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Tk_ComputeTextLayout(3)	     Tk Library Procedures     Tk_ComputeTextLayout(3)

______________________________________________________________________________

NAME
       Tk_ComputeTextLayout,  Tk_FreeTextLayout,  Tk_DrawTextLayout, Tk_Under‐
       lineTextLayout, Tk_PointToChar,	Tk_CharBbox,  Tk_DistanceToTextLayout,
       Tk_IntersectTextLayout, Tk_TextLayoutToPostscript - routines to measure
       and display single-font, multi-line, justified text.

SYNOPSIS
       #include <tk.h>

       Tk_TextLayout
       Tk_ComputeTextLayout(tkfont, string, numChars, wrapLength, justify, flags, widthPtr, heightPtr)

       void
       Tk_FreeTextLayout(layout)

       void
       Tk_DrawTextLayout(display, drawable, gc, layout, x, y, firstChar, lastChar)

       void
       Tk_UnderlineTextLayout(display, drawable, gc, layout, x, y, underline)

       int
       Tk_PointToChar(layout, x, y)

       int
       Tk_CharBbox(layout, index, xPtr, yPtr, widthPtr, heightPtr)

       int
       Tk_DistanceToTextLayout(layout, x, y)

       int
       Tk_IntersectTextLayout(layout, x, y, width, height)

       void
       Tk_TextLayoutToPostscript(interp, layout)

ARGUMENTS
       Tk_Font	       tkfont	      (in)	Font to use when  constructing
						and  displaying a text layout.
						The tkfont must	 remain	 valid
						for  the  lifetime of the text
						layout.	   Must	  have	  been
						returned by a previous call to
						Tk_GetFont.

       const char      *string	      (in)	Potentially multi-line	string
						whose  dimensions  are	to  be
						computed  and  stored  in  the
						text  layout.  The string must
						remain valid for the  lifetime
						of the text layout.

       int	       numChars	      (in)	The  number  of	 characters to
						consider from string.  If num‐
						Chars  is  less	 than  0, then
						assumes string is null	termi‐ │
						nated and uses Tcl_NumUtfChars │
						to  determine  the  length  of │
						string.

       int	       wrapLength     (in)	Longest	   permissible	  line
						length, in pixels.   Lines  in
						string	will  automatically be
						broken at word boundaries  and
						wrapped	 when  they reach this
						length.	 If wrapLength is  too
						small  for even a single char‐
						acter to fit  on  a  line,  it
						will  be expanded to allow one
						character to fit on each line.
						If  wrapLength	is <= 0, there
						is  no	 automatic   wrapping;
						lines will get as long as they
						need to be and only wrap if  a
						newline/return	 character  is
						encountered.

       Tk_Justify      justify	      (in)	How to justify the lines in  a
						multi-line  text layout.  Pos‐
						sible	values	 are   TK_JUS‐
						TIFY_LEFT,  TK_JUSTIFY_CENTER,
						or TK_JUSTIFY_RIGHT.   If  the
						text  layout  only  occupies a
						single line, then  justify  is
						irrelevant.

       int	       flags	      (in)	Various	   flag	  bits	 OR-ed
						together.	TK_IGNORE_TABS
						means	that   tab  characters
						should not be expanded to  the
						next tab stop.	TK_IGNORE_NEW‐
						LINES	 means	  that	  new‐
						line/return  characters should
						not cause a  line  break.   If
						either	   tabs	    or	  new‐
						lines/returns	are   ignored,
						then  they  will be treated as
						regular characters, being mea‐
						sured and displayed in a plat‐
						form-dependent	  manner    as
						described  in Tk_MeasureChars,
						and will not have any  special
						behaviors.

       int	       *widthPtr      (out)	If   non-NULL,	 filled	  with
						either the width,  in  pixels,
						of the widest line in the text
						layout, or the width, in  pix‐
						els,  of  the bounding box for
						the  character	specified   by
						index.

       int	       *heightPtr     (out)	If   non-NULL,	 filled	  with
						either the  total  height,  in
						pixels,	 of  all  the lines in
						the  text   layout,   or   the
						height,	  in  pixels,  of  the
						bounding box for the character
						specified by index.

       Tk_TextLayout   layout	      (in)	A  token  that	represents the
						cached	 layout	   information
						about  the single-font, multi-
						line, justified piece of text.
						This   token  is  returned  by
						Tk_ComputeTextLayout.

       Display	       *display	      (in)	Display on which to draw.

       Drawable	       drawable	      (in)	Window or pixmap in  which  to
						draw.

       GC	       gc	      (in)	Graphics  context  to  use for
						drawing text layout.  The font
						selected  in this GC must cor‐
						respond	 to  the  tkfont  used
						when   constructing  the  text
						layout.

       int	       x, y	      (in)	Point, in pixels, at which  to
						place the upper-left hand cor‐
						ner of the text layout when it
						is being drawn, or the coordi‐
						nates of a point (with respect
						to  the upper-left hand corner
						of the text layout)  to	 check
						against the text layout.

       int	       firstChar      (in)	The index of the first charac‐
						ter to	draw  from  the	 given
						text  layout.	The  number  0
						means to draw from the	begin‐
						ning.

       int	       lastChar	      (in)	The  index of the last charac‐
						ter up to which to draw.   The
						character     specified	    by
						lastChar itself	 will  not  be
						drawn.	 A  number less than 0
						means to draw  all  characters
						in the text layout.

       int	       underline      (in)	Index  of the single character
						to underline in the text  lay‐
						out,  or  a number less than 0
						for no underline.

       int	       index	      (in)	The  index  of	the  character
						whose bounding box is desired.
						The bounding box  is  computed
						with respect to the upper-left
						hand corner of the  text  lay‐
						out.

       int	       *xPtr, *yPtr   (out)	Filled	 with  the  upper-left
						hand corner, in pixels, of the
						bounding box for the character
						specified by index.  Either or
						both  xPtr  and	 yPtr  may  be
						NULL, in which case the corre‐
						sponding  value	 is not calcu‐
						lated.

       int	       width, height  (in)	Specifies   the	  width	   and
						height, in pixels, of the rec‐
						tangular area to  compare  for
						intersection  against the text
						layout.

       Tcl_Interp      *interp	      (out)	Postscript  code   that	  will
						print	the   text  layout  is
						appended to interp->result.
_________________________________________________________________

DESCRIPTION
       These routines are for measuring	 and  displaying  single-font,	multi-
       line,  justified text.  To measure and display simple single-font, sin‐
       gle-line strings,  refer	 to  the  documentation	 for  Tk_MeasureChars.
       There  is  no  programming  interface  in  the core of Tk that supports
       multi-font, multi-line text; support for that behavior must be built on
       top  of	simpler layers.	 Note that unlike the lower level text display │
       routines, the functions described here all  operate  on	character-ori‐ │
       ented  lengths  and  indices rather than byte-oriented values.  See the │
       description of Tcl_UtfAtIndex for more details  on  converting  between │
       character and byte offsets.

       The  routines described here are built on top of the programming inter‐
       face described in the Tk_MeasureChars  documentation.   Tab  characters
       and  newline/return characters may be treated specially by these proce‐
       dures, but all other characters are passed through to the lower level.

       Tk_ComputeTextLayout computes the layout information needed to  display
       a  single-font,	multi-line,  justified	string	of  text and returns a
       Tk_TextLayout token that holds this information.	 This token is used in
       subsequent  calls to procedures such as Tk_DrawTextLayout, Tk_Distance‐
       ToTextLayout, and Tk_FreeTextLayout.  The string and tkfont  used  when
       computing the layout must remain valid for the lifetime of this token.

       Tk_FreeTextLayout is called to release the storage associated with lay‐
       out when it is no longer needed.	 A layout should not be	 used  in  any
       other text layout procedures once it has been released.

       Tk_DrawTextLayout  uses	the information in layout to display a single-
       font, multi-line, justified string of text at the specified location.

       Tk_UnderlineTextLayout uses the information in  layout  to  display  an
       underline  below an individual character.  This procedure does not draw
       the text, just the underline.  To produce natively underlined text,  an
       underlined  font	 should	 be  constructed  and  used.   All characters,
       including tabs, newline/return characters, and spaces at	 the  ends  of
       lines,  can  be	underlined  using this method.	However, the underline
       will never be drawn outside of the computed width of layout; the under‐
       line  will  stop	 at  the edge for any character that would extend par‐
       tially outside of layout, and the underline will not be visible at  all
       for  any character that would be located completely outside of the lay‐
       out.

       Tk_PointToChar uses the information in layout to determine the  charac‐
       ter closest to the given point.	The point is specified with respect to
       the upper-left hand corner of the layout, which	is  considered	to  be
       located at (0, 0).  Any point whose y-value is less that 0 will be con‐
       sidered closest to the first character in the text  layout;  any	 point
       whose  y-value  is  greater  than the height of the text layout will be
       considered closest to the last character in the text layout.  Any point
       whose  x-value  is  less than 0 will be considered closest to the first
       character on that line; any point whose x-value	is  greater  than  the
       width of the text layout will be considered closest to the last charac‐
       ter on that line.  The return value is the index of the character  that
       was closest to the point.  Given a layout with no characters, the value
       0 will always be	 returned,  referring  to  a  hypothetical  zero-width
       placeholder character.

       Tk_CharBbox  uses  the information in layout to return the bounding box
       for the character specified by index.  The width of the bounding box is
       the  advance  width  of the character, and does not include any left or
       right bearing.  Any character that extends partially outside of	layout
       is considered to be truncated at the edge.  Any character that would be
       located completely outside of layout is considered to be zero-width and
       pegged  against	the  edge.  The height of the bounding box is the line
       height for this font, extending from the top of the ascent to the  bot‐
       tom  of	the descent; information about the actual height of individual
       letters is not available.  For measurement purposes, a layout that con‐
       tains no characters is considered to contain a single zero-width place‐
       holder character at index 0.  If index was not a valid character index,
       the  return  value is 0 and *xPtr, *yPtr, *widthPtr, and *heightPtr are
       unmodified.  Otherwise, if index did specify a valid, the return	 value
       is  non-zero,  and  *xPtr,  *yPtr, *widthPtr, and *heightPtr are filled
       with the bounding box information for the character.  If any  of	 xPtr,
       yPtr,  widthPtr,	 or heightPtr are NULL, the corresponding value is not
       calculated or stored.

       Tk_DistanceToTextLayout computes the shortest distance in  pixels  from
       the  given  point  (x,  y) to the characters in layout.	Newline/return
       characters and non-displaying space characters that occur at the end of
       individual  lines in the text layout are ignored for hit detection pur‐
       poses, but tab characters are not.  The return value is 0 if the	 point
       actually	 hits the layout.  If the point didn't hit the layout then the
       return value is the distance in pixels from the point to the layout.

       Tk_IntersectTextLayout  determines  whether  a  layout  lies   entirely
       inside,	 entirely  outside,  or	 overlaps  a  given  rectangle.	  New‐
       line/return characters and non-displaying space characters  that	 occur
       at  the end of individual lines in the layout are ignored for intersec‐
       tion calculations.  The return value is -1 if the  layout  is  entirely
       outside	of  the	 rectangle,  0 if it overlaps, and 1 if it is entirely
       inside of the rectangle.

       Tk_TextLayoutToPostscript outputs code consisting of a Postscript array
       of  strings  that  represent the individual lines in layout.  It is the
       responsibility of the caller to take the Postscript  array  of  strings
       and add some Postscript function operate on the array to render each of
       the lines.  The code that represents the Postscript array of strings is
       appended to interp->result.

DISPLAY MODEL
       When measuring a text layout, space characters that occur at the end of
       a line are ignored.  The space characters still exist and the insertion
       point  can  be  positioned  amongst them, but their additional width is
       ignored when justifying lines or returning the total width  of  a  text
       layout.	All end-of-line space characters are considered to be attached
       to the right edge of the line; this behavior is logical for left-justi‐
       fied text and reasonable for center-justified text, but not very useful
       when editing right-justified  text.   Spaces  are  considered  variable
       width  characters;  the	first  space that extends past the edge of the
       text layout is clipped to the edge, and any subsequent  spaces  on  the
       line  are  considered  zero  width  and pegged against the edge.	 Space
       characters that occur in the middle of a line  of  text	are  not  sup‐
       pressed and occupy their normal space width.

       Tab  characters are not ignored for measurement calculations.  If wrap‐
       ping is turned on and there are enough tabs on a	 line,	the  next  tab
       will  wrap  to the beginning of the next line.  There are some possible
       strange interactions between tabs and justification; tab positions  are
       calculated  and the line length computed in a left-justified world, and
       then the whole resulting line is shifted so it is  centered  or	right-
       justified, causing the tab columns not to align any more.

       When wrapping is turned on, lines may wrap at word breaks (space or tab
       characters) or newline/returns.	A dash or hyphen character in the mid‐
       dle  of	a  word	 is not considered a word break.  Tk_ComputeTextLayout
       always attempts to place at least one word on each line.	 If it	cannot
       because	the  wrapLength	 is  too small, the word will be broken and as
       much as fits placed on the line and the rest on subsequent line(s).  If
       wrapLength  is  so small that not even one character can fit on a given
       line, the wrapLength is ignored for that line and one character will be
       placed  on  the	line  anyhow.	When wrapping is turned off, only new‐
       line/return characters may cause a line break.

       When a text layout has been created using an  underlined	 tkfont,  then
       any  space  characters  that occur at the end of individual lines, new‐
       lines/returns, and tabs will not be displayed underlined when  Tk_Draw‐
       TextLayout is called, because those characters are never actually drawn
       - they are merely placeholders maintained in the layout.

KEYWORDS
       font

Tk				      8.1	       Tk_ComputeTextLayout(3)
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