Color Mode Application During Pre-press Image Processing
Color image processing is a procedure that usually arises in composing newspaper or magazine. When you open a color image, its mode may be RGB or CMYK. Which of them shall be used to process color image when using Photoshop? When Photoshop is used to process images, the first point is that an image, whether it is in CMYK mode or RGB mode, shall stay as it is opened. Don’t convert it from one mode to the other, let alone convert it for many times. Every color space conversion in matrix image editing software would lose some detail information of the original image. Multiple conversions between the two modes would cause great information loss. The important thing is: image mode used for plate-making and printing during the color newspaper publication must be in CMYK mode, otherwise it would fail to be printable. However, it is not necessarily good to process images in CMYK mode. You have to select a mode according to the actual situation. In case of using Photoshop to process images, RGB mode can produce better effect than CMYK. You can process image in RGB mode and then convert it into CMYK mode and output film for plate-making and printing. It is very favorable to composition of color newspaper if the opened image is in RGB mode, or RGB mode is selectable for scanning when use scanner to input original image, When use Photoshop to scan original image, you may select RGB setting option in the menu, the input color image through scanner would be in RGB mode. That is to say, when you can obtain RGB mode image without conversion at the first place, you shall process image in this mode, especially for images downloaded from internet. In order to ensure printing effect of images, RGB mode is strongly recommended. The following discussion will further convince you on it. (1) RGB mode is adopted by all optical-principle-based equipments, so it can display many colors, especially bright and vivid colors (color of the monitor shall be calibrated to avoid color distortion). These colors are hard to be printed, that is why images would be darkened after being changed from RGB to CMYK. When use Photoshop to edit RGB mode images, you have to select CMYK Preview instruction in the menu of View, i.e. adopt RGB to edit and process images, but use CMYK mode to display images, thus the image color the operator see on monitor is the needed color in actual printing. It is a very important image processing method in printing. When Photoshop works under CMYK mode, it has one more color channel than that under RGB, moreover, it has to use RGB display method to imitate CMYK monitor effect, and CMYK calculation method is different from that of optics-based RGB. Therefore, efficiency of image processing through CMYK mode is lower and so does the quality. (2) When process images by Photoshop, some filters do not support CMYK mode. Image editing and processing usually have to go through many subtle procedures, for example, you may have to combine content of several images together, while adjustment is needed due to different original hue of each component, or different parts shall be synthesized by a certain method and then be filtered etc. Whatever effect image processing shall achieve, operators all wish to produce and keep each subtle effect so as to furnish images with true and vivid details. Because the color range of RGB is much broader than that of CMYK, wider color space and more varied editing effect can be obtained by processing images in RGB mode. Most of those effects, if used well, can be preserved. Although the images shall be converted into CMYK mode and suffer from color loss at the end, it is much better than color loss at the beginning. (3) During the process of changing RGB mode image into CMYK mode image, color separation parameters are decisive to the effects of image conversion. Adjustment of color separation parameters also has great influence on image conversion. Photoshop image procession software is capable of controlling the parameters, i.e. when converting images processed in RGB mode to CMYK mode for output, color loss could be greatly reduced by adjustment of color separation parameters. (4) Images used in newspaper publication shall be preserved for a long time, so keeping image data by RGM mode is relatively ideal. RGB mode image data being calibrated and corrected could be effectively preserved for a long time, which enables future application of RGB mode images searched from archive store to different output equipments. It would be very convenient to adopt systemic color management method or image conversion method in Photoshop for color separation when future publication procedures need to re-use RGN mode image data. (5) When use different kinds of press, digital proofing equipment or computer monitor to conduct image printing, proofing and outputting, a major method for measuring the color difference of image reproduced by the abovementioned output equipments is to measure the volume of cyan, roseine and yellow that are needed to produce neutral grey, which is called as grey balance of reproducing system. When use different output equipments again to output images that are converted into CMYK mode, you have to adjust highlight, mid-tones and shadow dot, and change the overall grey balance and color saturation. Black color in the image shall be changed in volume to ensure no influence upon image printing quality. Image printed without correction of black data would have some defects. For example, color of CMYK mode image that has undergone color separation in high quality sheet-fed press would tend to increase in volume after being printed in roll-fed printing press. The only solution is to correct CMYK mode image, while RGB mode image can reproduce brighter and more saturated color through its wider RGB hue. However, after image is changed into CMYK mode by color separation, all pixels in the image would be within the hue scope of CMYK. From the above analysis we could know, processing color image by Photoshop shall adopt RGB mode if possible. But during the operation, we shall notice: the trueness of color of output image in CMYK mode shall be guaranteed; make sure that image data is fully processed in RGB mode before converting into CMYK mode image; better to leave a backup RGB mode image. Except for RGB mode, Lab color mode in Photoshop is also good. RGB mode is based on optic principle while CMYK mode is based on light-refraction. Lab mode compensates the mentioned two. There are too many intermediate colors between blue and green but not enough between green and red in RGB mode. There is more color loss in image editing and processing under CMYK mode. Lab mode can make up for the loss. Lab mode constitutes three channels, L, a and b. L channel, referring to brightness, controls image brightness and contrast; a channel covers colors from dark green (low brightness) to grey (middle brightness) and to bright pink (high brightness); b channel covers colors from bright blue (low brightness) to grey and then to sallow (high brightness). Lab mode resembles RGB mode in that its color-mix would produce brighter color. The brightness change of color is only influenced by brightness channels. Lab mode can be regarded as two-channel RGB mode plus a brightness channel. It is unrelated with equipment, and could be applied to edit and process any image (including grey-scale image), as quick as RGB mode and multiply that of CMYK mode. Lab mode is able to guarantee no color loss within CMYK scope in color mode conversion. When converting RGB mode image into CMYK mode, you have to add an intermediate procedure, i.e. converting into Lab mode. Lab mode is usually used for changing image into grey-scale image during achromatic color composition. In conclusion, Lab mode or RGB mode shall be given priority in image editing. If they are not available, convert image into CMYK, and once upon the conversion, it is not easy to restore it back. You can change image into Lab mode for procession if it is in need. If the input image for scanning is color image, but the image is used in grey-scale plate, do not directly input the original image in grey-scale mode when inputting image by scanner. Input and process it in RGB mode and convert it into Lab mode, then select L channel image as grey-scale image for printing through channel separation instruction. Most operators obtain needed image directly by scanner and begin plate-making after little procession. The printing result is betwixt and between. In fact, good quality depends on how much effort you have made, thus follow the standard operation procedures.
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