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bug#7887: Emacs will render some fonts under Mac OS X one pixel too tall

From: Anders Lindgren
Subject: bug#7887: Emacs will render some fonts under Mac OS X one pixel too tall (patch supplied)
Date: Sat, 22 Jan 2011 10:43:39 +0100


I have notices that Emacs will render some fonts under Mac OS X one
pixel too tall.

The current implementation for handling fonts under NextStep always
round the descender to the next full integer, to avoid clipping. (As
descenders under NextStep are negative, this is done using the
function "floor"). Unfortunately, some fonts do not specify the
descender as exactly as desired. Concretely, a 6x8 bitmap font could
report a descender of -2.0000040531158447 (represented by
0xc000000220000000), which Emacs rounds to -3. The result is that the
font will be drawn one pixel taller than expected.

The patch below is one example of how to handle this, it will
circumvent the problem by adding a small value to the descender,
making sure that values close enough to an integer will be rounded to
that integer.

    -- Anders Lindgren

2011-01-22  Anders Lindgren  <address@hidden>

        * nsfont.m (nsfont_open): Ensure that fonts with inexact
        descenders would not become one pixel too tall.

=== modified file 'src/nsfont.m'
--- src/nsfont.m        2011-01-19 22:11:33 +0000
+++ src/nsfont.m        2011-01-22 09:03:56 +0000
@@ -809,6 +809,15 @@
     const char *fontName = [[nsfont fontName] UTF8String];
     int len = strlen (fontName);

+    /* The values specified by fonts are not always exact. For
+     * example, a 6x8 font could specify that the descender is
+     * -2.00000405... (represented by 0xc000000220000000).  Without
+     * adjustment, the code below would round the descender to -3,
+     * resulting in a font that would be one pixel higher than
+     * intended. */
+    CGFloat adjusted_descender = [sfont descender] + 0.0001;
     font_info->nsfont = sfont;
@@ -830,7 +839,7 @@

     brect =  [sfont boundingRectForFont];
     full_height = brect.size.height;
-    min_height = [sfont ascender] - [sfont descender];
+    min_height = [sfont ascender] - adjusted_descender;
     hd = full_height - min_height;

     /* standard height, similar to Carbon. Emacs.app: was 0.5 by default. */
@@ -845,10 +854,10 @@
     /* max bounds */
     font_info->max_bounds.ascent =
       lrint (hshrink * [sfont ascender] + expand * hd/2);
-    /* [sfont descender] is usually negative.  Use floor to avoid
-       clipping descenders. */
+    /* Descender is usually negative.  Use floor to avoid clipping
+       descenders. */
     font_info->max_bounds.descent =
-      -lrint (floor(hshrink* [sfont descender] - expand*hd/2));
+      -lrint (floor(hshrink * adjusted_descender - expand * hd/2));
     font_info->height =
       font_info->max_bounds.ascent + font_info->max_bounds.descent;
     font_info->max_bounds.width = lrint (font_info->width);
@@ -884,7 +893,7 @@

     /* set up metrics portion of font struct */
     font->ascent = lrint([sfont ascender]);
-    font->descent = -lrint(floor([sfont descender]));
+    font->descent = -lrint(floor(adjusted_descender));
     font->min_width = ns_char_width(sfont, '|');
     font->space_width = lrint (ns_char_width (sfont, ' '));
     font->average_width = lrint (font_info->width);

In GNU Emacs (x86_64-apple-darwin10.5.0, NS apple-appkit-1038.35)
 of 2011-01-14 on macpro.local
Windowing system distributor `Apple', version 10.3.1038
configured using `configure  '--with-ns''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: nil
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default enable-multibyte-characters: t

Major mode: ObjC/lw

Minor modes in effect:
  diff-auto-refine-mode: t
  subword-mode: t
  global-auto-revert-mode: t
  global-cwarn-mode: t
  minibuffer-electric-file-mode: t
  recentf-mode: t
  msb-mode: t
  display-time-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Recent input:
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> C-SPC <down> C-w <down> C-SPC
<down> <down> <down> C-w <up> C-e <left> <left> <left>
<M-backspace> <backspace> <backspace> <backspace> C-a
C-x C-s C-M-a C-s e p s C-a C-x C-f s a <backspace>
l a s j . <backspace> <backspace> k . c <return> <C-tab>
C-x o <C-S-tab> C-x p C-M-a <up> <up> <up> <up> <up>
<up> <up> <up> <up> C-SPC <down> <down> <down> <down>
<down> <down> <down> C-w C-x C-s C-x o C-y C-x p <down>
C-v C-l C-v <down> <down-mouse-1> <mouse-1> C-a C-SPC
<down> <down> <down> <down> <down> <down> <down> C-w
C-x C-s C-x o C-y C-x C-s C-x p <down-mouse-1> <mouse-1>
M-d M-d M-d c o u l y <backspace> d SPC <backspace>
M-f <backspace> <backspace> <backspace> y <escape>
q <M-left> <M-left> <M-backspace> <escape> q <down>
<down> M-f M-f M-f <M-backspace> t h e SPC d e s c
M-- <escape> q C-x C-s <down-mouse-1> <mouse-1> <down-mouse-1>
<mouse-1> <backspace> C-a C-x C-s <down-mouse-1> <mouse-1>
SPC <down-mouse-1> <mouse-1> SPC C-a C-x C-s <down-mouse-1>
<mouse-1> SPC <down-mouse-1> <mouse-1> SPC C-a C-x
C-s <down-mouse-1> <mouse-1> C-a <down> SPC <backspace>
<down> <down> <down> <down> <down> <up> SPC <backspace>
C-a C-x C-s <up> <up> <up> <up> M-f <right> <right>
C-s C-w C-s C-a <down-mouse-1> <mouse-1> a <backspace>
a d j u s t m e n t <escape> d <escape> q <up> <up>
<up> C-a C-SPC <down> <down> <down> <down> <down> <down>
<escape> x i s p l l <backspace> <backspace> e l l
- r e g <tab> ' <backspace> <return> q <up> C-g <up>
<down-mouse-1> <mouse-1> c <escape> q <down-mouse-1>
<mouse-1> C-d <backspace> <backspace> <backspace> i
f y C-a C-x C-s <menu-bar> <help-menu> <send-emacs

Recent messages:
Wrote /Users/anders/build/emacs/trunk/src/nsfont.m
Mark saved where search started
Mark set
Starting new Ispell process [default] ...
Entering debugger...
Back to top level.
Saving file /Users/anders/build/emacs/trunk/src/nsfont.m...
Wrote /Users/anders/build/emacs/trunk/src/nsfont.m

Load-path shadows:
~/emacs/lisp/table hides
~/emacs/lisp/ruby-mode hides
~/emacs/src/asm-mode-new/src/asm-mode hides

(ispell smerge-mode diff-mode texinfo autoconf autoconf-mode conf-mode
shadow sort gnus-util mail-extr message rfc822 mml mml-sec mm-decode
mm-bodies mm-encode mail-parse rfc2231 rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mailabbrev mail-utils gmm-utils mailheader emacsbug
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autorevert folding-isearch folding view rdebug cwarn prepaint cc-mode
cc-fonts cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
tempo edg-mode split-nways toggle-file-read-only lockdir uniquify
ange-ftp comint regexp-opt ring paren mic-paren iso-insert
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dnd fontset image fringe lisp-mode register page menu-bar rfn-eshadow
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