>Psychtoolbox>PsychTests

TextBoundsTest([string] [, font] [, textSize] [, rotAngle])

Test Screen(‘TextBounds’) as a fast way of determining the bounding
rectangle (aka bounding box) of a string of text, as well as the very
precise but slow TextBounds.m function.

Screen(‘TextBounds’) uses information from the operating system about the
bounding polygons of a string of text to compute the bounding box. This
is fast, but it relies on the typesetting algorithms of the OS and the
information encoded in a font definition file playing well with each
other. Usually that’s the case, but there are a few special fonts which
either don’t encode their own boundaries correctly, or which are typeset
in an unusual way that the OS facilities can’t handle correctly. In such
cases, textbounds will be way to big or too small and text may appear cut
off.

The TextBounds() function implemented in TextBounds.m uses a different
approach: It draws the text string into a provided window - usually an
offscreen window which is so big that it can easily accomodate the text
string in question, ie., it is way too big for the string. Then it reads
back the image of that string and computes the exact bounding box of the
non-background pixels. This method is obviously independent of OS
facilities and (faulty) information in the font files - thereby highly
robust and accurate. However the approach is very slow, so you should
only use this method if you really need it.

This test script both tests functioning of both methods and provides a
code example of how to use the information returned by both methods to
either frame drawn text, or to actually draw unusual difficult text by
first predrawing it into an offscreen window of sufficient size, then
blitting the really “meaningful” portions of that offscreen window into
the real image window.

Optional parameters: (All have meaningful defaults)
‘string’ The text string to draw. Default: “Wordy”
‘font’ Name of the font definition file. Default: “Chicago”
‘textSize’ Size of text font. Default: 64 pts.
‘rotAngle’ Orientation of test-blitted text. Default: 45 degrees.