Comparison of RGB and CMYK Images in Scanning and Color Correction (I)

Advances in color management, digital photography, and color scanning have prompted new and old scanner operators to carefully consider when to perform color corrections and when to perform color separations. Drum scanner operators use conventional methods to generate scanned images consisting of yellow, magenta, cyan, and black, but today's new tools lead to the widespread adoption of new workflows—that is, scanning before separation into CMYK. School color. This article explains the advantages of this method as well as some background knowledge about scanning, color correction, and color separation.

Both scanning and digital photography capture red, green, and blue information about the image, but various methods of image capture generate different amounts of information depending on their depth.

Although most scanners use 1 byte (8 bits) of information in each color channel, it has become increasingly common for scanners and digital cameras to use more than 8-bit bytes to describe each basic color. These additional bits are used to capture a large number of dark shades for each pixel, producing a subtle description (mostly grey tone) between the multicolor and the maximum color of each channel. The number of bits used for each channel is what we call the bit depth of the digital image.

For example, in an RGB mode with an 8-bit depth per channel, a total of 24 bits are used for scanning or digital photographs to describe the color of each pixel, which is called 24-bit color, since 8 bits per channel count, 3 The channel (red, green, blue) is a total of 24 bits per pixel location. Other common configurations for capturing RGB data include:

10 bits per channel (also known as 30-bit color because there are 3 channels in 10 digits);

12 bits per channel (36-bit color);

16-bit per channel (48-bit color).

These additional bits of data are very useful when the image is enlarged after scanning or capturing because the additional bit depth is suitable for better interpolation.

Color separation
The so-called color separation refers to a process in which RGB image data is converted to the closest equivalent cyan, magenta, yellow, and black (CMYK) values. This is very necessary for a general print reproduction process because most printing apparatuses use cyan, magenta, yellow subtractive primary colors and black (it is not a basic color). Black is used to compensate for less-than-ideal absorption characteristics of printing inks (ie, colorants). The use of black expands the printed tonal range, resulting in a darker, richer dark tone.

Color separation depends on how much CMYK is needed for accurate calculations to get close to RGB scans. Traditionally, this is done by pre-installing an on-board computer attached to a roller scanner. For decades, these "high-end" scanners have captured RGB data during the scan and converted it to "CMYK data" while "running" (while scanning the image). In today's printing industry, this separation method is rapidly being replaced by a workflow that captures RGB data and stores it as RGB on disk. Separation and conversion to CMYK is done at a later time using software or any software program that can connect a digital camera.

However, both color separation methods severely limit the flexibility of outputting the same color separation data to various different devices because color separation is performed specifically for a particular printing reproduction system. A document that is color-coded for lithographic printing will not look the same when it is output to a color copier, even if both are CMYK output devices.

CMYK color separation is specific to a single device for a variety of reasons: First, each device has its unique gray balance and tone reproduction (including dot gain) characteristics. In addition, the operator setting the color separation control can change the amount of black during the conversion from RGB to CMYK.

Black version information
As previously mentioned, the amount of black required to produce an approximate tonal range depends primarily on the light absorption characteristics of the printing ink used. The user's choice of substrate is also part of this factor. However, skilled printer operators can also change the thickness of the ink they choose. The thicker the ink layer, the higher the density, generally giving the printed image a more saturated appearance. Increasing the thickness of the ink layer will make it difficult to maintain the desired ink balance. Some printers therefore prefer the separation of thinner ink prints to ensure consistent print quality throughout the printing process.

The effect of all this on color separation is that an image prepared for thick ink layer printing will require a reduction in black in the dark tone area because dark shades can be produced by printing a high percentage of cyan, magenta, and yellow inks. The color separation process for determining the amount of black information in the color separation includes UCR (Background Removal) and GCR (Grey Component Substitution).

Increased tone value
When considering the increase in the hue value (the dot gain), the difference between the CMYK images prepared for various print copying systems is increased. Scanners and printer operators understand that ink dots printed on substrates produce images that are much darker than the original digital data - an effect known as dot gain.

In addition to factors such as the surface of the paper and the viscosity of the ink, each printer also plays a role in determining the dot enlargement of the printed image. Compensating for dot gains during color separation means that the darkening that occurs during printing can be offset, making the image brighter when converted to CMYK.

Moving an image from one printing state to another without compensating for changes in the tonal value will make the image too dark or too bright, which will cause color shifts because the gray balance of highlights, midtones, and shadows increases dot gain. Big plays a different role.

(to be continued)