors y, m, and c is replaced by black ink. This is the undercolor removal (UCR) method used by electronic color scanners and Apple computers. Undercolor removal has many necessities: ① It improves printability. Generally, it is used in the dark tone area. If the three primary colors are used for printing in the dark tone area, the dot area is often very high, and it is easy to produce problems such as excessive ink thickness and plate sticking in high-speed multi-color printing. If undercolor removal is used, the ink volume will be greatly reduced, mainly saving the amount of y, m, and c color inks and improving printability. ② It is beneficial to control the neutral gray balance.
The length of the black plate tone depends on the amount of undercolor removal of the three primary colors y, m, and c. If the amount of undercolor removal is large, the mid-tone and dark tone gradation will be relatively flat, and the black plate tone can be appropriately extended to compensate for the lost gradation of the three-color plate. If the amount of undercolor removal of the three-color plate is small, the black plate tone can be appropriately shortened to avoid dull mid-tones. The dark tone is generally calibrated according to the maximum density of the image, mainly to meet the image gradation. The calibration should be based on the characteristics of the original, paying attention to the color balance of the dark tone, and the dark tone calibration should also consider the relationship with undercolor removal (UCR).
093 487- 07-07
095 487- 07-07
097 487- 07-07
