[BASIC] Use of Font_ScanString

Hello

I am struggling with the use of Font_ScanString to find out the extent of a string on the screen. I want the width and the height of a string located at textbuf%.

Here is the code snippet I wrote

DIM c% 40
plot_type%=&40120:REM bit 5, 8 and 18 are set to use current font handler and return bounding box
$textbuf%="Paths passing through"
!c%=0:c%!4=0:c%!8=0:c%!12=0:c%!16=-1
SYS "Font_ScanString",,textbuf%,plot_type%,,,c%

After the call, the coordinates returnd in c%!20 and onwards are as follow:
c%!20 —> 536870912
c%!24 —> -536870912
c%!28 —> 536870912
c%!32 —> -536870912

The result is the same if I REM the line !c%=0:c%!4=0:c%!8=0:c%!12=0:c%!16=-1

Any help would be much appreciate.

Regards

François

One thing that looks wrong – you are not setting r3 and r4 on entry. I assume basic will set both registers to zero, which will mean stop the scan as soon as x or y exceeds 0, ie. scan will terminate immediately. I would suggest you use a large value, eg. 0×7fffffff for r3 and r4. I have also found that it is best to set the bbox values to zero before entry.

I use font_scanstring a lot (from c) and it works fine for ‘simple’ strings. I have only had problems when the string contains vertical shift sequences.

Hi Chris

Thanks for your answer, this indeed seems to fix my problem!
I just cannot understand why I get negative value for the y0 coordinate, if we state that (x0,y0) is the bottom-left corner of the bounding box… I would have expect y0 to be 0…

Cheers

François

I just cannot understand why I get negative value for the y0 coordinate

The y0 is relative to the baseline of the string so bounding box y0 is usually negative because of decenders ie the bottom a y is a negative coordinate. You can also get -ve x0 coordinates because the x origin of a character can be inside its bounding box. If you measure the width of a string it may be shorter than the bounding box x1 because character widths can be narrower than a box covering the characters.