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
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
It is however very simple to create vector masks for images. Just follow the instructions below step-by-step:
somehow. This can be done with Calamus and Bridge for instance by first making a copy of the picture as a greyscale image frame:
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:
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.