label: Use new, optional 'ascent', 'descent' properties

Each time new glyphs have to be loaded from a font, it can take a long
time (hundreds of milliseconds).  In a parallel commit, 'ascent' and
'descent' properties will be added to BDF font objects, are essentially
free to compute, and are in any case much quicker than entering
load_glyphs.

This may change the layout of text slightly.  For instance, the height
of the "M" glyph in GothamBlack-50.bdf is 35, but the Ascent of the font
is 40 (and some characters, such as Å, are taller than the ascent at 45
pixels high)

Author: jepler

adafruit_display_text/label.py

@@ -147,28 +147,17 @@ def _create_background_box(self, lines, y_offset):
             y_box_offset = self._boundingbox[1]
 
         else:  # draw a "loose" bounding box to include any ascenders/descenders.
-
-            # check a few glyphs for maximum ascender and descender height
-            # Enhancement: it would be preferred to access the font "FONT_ASCENT" and
-            # "FONT_DESCENT" in the imported BDF file
-            glyphs = "M j'"  # choose glyphs with highest ascender and lowest
-            # descender, will depend upon font used
-            ascender_max = descender_max = 0
-            for char in glyphs:
-                this_glyph = self._font.get_glyph(ord(char))
-                if this_glyph:
-                    ascender_max = max(ascender_max, this_glyph.height + this_glyph.dy)
-                    descender_max = max(descender_max, -this_glyph.dy)
+            ascent, descent = self._get_ascent_descent()
 
             box_width = self._boundingbox[2] + self._padding_left + self._padding_right
             x_box_offset = -self._padding_left
             box_height = (
-                (ascender_max + descender_max)
+                (ascent + descent)
                 + int((lines - 1) * self.height * self._line_spacing)
                 + self._padding_top
                 + self._padding_bottom
             )
-            y_box_offset = -ascender_max + y_offset - self._padding_top
+            y_box_offset = -ascent + y_offset - self._padding_top
 
         box_width = max(0, box_width)  # remove any negative values
         box_height = max(0, box_height)  # remove any negative values
@@ -183,6 +172,25 @@ def _create_background_box(self, lines, y_offset):
 
         return tile_grid
 
+    def _get_ascent_descent(self):
+        if hasattr(self.font, "ascent"):
+            return self.font.ascent, self.font.descent
+
+        # check a few glyphs for maximum ascender and descender height
+        glyphs = "M j'"  # choose glyphs with highest ascender and lowest
+        self._font.load_glyphs(glyphs)
+        # descender, will depend upon font used
+        ascender_max = descender_max = 0
+        for char in glyphs:
+            this_glyph = self._font.get_glyph(ord(char))
+            if this_glyph:
+                ascender_max = max(ascender_max, this_glyph.height + this_glyph.dy)
+                descender_max = max(descender_max, -this_glyph.dy)
+        return ascender_max, descender_max
+
+    def _get_ascent(self):
+        return self._get_ascent_descent()[0]
+
     def _update_background_color(self, new_color):
 
         if new_color is None:
@@ -196,7 +204,7 @@ def _update_background_color(self, new_color):
         self._background_color = new_color
 
         lines = self._text.rstrip("\n").count("\n") + 1
-        y_offset = int((self._font.get_glyph(ord("M")).height) / 2)
+        y_offset = self._get_ascent() // 2
 
         if not self._added_background_tilegrid:  # no bitmap is in the self Group
             # add bitmap if text is present and bitmap sizes > 0 pixels
@@ -248,13 +256,7 @@ def _update_text(
             i = 0
         tilegrid_count = i
 
-        try:
-            self._font.load_glyphs(new_text + "M")
-        except AttributeError:
-            # ignore if font does not have load_glyphs
-            pass
-
-        y_offset = int((self._font.get_glyph(ord("M")).height) / 2)
+        y_offset = self._get_ascent() // 2
 
         right = top = bottom = 0
         left = None