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pdfclipping

Clipping path seems to be outside of text

recently I wanted to construct a PDF document which should have text clipping: With 4 Tr I tried to define the text as clipping area. But when I wanted to fill the lower part of the text with red color, the result was reversed.

Do anyone knows, why?

Thanks for any answer!

stream
BT
 4 8 Td
 0.8 0.2 0.7 rg    % Writing lila.
 4 Tr              % Fill & Use text as clipping area.
 /TR 32 Tf
 (Hallo Welt) Tj
 1 0 0 rg          % Fill in red.
 0 0 200 20 re F   % <- Mistake?
ET

What I wanted to have:

What I wanted to have

What I got:

What I got

Solution

  • Have a look at the specification ISO 32000-1:

    The behaviour of the clipping modes requires further explanation. Glyph outlines shall begin accumulating if a BT operator is executed while the text rendering mode is set to a clipping mode or if it is set to a clipping mode within a text object. Glyphs shall accumulate until the text object is ended by an ET operator; the text rendering mode shall not be changed back to a nonclipping mode before that point.

    (section 9.3.6 Text Rendering Mode )

    In your sample you don't wait until the ET for the clipping path to take effect. So, when you are painting the red rectangle, your special clipping path is not yet in effect.

    Furthermore your operation sequence actually is invalid! Neither path construction nor path painting operators (i.e. neither your 0 0 200 20 re nor your F) are allowed inside a text object, cf. Figure 9 – Graphics Objects in the specification:

    Figure 9 – Graphics Objects

    Thus, strictly speaking your PDF viewer had better refuse to draw your content stream at all.