Dot for dot – the Raster generator module
Some of the principles underlying rasterization were already
described in the
Fundamentals chapter. The process is always
the same: Different brightness levels are converted to different sizes
of raster points. Depending on the resolution of the output device,
you can reproduce more or less good quality images this way.
With the Raster generator module described here you can decide which rasters are to be used for which parts of your document. You can manipulate the shape, angle and size of raster points – important elements used in the image processing stage of desktop publishing.
Calamus follows the hierarchical principle for this: At first the
same (preset) rasters are valid for the whole document. If dealing
with a master that will be printed only in black and white, then there
will be only one raster. For documents in colour, the various colour
separations have to be printed with different rasters to prevent ugly
Moiré effects (see
Fundamentals). But you can also allocate to
each page and even to each frame its own special raster. Pages or
frames for which no user-defined raster has been set will then be
rasterized using the defaults preset for the complete document.
The Raster generator module creates all required rasters in real-time, i.e. while building up the image. Normally it works so quickly that you will not notice it at all; the calculation time may become apparent only with larger rasters (see below). Here too the philosophy that has the top priority everywhere in Calamus applies: The user should see the document on the screen exactly as it will appear later on paper. Due to the different resolutions of the monitor screen and the output device this is of course possible only in a limited way. But if you set the screen magnification so that it corresponds to that of the printer, you will see each printer pixel 1:1 on the screen as usual.
By using suitable focusing systems and good support materials, many laser printers and practically all imagesetters nowadays can separate the raster pixels in an optimum way, so in general these pixels do not blur or coalesce. Matters are different later during printing. Here one is working with liquid ink colours that can produce unpleasant side effects due to their physical properties: Their (necessary) surface tension will make two closely spaced dots coalesce, just like two water droplets flow together when the distance between them is small enough. In addition, the same effect creates difficulties in printing very small dots of the order of 1/100 mm. For these reasons one should always set the control curves for images to taper off at their extremities – in other words so that each line becomes a slightly S-shaped curve. Even though it appears that this would result in a drop in contrast, due to the effects described this is the only way to ensure reproduction of all brightness levels of the image.