[Rd] Tight bounding box around text in graphics?

[Paul Murrell]

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

On 26/09/23 08:06, Duncan Murdoch wrote:
> 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 using stat.auckland.ac.nz
www.stat.auckland.ac.nz/~paul/