[ft-devel] Light Autofit + RGB Filtering

[James Cloos]

A few months back aging eyes and displays forced me to change my UI font
preferences.  Up until then I primarily used fonts which had stem widths
of one pixel.  In fontconfig pattern syntax, I used faces such as:

  DejaVu Sans Mono:pixelsize=15:minspace=true:autohint=false
  DejaVu Serif:pixelsize=15:minspace=true:autohint=false

After considerable testing, I ended up with:

  DejaVu Sans Mono:pixelsize=20:minspace=true:autohint=true
  DejaVu Serif:pixelsize=20:minspace=true:autohint=true

as my preferences.

I had already had fontconfig configured to suggest freetype’s legacy
lcd filter for instructed sfnt/glyf faces and its default filter for
all other faces.

The buggest annoyance I hit was the fact that the autofitter adjusts
glyphs’ advance widths.  The resulted in DejaVu Sans Mono’s asterisk
glyph being a (sub-?)pixel wider than its space glyph, causing text
to reposition in my Gnus Summary buffers when I *-tagged messages.

Fontconfig and freetype make it easy to force lcd filtering and autofit,
but I do not see any easy way to get lcd filtering with light autofit.

I first tested a ft patch which only avoided changes to advance width.
That produced ugly output.

The below patch changes every test for (mode == FT_RENDER_MODE_LIGHT)
to (true) and (mode != FT_RENDER_MODE_LIGHT) to (false).

To my eyes, on this display, the results look substantially better than
the defaults for autofit+lcd.  The horizontal stems tend to get fit to a
single pixel tall and the vertical stems tend to be one of 1+3+1, 1+3+2,
2+3+1 or 2+3+2 sub-pixels wide.  Which looks crisp; I cannot see any
colour fringing at all.

IIRC, David at some point expressed a preference for the light autofit.

I’ve come to strongly agree.

I think we should look into making autofit fit in a single direction
by default.

The patch is not meant for upstreaming, but rather to allow easy testing
and to be mostly self-documenting:


diff --git a/src/autofit/afcjk.c b/src/autofit/afcjk.c
index d7670f4..ae2a18a 100644
--- a/src/autofit/afcjk.c
+++ b/src/autofit/afcjk.c
@@ -1303,7 +1303,7 @@
     /*
      *  We adjust stems to full pixels only if we don't use the `light' mode.
      */
-    if ( mode != FT_RENDER_MODE_LIGHT )
+    if ( 0 )
       other_flags |= AF_LATIN_HINTS_STEM_ADJUST;
 
     if ( mode == FT_RENDER_MODE_MONO )
diff --git a/src/autofit/aflatin.c b/src/autofit/aflatin.c
index b939938..9b25781 100644
--- a/src/autofit/aflatin.c
+++ b/src/autofit/aflatin.c
@@ -1495,7 +1495,7 @@
     /*
      *  We adjust stems to full pixels only if we don't use the `light' mode.
      */
-    if ( mode != FT_RENDER_MODE_LIGHT )
+    if ( 0 )
       other_flags |= AF_LATIN_HINTS_STEM_ADJUST;
 
     if ( mode == FT_RENDER_MODE_MONO )
@@ -1505,7 +1505,7 @@
      *  In `light' hinting mode we disable horizontal hinting completely.
      *  We also do it if the face is italic.
      */
-    if ( mode == FT_RENDER_MODE_LIGHT                      ||
+    if ( 1                      ||
          ( face->style_flags & FT_STYLE_FLAG_ITALIC ) != 0 )
       scaler_flags |= AF_SCALER_FLAG_NO_HORIZONTAL;
 
@@ -2235,7 +2235,7 @@
 
     /* analyze glyph outline */
 #ifdef AF_CONFIG_OPTION_USE_WARPER
-    if ( metrics->root.scaler.render_mode == FT_RENDER_MODE_LIGHT ||
+    if ( 1 ||
          AF_HINTS_DO_HORIZONTAL( hints )                          )
 #else
     if ( AF_HINTS_DO_HORIZONTAL( hints ) )
@@ -2260,7 +2260,7 @@
     {
 #ifdef AF_CONFIG_OPTION_USE_WARPER
       if ( dim == AF_DIMENSION_HORZ                                 &&
-           metrics->root.scaler.render_mode == FT_RENDER_MODE_LIGHT )
+           1 )
       {
         AF_WarperRec  warper;
         FT_Fixed      scale;
diff --git a/src/autofit/aflatin2.c b/src/autofit/aflatin2.c
index d84e53a..efdd74f 100644
--- a/src/autofit/aflatin2.c
+++ b/src/autofit/aflatin2.c
@@ -1512,7 +1512,7 @@
     /*
      *  We adjust stems to full pixels only if we don't use the `light' mode.
      */
-    if ( mode != FT_RENDER_MODE_LIGHT )
+    if ( 0 )
       other_flags |= AF_LATIN_HINTS_STEM_ADJUST;
 
     if ( mode == FT_RENDER_MODE_MONO )
@@ -1522,7 +1522,7 @@
      *  In `light' hinting mode we disable horizontal hinting completely.
      *  We also do it if the face is italic.
      */
-    if ( mode == FT_RENDER_MODE_LIGHT                    ||
+    if ( 1                    ||
          (face->style_flags & FT_STYLE_FLAG_ITALIC) != 0 )
       scaler_flags |= AF_SCALER_FLAG_NO_HORIZONTAL;
 
@@ -2286,7 +2286,7 @@
 
     /* analyze glyph outline */
 #ifdef AF_CONFIG_OPTION_USE_WARPER
-    if ( metrics->root.scaler.render_mode == FT_RENDER_MODE_LIGHT ||
+    if ( 1 ||
          AF_HINTS_DO_HORIZONTAL( hints ) )
 #else
     if ( AF_HINTS_DO_HORIZONTAL( hints ) )
@@ -2311,7 +2311,7 @@
     {
 #ifdef AF_CONFIG_OPTION_USE_WARPER
       if ( ( dim == AF_DIMENSION_HORZ &&
-             metrics->root.scaler.render_mode == FT_RENDER_MODE_LIGHT ) )
+             1 ) )
       {
         AF_WarperRec  warper;
         FT_Fixed      scale;
diff --git a/src/autofit/afloader.c b/src/autofit/afloader.c
index 966a0df..00c4acc 100644
--- a/src/autofit/afloader.c
+++ b/src/autofit/afloader.c
@@ -181,7 +181,7 @@
 
       /* we now need to adjust the metrics according to the change in */
       /* width/positioning that occurred during the hinting process   */
-      if ( scaler->render_mode != FT_RENDER_MODE_LIGHT )
+      if ( 0 )
       {
         FT_Pos        old_rsb, old_lsb, new_lsb;
         FT_Pos        pp1x_uh, pp2x_uh;
diff --git a/src/pshinter/pshalgo.c b/src/pshinter/pshalgo.c
index d798978..847763b 100644
--- a/src/pshinter/pshalgo.c
+++ b/src/pshinter/pshalgo.c
@@ -658,7 +658,7 @@
   }
 
 
-#if 0  /* not used for now, experimental */
+#if 1  /* not used for now, experimental */
 
  /*
   *  A variant to perform "light" hinting (i.e. FT_RENDER_MODE_LIGHT)
@@ -2260,7 +2260,7 @@
       glyph->do_vert_snapping = FT_BOOL( hint_mode == FT_RENDER_MODE_MONO  ||
                                          hint_mode == FT_RENDER_MODE_LCD_V );
 
-      glyph->do_stem_adjust   = FT_BOOL( hint_mode != FT_RENDER_MODE_LIGHT );
+      glyph->do_stem_adjust   = FT_BOOL( 0 );
 
       for ( dimension = 0; dimension < 2; dimension++ )
       {


-JimC
-- 
James Cloos <address@hidden>         OpenPGP: 1024D/ED7DAEA6