If you want all elements in a frame to be output as transparent,
select the frame and then click on this icon. This assigns the write
attribute Transparent
to the frame.
When a frame lies partially or entirely on top of another one, all colour and transparent areas will be transparent and you will be able to see the frame underneath through them. The result is that the colours contained in the transparent top frame may mix with the colours of the frame lying underneath when printed.
You can alter this assignment at any time.
A very common application for transparent output is, for instance,
for text frames with black text that lie on a light background. If the
output of the text frame were opaque, then the light background colour
would have to be cut out everywhere a letter is to be placed. With
offset printing in particular this would quickly lead to white
show-through flashes
as the colours can never be printed 100 %
in register. If there is only very slight misregistration here, then
the white of the paper will show through partly at the character edges
– giving flashes.
However, if you set the text frame to transparent, nothing will be cut out of the light background, and the black of the text frame (which in any case covers everything) can be printed without problems slightly out of its correct position on the light background – no flashes.
The screen display of transparent frame contents attempts to mimic the mixing of the colours in offset printing as closely as possible. As Calamus converts all colour values that are to be output on the screen to RGB first (as this is the colour space of monitor screens) and can determine only then if and how the various write modes of various frames affect each other, the screen display can not imitate the printed output absolutely 100 % at present. In places where, say, a transparent 30 % yellow overlies a transparent 30 % grey, then (only in the screen output!) it will not be recognized that a grey value of 30 % is a part of the mix, as this was already converted previously to RGB values. The addition of the colours can not therefore take place here (and only in this special case, where any grey values are in play).
30 % yellow = R100 G100 B070 30 % grey = R070 G070 B070
The lowest values (i.e. the darkest colours) will take precedence so the result will be:
Target colour = R070 G070 B070
And one can see immediately that there is a mathematical problem here. But this applies only in cases where mixing with grey values occurs, and only on the screen ... and may perhaps be solved in a future version of Calamus.
Transparency on the screen:
Transparency problems with grey values:
This is the output on paper:
