In printing, t rap states how far ink has been printed on the substrate receiving another layer printed on it compared to how well the substrate (eg, paper) receives the ink.
However, in the era of prepress software, the term refers to compensation for registration errors (when two ink layers are not perfectly aligned) traditionally known as "chokes and spreads". Registration error caused a gap or free space on the final printout. Correcting this involves creating overlapping (spread) or overlapping (choking) objects during the printing process.
Video Trap (printing)
Registration
Errors in the graphics workflow may be caused by human error, inaccuracies in image tuning, film-to-plate or film-to-film copying steps or image-carrying instability (for example, stretching in films or plates), the latest press or media.
These problems can be minimized, but not eliminated - every mechanical process produces several levels of error. The gaps that arise as a result can be hidden by creating an overlap between two adjacent colors.
Maps Trap (printing)
Method
In general, prepress software provides some level of trap, through the default value of the application. Additional traps may be required.
Manual traps can be applied at several stages in the workflow, using vector-based or raster-based settings. The choice depends on the type of output (packaging applications including flexo-printing have other requirements than commercial printing on the offset system) and the level of interactivity or automation.
In-RIP trapping moves the trap to the Raster image processor (RIP) processor at the last minute. The process is automated, although it is possible to set up zones to allow different rules for different areas, or to disable traps for manually trapped areas.
Trap decision
When the two colors in question are spot colors, traps are always needed: from when the artwork is imaged on a film or plate, the colors are handled separately and printed on two different printing units. The same approach applies if one color is the point color and other process colors.
Traps become more difficult if both colors are the color of the process and each is printed as a combination of the basic colors of cyan, magenta, yellow and black. In this case, the decision of the trap depends on the number of 'general' colors.
Another factor that affects the trap's visibility is the trap direction. Decisions about which color to spread or choking are usually based on the relative luminance. Lighter colors (higher luminance) are scattered to the dark. It responds to the way the human eye perceives color: the darker color determines the shape, therefore the distortion of the lighter color results in less visible distortion in its entirety. The degree of light or darkness of a color is defined as its neutral density.
The big exception to this is the case when blurred (colors that completely obscure the colors printed underneath) spot colors are used. Other colors, regardless of their relative luminance, are always trapped (scattered below) the color of this point. If some of these spot colors are used (common practice in the packaging market), the order of the printing layers rather than the luminance is the decisive element: the first color to be printed is scattered under the next color.
The thinner trap is less visible. Therefore, the trap width is set to a tight minimum, dictated by the maximum number of misregistration of the entire workflow through to the press. When printing at 150 lpi, the trap is usually between 1/150 and 1/300 inch (0.48 pt and 0.24 pt, 0.16 mm and 0.08 mm). These values ​​are usually multiplied by 1.5 or 2 when one color is black. The trap is invisible because the lighter color is scattered below - the black is almost opaque. For the same reason, in many cases, black ink is set to "exaggerate" colors in the background, thus eliminating more complicated spreading or strangulation processes. Because black is a dark color, white gaps caused by registration errors are more noticeable. On top of that, in wet-in-wet offset printing, black is the first printed color, causing relatively more distortion, thus increasing the risk of being mistakenly seen.
When the traps between the two colors are made, it contains the sum of the two colors being questioned when at least one of them is the dot color. When two colors are the color of the process, the trap contains the highest value of each CMYK component. This trap color is always darker than the two bordering colors. In some cases, more specifically when two colors are pastel-like colors, this may produce a trap that is considered too visible. In this case, reducing the number of colors in the trap can help. However, the traps should never be brighter than the darkest colors because this will have the same effect as the wrong registration - creating a bright 'gap' between the two colors. Trap color reduction is not recommended when solid dot color is used. In this case, the reduction will cause the spot color in the trap to be printed, not as solid, but as filtered color.
Trap in the rich black direction (black with other color support screens added to it to give it a 'deeper' look and make it more opaque - often called "undercolor"), following the same rules as trapping into 'normal' black.
Blending or 'sketch' offers a special challenge to trap. The lighter part of the mixture needs to spread to the background, while the darker part should be choked up. If a trap over the entire length of the mixture is required, this produces a visible 'ladder' effect. The solution is to use a sliding trap: a trap that gradually changes color and position. The trap can slide all the way. It may not be distorted to distort the original artwork. Often, the shear factor is set to the point where the mixed neutral density and background reach the corresponding difference.
References
Source of the article : Wikipedia
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