Subject: Emacs will render some fonts under Mac OS X one pixel too tall (patch supplied)

From: Anders Lindgren <[email-masked]>
Date: Sat, 22 Jan 2011 10:43:39 +0100

Hi!

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  <[email-masked]>

	* 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;
+
 #ifdef NS_IMPL_GNUSTEP
     font_info->nsfont = sfont;
 #else
@@ -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 24.0.50.1 (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
-bug-report>

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.
Quit
Auto-saving...done
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
/Applications/Emacs24.app/Contents/Resources/lisp/textmodes/table
~/emacs/lisp/ruby-mode hides
/Applications/Emacs24.app/Contents/Resources/lisp/progmodes/ruby-mode
~/emacs/src/asm-mode-new/src/asm-mode hides
/Applications/Emacs24.app/Contents/Resources/lisp/progmodes/asm-mode

Features:
(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
subword newcomment ctypes grep compile etags jka-compr find-func follow
my-end-of-buffer-log c-electric-operator c-indent-operator dired-aux
dired vc-bzr sha1 hex-util add-log vc-dispatcher vc-svn pp apropos
multi-isearch debug dabbrev help-mode eldoc ps-print ps-def lpr
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
minibuf-elfile easy-mmode recentf tree-widget wid-edit easymenu msb
edmacro kmacro warnings disp-table time advice help-fns advice-preload
cl tooltip ediff-hook vc-hooks lisp-float-type mwheel ns-win tool-bar
dnd fontset image fringe lisp-mode register page menu-bar rfn-eshadow
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core frame cham georgian utf-8-lang misc-lang vietnamese tibetan
thai tai-viet lao korean japanese hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese case-table epa-hook
jka-cmpr-hook help simple abbrev loaddefs button minibuffer faces
cus-face files text-properties overlay md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote
make-network-process ns multi-tty emacs)