Sunday, 30 October 2011

What is design for print?//Trapping/Knock Out/Overprinting.

Researching specific information on trapping, knock out and overprinting methods used in professional print production. All research will help to generate both general knowledge and content for my ISSUU based publication in which I will explore and note useful tips and information for designing for print- the development of which can be found on my Design Practice blog over the forthcoming days and weeks.


Trap (printing)

without trapping
with trapping

Comparison of a knock-out with and without trapping, and overprinting for perfect and imperfect registration. 
Rows are as follows:
1. The cyan (lighter) plate,
2. The magenta (darker) plate,
3. Result with perfect registration (some monitors show slight misalignment), and
4. Result with imperfect registration.

Trapping is a term most commonly used in the prepress industry to describe the compensation for misregistration between printing units on a multicolor press. This misregistration causes unsightly gaps or white-space on the final printed work. Trapping involves creating overlaps (spreads) or underlaps (chokes) of objects during the print production process to eliminate misregistration on the press.


Misregistration in the graphical workflow may be caused by a number of reasons:
  • inaccuracies in the image setter
  • instability of the image carrier, eg stretch in film or plate
  • inaccuracy in the film to plate or film to film copying steps
  • instability of the press
  • instability of the final media
  • human error
These inaccuracies are inherent to the graphical production process and although they can be minimized they will never completely disappear - any mechanical process will always show some margin of error. The small gaps showing up as a result can however be hidden by creating overlaps between two adjacent colors.

Trapping methods

One approach to trapping is to change the submitted artwork. In general, all digital files produced using any current professional software have some level of trapping provided already, via application default values. Additional trapping may also be necessary, but all traps should be as unobtrusive as possible.
Traps can be applied at several stages in the digital workflow, using one of two trapping technologies: vector-based and raster-based. The right choice will depend on the type of products (packaging applications including flexo-printing have other requirements than commercial printing on offset systems) and the degree of interactivity or automation that is wanted.
In-RIP trapping moves the trapping to the RIP so that it is done at the last moment. The process is automatic, though it is possible to set up zones to allow different automatic rules for different areas, or to disable trapping for areas previously manually trapped.

Trapping decision making

Certain basic rules have to be observed.
First the decision should be made if a trap is needed between two specific inks, in other words, if these two abutting colors are printed is there a risk of gaps showing up when misregistration happens.
In case the two colors in question are spot colors, trapping is always needed: from the moment the artwork is imaged on film or plate, they are handled separately and ultimately will be printed on two different printing units. The same applies if one of the colors is a spot, the other a process color.
The decision becomes a bit more tricky if the two colors are process colors and will each be printed as a combination of the basic printing colors Cyan, Magenta, Yellow and Black. In this case the decision whether to trap or not will be defined by the amount of ‘common’ color.
Another factor that will influence the visibility of the traps is the direction of the trap. The decision which color should be spread or choked is usually decided upon the relative luminance of the colors in question. The ‘lighter’ color should always be spread into the darker. Again this reflects the way the human eye perceives color: since the darker colors define the shapes we see, distortion of the lighter color will result in less visible distortion overall. The ‘lightness’ or ‘darkness’ of a color is usually defined as its ‘neutral density’.
A major exception to this rule should be applied when opaque spot colors are used. Other colors, regardless of the relative luminance should always be trapped to (spread under) these spot colors, If several of these spot colors are used (a common practice in the packaging market), it is not the luminance of the color but the order of printing that will be the decisive element: the first color to be printed should always spread under the next color.

Example use of a trap.
The thinner the traps created, the less visible they will be. Therefore the trap width should be set to the strict minimum, dictated by the maximum amount of misregistration or error margin of the whole production workflow up to the printing press. Since the printing technology and the quality of the paper are the most important causes for misregistration it is possible to come up with some rules of thumb. E.g. for quality offset printing it is generally accepted that the trapping width should be between 1 and 1/2 print dots. When printing at 150 lpi the traps should be 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 with a factor of 1.5 or 2 whenever one of the colors is Black. First of all the trap will not be visible since the lighter color will be spread underneath the -almost- opaque black. For the same reason, in many cases, black ink will be set to "overprint" colors in the background, eliminating the more complex process of "spreading or choking". Since black is a very dark color, white gaps caused by misregistration will be the more visible. On top of that -in wet-in-wet offset printing- black is the first color to be laid down on paper, causing relatively more distortion of the paper and thus at higher risk of showing misregistration.
Whenever a trap between two colors is created this trap will contain the sum of the two colors in question whenever at least one of them is a spot color. In case the two colors are process colors, the trap will contain the highest value of each of the CMYK-components. This trap color is always darker than the darker of the two abutting colors. In some cases, more specifically when the two colors are light pastel-like colors, this might result in a trap that is perceived as too visible, In this case it might be desirable to reduce the amount of color in the trap. This should however be limited: the trap should never be lighter than the darkest color since this would have the same effect as misregistration: a light colored ‘gap’ between the two colors. Trap color reduction is also not recommended when solid spot colors are used. In this case reduction would cause the spot color in the trap to be printed not as a solid but as a screened tint.
Trapping towards a rich black (a black with a support screen of another color added to it to give it a ‘deeper’ look and making it more opaque - often called "undercolor" ), will follow the same rules as trapping to a ‘normal’ black. However, a stay-away should be created for the supporting color. This will prevent misregistration from revealing the undercolor at the edges of the rich black object. In short, a stay-away pushes the undercolor away from the edge of the rich black, and is usually created with a single color black stroke, set to "knock-out".
Blends or ‘vignettes’ often offer special challenges to trapping. The lighter part of a blend needs to spread into the background, the darker part needs to be choked. If a trap over the full length of the blend is needed, this would result in a very visible ‘staircase’. The solution here is the creation of a sliding trap: a trap that should not only gradually change color but also position. The trap can be created so that it ‘slides’ all the way, but this not often the desired effect either since it might distort the original artwork too much. Often the ‘sliding’ factor is set to a point where the neutral densities of blend and background reach a certain difference.


without trapping
with trapping

Comparison of a knock-out with and without trapping, and overprinting for perfect and imperfect registration. 
Rows are as follows:
1. The cyan (lighter) plate,
2. The magenta (darker) plate,
3. Result with perfect registration (some monitors show slight misalignment), and
4. Result with imperfect registration.

Overprinting refers to the process of printing one colour on top of another in reprographics. This is closely linked to the reprographic technique of 'trapping'. Another use of overprinting is to create a rich black (often regarded as a color that is "blacker than black") by printing black over another dark colour.
It is also the term used in the production of envelopes customised to order by printing images (such as logos) and texts (such as slogans) on mass-produced machine-made envelopes; the alternative way of producing such envelopes is to print "on the flat" and then cut out the individual shapes and fold them to form the envelopes. However the latter method is generally only economically viable for large print runs offering returns to scale.

1. Commercial printing: Process of creating slight overlap between abutting colors of a multicolor job in commercial (multipass) printing machines, to compensate for errors in registration of color plates. No trapping is required in inkjet (and other single pass) printing because there are no registration problems.

Preparing color documents for commercial printing

You can print each page of a document as a series of color separations. Color separating splits color images into several pages. Each page contains one component color. A commercial printer uses the separations to make printing plates, one for each color.
You can also print mirror and negative images, which is sometimes required when commercially printing to film.
If color objects overlap in your document, you can overprint. Overprinting prints an object (most often a dark one) on top of another color object. You can also create a knockout, in which the top color is printed but colors behind it are not. You can use trapping to ensure that no gap exists between objects.

Print color separations

When you print color separations, choose which colors to print as spot colors, which to print as process colors, and which not to print at all. Text and graphic objects are printed in black for each separation, with shades of gray indicating the percentages of color saturation. The color name of each separation is printed outside the registration marks (if registration marks are on and if there’s room on the page).
Note: For best results, print CMYK colors as process rather than spot-color inks. You can check plate assignments in the Separations Setup dialog box (File > Print and click Separations Setup).
Composite image
Black separation
Spot color separation
Imported color graphics are separated if they are in CMYK, TIFF, DCS, or EPS line art format. Also, bitmap images in EPS graphics can be separated as long as they can also be separated in Adobe Illustrator.
You can also separate a document by printing to a single PostScript file and then having a commercial printer separate the file for you.
Make sure that spot colors with identical definitions have the same names. Spot colors with the same definition but with different names appear on different plates when you print color separations.
Process color separations are printed using grids of black dots for each color—the larger the dots, the more color is printed. The halftone screen settings control how close together the dots appear, the orientation of the grid (the screen angle), and the dot shape. For information, consult your printer documentation and your commercial printer.

Creating color separations

  1. Make sure that you have not set colors to print as black and white instead of shades of gray.
  2. Choose File > Print.
  3. Choose an option from the Registration Marks pop-up menu.
  4. To print all pages for one plate and then all pages for the next plate, deselect Collate. Select Collate to print all plates for one page before printing all plates for the next page, and so on.
  5. Click Separations Setup. If necessary, move the color names to the appropriate scroll lists and click Set. To move a color, double-click the color. To move all colors, select a color in the list and Shift-click an arrow.
    Tints do not appear in this dialog box; they print on the same plate as the base color.
    If your printer can’t print process color separations, the dialog box contains only the Print As Spot and Don’t Print scroll lists. The scroll list where each color appears by default depends on how the color was defined on its Print As setting.
  6. To specify halftone screen settings, click Halftone Screens, adjust the settings, and then click Set.
  7. In the Print dialog box, select Print Separations, set the remaining print options as necessary, and then click Print.

Knock out and overprint colors

When one color object overlaps another, FrameMaker normally knocks out the overlapped portion so that it does not print in a color separation. It appears as the color of the paper. If you’re producing color separations and printing commercially, registration errors sometimes occur, and small gaps between colors appear.
Accurate and inaccurate registration
To avoid gaps between colors, you can apply overprinting to the top object so that the overlapped portion is not knocked out.
Knocked out (left) and overprinted (right)
You can also use overprinting to combine two colors for special effects.

Knock out or overprint all objects of a particular color

  1. Define a color.
  2. To make this color print on top of other colors when printing separations, do one of the following:
    • To have any object that uses this color overprint, choose Overprint.
    • To have any object that uses this color knock out, choose Knock Out.

Apply knock out or overprint to objects

  1. Select the object and open the Tools palette or choose Graphics > Object Properties.
  2. Choose one of the following from the Overprint pop-up menu:
    • To have this object overprint objects beneath it, choose Overprint.
    • To have this object knock out objects beneath it, choose Knock Out from.
    • To have this object use the overprint setting defined for the color, choose From Color. This option is the recommended setting.

Overprinting images created in other applications

If you want images created in other applications to overprint other objects when printed from FrameMaker, note these special cases:
  • CMYK TIFF files overprint objects on spot color plates under all of the following conditions: if printed as separations, if their Overprint attribute is set to True, and if their Fill property is set to None.
  • EPS files do not overprint other objects when printed as separations.

Print negative and mirror images

Your commercial printer sometimes asks you to print negative images in which all text and objects are inverted when you submit printing files on film. Sometimes you print flipped images with the emulsion side down. Emulsion is the photosensitive substance on the film surface. These flipped images mirror the normal appearance of the pages.
Note: Sometimes the settings differ depending on the printer driver you’re using, and are unavailable with non-PostScript printer drivers.
 Choose File > Print Setup. Click Properties to access the printer driver options. Locate and set the options that control negative and mirror images.

Trap objects

In commercial printing, overprinting to compensate for registration errors sometimes produces unacceptable color mixing. In this case, perhaps you trap the object instead of overprinting it. A trap is a line bordering the object on top. It is wide enough to fill the color gap and to overprint the other object along the border of its cutout.
Without trapping and with trapping
Having a commercial printer trap your documents saves you the effort of hand-trapping each object individually. Also, manual trapping sometimes must be undone if you later decide to have a commercial printer do the trapping.
  1. Create a border for the object on top by duplicating the object and setting the duplicate fill pattern to None. Be sure that the duplicated object is exactly over the original.
  2. Set the line width of the border as appropriate. Consider the size and contour of the object, the type of printing paper, and the accuracy of the printing press. Consult your commercial printer for information on suitable line widths for trapping.
    The stroke of a line is always centered on an object edge. Therefore, double the line width specification that the commercial printer gives you.
  3. Select the border and choose Overprint from the Tools palette.

Processing color documents using OPI

You can have a document color separated, or have all objects in a document or book trapped by printing to a PostScript file. Then have a commercial printer process the PostScript file for you. Creating a PostScript file in this way embeds instructions in the file. These instructions conform to the Adobe Document Structuring Convention (DSC). DSC enhances the performance of postprocessing products that perform trapping and imposition. DSC also lets you take full advantage of products that support Open Prepress Interface (OPI) version 1.3. OPI reduces the hardware overhead for working with high-resolution color images.
OPI-enhanced PostScript files contain information that the commercial printer software uses to match the placeholder images with the high-resolution ones at print time.
Note: Before beginning, ask your commercial printer for any special instructions for producing PostScript files.
  1. Ask your service bureau or commercial printer to make high-quality scans of your artwork and keep the high-resolution images. You take the OPI-ready low resolution EPS or TIFF versions of the images to work with.
  2. Import (by reference or by copying) the low resolution images into your document.
  3. Create a print file, or a series of print files for a book.

Printing to Linotronic typesetters

Here are some tips for printing to Linotronic typesetters:
  • Some Linotronic typesetters automatically place registration marks on the page. Check with your service bureau to see whether to use the Registration Marks setting when you print.
  • Not all Linotronic models support thumbnails. Test various thumbnail settings to find the optimum setting. Some Linotronic models print 1 x 2 thumbnails correctly, but not 2 x 2 thumbnails.
  • Some Linotronic drivers fail to let you enter a custom paper size even after you have chosen User Defined Size in the Paper Size pop-up menu. To enter a custom paper size, right-click the Linotronic driver and choose Properties. Click the Paper tab, and then select the custom paper size icon from the scroll list of icons.

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