Bridge 7 > Bridge: Tipps & Tricks > Preparing documents for PostScript Index

Transparencies

Calamus uses three different writing modes to represent the contents of various frames: Transparent, Opaque and Inverse. You can define these writing modes in the Frame editing module, as already described:

It is imperative that before exporting a document as a PostScript file you check whether you have used any frames that were switched as Transparent. The simplest way is to set all frames to Opaque and check if any changes can be seen on the screen. This is easiest with the (optionally available) Select module, which among other things was optimized for this task and is able to switch all frames of any size of document to opaque at the touch of a button.

Example 1: You have set, say, black text over a yellow background and have set the text frame to transparent, so that the black text will overprint and so not show white flashes where it meets the yellow background after imagesetting the K and Y separations – in PostScript this black text is always output as opaque, and even masking brings no advantages since masks can not define opaque or transparent writing modes.

Example 2: You have a picture subject with a white surround that is to be placed on top of a light grey background. As soon as you set the image frame to the transparent write mode, the white surround will disappear in Calamus (becauses it is transparent) and only a little of the light grey background will shine through during output. Calamus does not support true semi-transparency at present, where you could define that really only the white parts of the image are transparent with the rest of the picture opaque; therefore – in this and all similar cases where the complete frame contents should not cover everything lying under it – you have to resort to masking to obtain the desired effect.

So that these masks are also exported correctly in PostScript, it is imperative that you create vectorial masks only! Raster graphic frames with b&w copies of a coloured image, for instance, would be output simply as what they are, namely monochrome images, but not as masks or as PostScript jargon has it clipping paths.

It is however very simple to create vector masks for images. Just follow the instructions below step-by-step:

• In order to cut out a portrait from its background, say, the background must be removed somehow. This can be done with Calamus and Bridge for instance by first making a copy of the picture as a greyscale image frame:

  

• After this use FrankLIN to alter the control curve of the greyscale image so that the result comes as close as possible to the desired effect, namely a complete b&w mask:

  

  

• The mask raster graphic frame now has to be vectorized. To do this, just create a vector frame that overlaps the mask image slightly:

  

• Now use the Magnifier to zoom as tight as possible to the picture, because the vector tracing module Speed~Line uses the current screen resolution for vectorizing. Call that module and use the settings as shown in the following dialog box illustration, preferably without altering any of the Speed~Line parameters (apart from changing from Line to Curve tracing and setting the Curve precision):

  

• After Speed~Line has traced and vectorized the mask image you can check and clean it up in the Vector module. We recommend that, as in our example, you correct the vector object that represents the main part of the mask and then store it temporarily in the clipboard. After this select all other vector objects (Options menu, Select all) and delete them. After inserting the main object back into the vector frame from the clipboard you will get the finished, vectorial mask:

  

• With this vector mask you can produce the background-free raster image:

  

On PostScript output the raster image is written quite normally as a rectangular image frame, with the vector data of the mask written as a clipping path definition. In this way you can also use several masks on a page, even masks within masks. However the mask objects must always be vectorial ones, i.e. all Calamus frame types that do not contain raster graphic data.