Color Trapping in CorelDRAW
Offset lithography is the most common printing process used today to transfer digital documents to paper in commercial press. This method produces fine print quality, great printing speed and has reasonable cost. However an offset printing press only produces solid colors, not shading and applies only one color of ink to a sheet of paper at a time. In order to print images that contain shading (called continuous-tone images), shading is simulated using tiny solid color dots (halftone images). To overcome the second problem of reproducing multicolor images on printing press, each ink must be applied separately, so the image must be broken down into its component colors by creating color separations.
Each color separation is output to a photographic film that is used to create printing plates. Generally, full color documents (those containing many colors, such as photographs) are separated into four basic process colors - Cyan, Magenta, Yellow and Black. Thus, this kind of color separation is usually called CMYK separation.
Original
 Cyan |
 Magenta |
 Yellow |
 Black |
The four separation plates are then printed with its own corresponding ink color:
 Cyan |
 Magenta |
 Yellow |
Black |
CMYK separation is great but not so precise in color rendering and has limited color gamut. If you need one or more specific colors, or color that can't be rendered in CMYK, you need to use Spot colors that are separated to additional plates.
After the film is produced, the color sheets must be aligned (registered) precisely. If the colors are not aligned properly on a page, unintentional space appears between adjoining colors. This problem is called misregistration, and no matter how much care is taken, some amount of misregistration will occur. To compensate for misregistration you should overlap colors a little. Overlapping colors is called color trapping.
For example, the cyan circle on magenta background on the picture below will separate into two color plates - Cyan and Magenta (the other two, Yellow and Black, are empty and usually are not printed at all).
 Original |
|
 Cyan plate |
 Magenta plate |
Note that the Magenta plate contains white hole in the place where the circle are on the Cyan plate. It is said that the Cyan circle knocks out the magenta background. When the color plates are aligned on paper, any misregistration produces small gaps between colors and makes the paper to look through. However, you can increase the size of the circle, remaining the hole the same size. This will overlap colors filling gaps and thus producing color trap.
 Knockout |
 Trap |
There are two basic methods of color trapping: Spread and Choke. Spread trapping is created by increasing the size of the overlying object as in the above example. Choke overlaps color in opposite direction - the background hole is reduced while the overlying object remains the same size. Generally, one of there two trapping methods should be chosen according to colors of the overlapping objects. As a rule of thumb, a lighter object should be increased in size. So, if a darker object is on top, then choke should be used. If the background is darker, then you should use spread.
| Examples of different trapping methods | ||
|---|---|---|
 |
 |
 |
| Original with lighter object on top |
Spread (correct) | Choke (incorrect) Object looks smaller |
 |
 |
 |
| Original with darker object on top |
Spread (incorrect) Object looks larger |
Choke (correct) |
Spreading dark objects can influence the document look. It is not recommended to apply spread trapping to small text objects. Usually a differential trapping is used. This means that lighter objects are spread more than darker.
There are several techniques to produce traps. CorelDRAW can automatically create spread trapping. When you click Print in CorelDRAW, you have an option of auto trapping found on Separation tab of the dialog. In the Auto Trapping control group check Auto-spreading checkbox and specify the maximum amount of color overlap. As noted above, usually color overlap depends on the color of objects. Lighter objects are spread farther than darker ones.
CorelDRAW creates spreads by adding an outline to the object. The color of the outline is set to that of the object's fill and overprint outline is applied to it. When overprint is applied to an objects, it doesn't knock out the background when printing separations. In fact, knockout is performed by printing the same object filled with white on the plates where the object doesn't contain the corresponding color component. For example, a circle filled with CMYK(100,50,20,0) will be printed as 100% gray (black) on cyan plate, 50% gray on magenta plate, 20% gray on yellow plate and 0% (white) on black plate. If the overprint is specified, objects with 0% gray are just omitted and not printed, therefore failing to knock out the background.
In the above examples with cyan circle over the magenta background if we apply Overprint Fill to the circle, then the circle will be printed as before on the cyan plate (100% black) but will not be printed as 0% on magenta. The magenta plate will be 100% gray as if there were no circle in the document. When the two colors are combined on paper, the circle will appear to have (100,100,0,0) fill. Hence, the overprint can be simulated by applying the mixture of its own and the background color components (you should replace the channels with 0% gray with the corresponding channel values of the background).
Returning to auto-trapping, when CorelDRAW applies an outline to an object and applies Overprint Outline to it, the outline doesn't knock out the background right under it but the object's fill does. Therefore, the colors overlap only under the added outline. The width of the outline determines the spread amount.
There are several limitation for auto-trapping in CorelDRAW. First, it can't spread objects already with outlines. Also, only objects with uniform fills can be spread automatically because the outline added to the object can be colored only with a solid color, not a fountain or bitmap fills. CorelDRAW will not autotrap objects that already have overprint fill to let you control trapping manually.
CorelDRAW's automatic trapping is good but it can't create chokes. That is, it only increases the size of the top object and can't decrease the size of the hole under the object to make the background bleed into the object. However in several cases you can do choke trapping manually by manipulating outlines and overprints.
As in the example above, if you color the outline with magenta rather than cyan and apply Overprint outline to it, then the exterior areas of the overlap will not be affected because the magenta outline on magenta background is not seen. However the interior area of the overlap will mix the cyan fill of the object with the magenta outline producing color overlap. This will look as if the background bleeds inside the object, i.e. the choke trap.
But this method is not acceptable if the top object overlaps two or more background objects with different colors or if the background is something like a bitmap or color blend. In this case you have to create the knockout hole (a white object) with the Contour effect, setting it to one step inside. Then separate the contour, the smaller generated object should be filled with white and placed behind the original object. Then apply overprint fill to the original. The white smaller copy will knock out the background while the original will overlap colors producing the trap. But you must bear in mind that creating contours on a complex object like text will take some time as it is very resource consuming operation and it is not so precise to assure accurate trap area.
And the last note, don't expect that if you set overprint fill to an orange circle (0,60,100,0), it will mix with the magenta background because the orange already has non-zero magenta component (60%). Overprint doesn't actually mix colors. It only omits empty channels when printing separations.