Tight bounding box around text in graphics?

Hi

strheight(), which is based GEStrHeight(), is pretty crude, not only 
ignoring descenders, but also only considering the ascent of the overall 
font (capital "M").

There is a GEStrMetric(), which returns character-specific ascent and 
descent, but that is only currently exposed via grid::stringAscent() and 
grid::stringDescent().  There is also grid::stringHeight(), which is as 
unsubtle as strheight().

For example, these are all the same (just font ascent) ...

 > strheight("y", "in")
[1] 0.1248031
 > strheight("x", "in")
[1] 0.1248031
 > strheight("M", "in")
[1] 0.1248031

... and these are all the same ...

 > convertHeight(stringHeight("y"), "in")
[1] 0.124803149606299inches
 > convertHeight(stringHeight("x"), "in")
[1] 0.124803149606299inches
 > convertHeight(stringAscent("M"), "in")
[1] 0.124803149606299inches

... but these have more detail ...

 > convertHeight(stringAscent("y"), "in")
[1] 0.0936023622047244inches
 > convertHeight(stringDescent("y"), "in")
[1] 0.0416010498687664inches
 > convertHeight(stringAscent("x"), "in")
[1] 0.0936023622047244inches
 > convertHeight(stringDescent("x"), "in")
[1] 0inches
 > convertHeight(stringHeight("M"), "in")
[1] 0.124803149606299inches
 > convertHeight(stringDescent("M"), "in")
[1] 0inches

In theory, it should not be difficult to add a graphics::strascent() and 
graphics::strdescent() if that would help.

Paul

I've mentioned in previous messages that I'm trying to redo rgl text.

Part of what I need is to measure the size of strings in pixels when
they are drawn by base graphics.

It appears that

strwidth(texts, "user", cex = cex, font = font, family = family)

gives accurate measurements of the width in user coordinates. I've got
those set up to match pixels, so I'm fine here.

However, the equivalent call for strheight() only measures height above
the baseline according to the docs, and indeed the number is smaller
than the size of what's displayed. Descenders (e.g. the tail of "y")
aren't counted.

Is there a way to measure how far a character might descend? Is it
valid to assume it won't descend more than a line height below the top
of the char?

I have a partial solution -- textshaping::shape_text gives a "height"
value that includes lots of space below the character, and a
"top_border" value that measures from the top of the textbox to the
baseline. So I think `height - top_border` would give me what I'm
asking for. But this only works with graphics devices in the ragg
package. Is there a general solution?

Duncan Murdoch

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Dr Paul Murrell
Te Kura Tatauranga | Department of Statistics
Waipapa Taumata Rau | The University of Auckland
Private Bag 92019, Auckland 1142, New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz
www.stat.auckland.ac.nz/~paul/