Tk_ComputeTextLayout(3tk) Tk Library Procedures Tk_ComputeTextLayout(3tk)
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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 re-
turned 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 as-
sumes 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 to-
gether. TK_IGNORE_TABS means
that tab characters should not
be expanded to the next tab
stop. TK_IGNORE_NEWLINES
means that newline/return
characters should not cause a
line break. If either tabs or
newlines/returns are ignored,
then they will be treated as
regular characters, being mea-
sured and displayed in a plat-
form-dependent manner as de-
scribed in Tk_MeasureChars,
and will not have any special
behaviors.
int *widthPtr (out) If non-NULL, filled with ei-
ther 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 in-
dex.
int *heightPtr (out) If non-NULL, filled with ei-
ther the total height, in pix-
els, 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 ap-
pended to the result of inter-
preter interp.
______________________________________________________________________________
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 un-
derline 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, in-
cluding 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 lo-
cated 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, or one more than the index of any character
(to indicate that the point was after the end of the string and that
the corresponding caret would be at the end of the string). Given a
layout with no characters, the value 0 will always be returned, refer-
ring 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 did not 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 in-
side, entirely outside, or overlaps a given rectangle. Newline/return
characters and non-displaying space characters that occur at the end of
individual lines in the layout are ignored for intersection calcula-
tions. 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 rec-
tangle.
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 interpreter interp's 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 ig-
nored when justifying lines or returning the total width of a text lay-
out. All end-of-line space characters are considered to be attached to
the right edge of the line; this behavior is logical for left-justified
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 lay-
out 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 suppressed and oc-
cupy 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 al-
ways 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(3tk)
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