[util/hb-view] Add --font-extents

Fixes https://github.com/harfbuzz/harfbuzz/issues/2402

util/options.cc

@@ -163,6 +163,28 @@ option_parser_t::parse (int *argc, char ***argv)
 }
 
 
+static gboolean
+parse_font_extents (const char *name G_GNUC_UNUSED,
+		    const char *arg,
+		    gpointer    data,
+		    GError    **error G_GNUC_UNUSED)
+{
+  view_options_t *view_opts = (view_options_t *) data;
+  view_options_t::font_extents_t &e = view_opts->font_extents;
+  switch (sscanf (arg, "%lf%*[ ,]%lf%*[ ,]%lf", &e.ascent, &e.descent, &e.line_gap)) {
+    case 1: HB_FALLTHROUGH;
+    case 2: HB_FALLTHROUGH;
+    case 3:
+      view_opts->have_font_extents = true;
+      return true;
+    default:
+      g_set_error (error, G_OPTION_ERROR, G_OPTION_ERROR_BAD_VALUE,
+		   "%s argument should be one to three space-separated numbers",
+		   name);
+      return false;
+  }
+}
+
 static gboolean
 parse_margin (const char *name G_GNUC_UNUSED,
 	      const char *arg,
@@ -399,6 +421,7 @@ view_options_t::add_options (option_parser_t *parser)
     {"background",	0, 0, G_OPTION_ARG_STRING,	&this->back,			"Set background color (default: " DEFAULT_BACK ")",	"rrggbb/rrggbbaa"},
     {"foreground",	0, 0, G_OPTION_ARG_STRING,	&this->fore,			"Set foreground color (default: " DEFAULT_FORE ")",	"rrggbb/rrggbbaa"},
     {"line-space",	0, 0, G_OPTION_ARG_DOUBLE,	&this->line_space,		"Set space between lines (default: 0)",			"units"},
+    {"font-extents",	0, 0, G_OPTION_ARG_CALLBACK,	(gpointer) &parse_font_extents,	"Set font ascent/descent/line-gap (default: auto)","one to three numbers"},
     {"margin",		0, 0, G_OPTION_ARG_CALLBACK,	(gpointer) &parse_margin,	"Margin around output (default: " G_STRINGIFY(DEFAULT_MARGIN) ")","one to four numbers"},
     {nullptr}
   };

util/options.hh

@@ -125,6 +125,8 @@ struct view_options_t : option_group_t
     fore = nullptr;
     back = nullptr;
     line_space = 0;
+    have_font_extents = false;
+    font_extents.ascent = font_extents.descent = font_extents.line_gap = 0;
     margin.t = margin.r = margin.b = margin.l = DEFAULT_MARGIN;
 
     add_options (parser);
@@ -141,6 +143,10 @@ struct view_options_t : option_group_t
   char *fore;
   char *back;
   double line_space;
+  bool have_font_extents;
+  struct font_extents_t {
+    double ascent, descent, line_gap;
+  } font_extents;
   struct margin_t {
     double t, r, b, l;
   } margin;

util/view-cairo.cc

@@ -40,17 +40,25 @@ view_cairo_t::render (const font_options_t *font_opts)
   int y_sign = font_opts->font_size_y < 0 ? -1 : +1;
 
   hb_font_t *font = font_opts->get_font();
-  hb_font_extents_t extents;
-  hb_font_get_extents_for_direction (font, direction, &extents);
 
-  double ascent = y_sign * scalbn ((double) extents.ascender, scale_bits);
-  double descent = y_sign * -scalbn ((double) extents.descender, scale_bits);
-  double font_height = y_sign * scalbn ((double) extents.ascender - extents.descender + extents.line_gap, scale_bits);
-  double leading = font_height + view_options.line_space;
+  view_options_t::font_extents_t extents = view_options.font_extents;
+  if (!view_options.have_font_extents)
+  {
+    hb_font_extents_t hb_extents;
+    hb_font_get_extents_for_direction (font, direction, &hb_extents);
+    extents.ascent = scalbn ((double) hb_extents.ascender, scale_bits);
+    extents.descent = -scalbn ((double) hb_extents.descender, scale_bits);
+    extents.line_gap = scalbn ((double) hb_extents.line_gap, scale_bits);
+  }
+
+  double ascent = y_sign * extents.ascent;
+  double descent = y_sign * extents.descent;
+  double line_gap = y_sign * extents.line_gap + view_options.line_space;
+  double leading = ascent + descent + line_gap;
 
   /* Calculate surface size. */
   double w = 0, h = 0;
-  (vertical ? w : h) = (int) lines->len * leading - view_options.line_space;
+  (vertical ? w : h) = (int) lines->len * leading - (extents.line_gap + view_options.line_space);
   (vertical ? h : w) = 0;
   for (unsigned int i = 0; i < lines->len; i++) {
     helper_cairo_line_t &line = g_array_index (lines, helper_cairo_line_t, i);
@@ -79,8 +87,7 @@ view_cairo_t::render (const font_options_t *font_opts)
   cairo_translate (cr, view_options.margin.l, view_options.margin.t);
   if (vertical)
     cairo_translate (cr,
-		     w /* We stack lines right to left */
-		     -font_height * .5 /* "ascent" for vertical */,
+		     w - ascent, /* We currently always stack lines right to left */
 		     y_sign < 0 ? h : 0);
   else
    {