Weighted average check

Author: simoncozens

profile-universal/src/checks/os2.rs

@@ -3,6 +3,7 @@ use read_fonts::{
     tables::{head::MacStyle, os2::SelectionFlags},
     TableProvider,
 };
+use skrifa::{GlyphId, MetadataProvider};
 
 #[check(
     id = "opentype/fsselection",
@@ -160,3 +161,102 @@ fn check_vendor_id(f: &Testable, context: &Context) -> CheckFnResult {
         ))
     }
 }
+
+const AVG_CHAR_WEIGHTS: [(char, u32); 27] = [
+    ('a', 64),
+    ('b', 14),
+    ('c', 27),
+    ('d', 35),
+    ('e', 100),
+    ('f', 20),
+    ('g', 14),
+    ('h', 42),
+    ('i', 63),
+    ('j', 3),
+    ('k', 6),
+    ('l', 35),
+    ('m', 20),
+    ('n', 56),
+    ('o', 56),
+    ('p', 17),
+    ('q', 4),
+    ('r', 49),
+    ('s', 56),
+    ('t', 71),
+    ('u', 31),
+    ('v', 10),
+    ('w', 18),
+    ('x', 3),
+    ('y', 18),
+    ('z', 2),
+    (' ', 166),
+];
+
+#[check(
+    id = "opentype/xavgcharwidth",
+    proposal = "https://github.com/fonttools/fontbakery/issues/4829",
+    title = "Checking OS/2 fsSelection value.",
+    rationale = "
+        The OS/2.xAvgCharWidth field is used to calculate the width of a string of
+        characters. It is the average width of all non-zero width glyphs in the font.
+
+        This check ensures that the value is correct. A failure here may indicate
+        a bug in the font compiler, rather than something that the designer can
+        do anything about.
+    "
+)]
+fn xavgcharwidth(f: &Testable, _context: &Context) -> CheckFnResult {
+    let font = testfont!(f);
+    let os2 = font.font().os2()?;
+    let hmtx = font.font().hmtx()?;
+    let charmap = font.font().charmap();
+    let (rule, expected) = if os2.version() >= 3 {
+        let advances = hmtx
+            .h_metrics()
+            .iter()
+            .map(|metric| metric.advance.get() as u32)
+            .filter(|&w| w > 0)
+            .collect::<Vec<_>>();
+        (
+            "the average of the widths of all glyphs in the font",
+            advances.iter().sum::<u32>() / advances.len() as u32,
+        )
+    } else {
+        let ids: Vec<Option<GlyphId>> = AVG_CHAR_WEIGHTS
+            .iter()
+            .map(|(c, _)| charmap.map(*c))
+            .collect();
+        if ids.iter().any(|id| id.is_none()) {
+            return Err(CheckError::Error(
+                "Missing glyph in font for average character width calculation".to_string(),
+            ));
+        }
+        #[allow(clippy::unwrap_used)] // We know all the characters are in the font
+        let advances = ids
+            .iter()
+            .zip(AVG_CHAR_WEIGHTS.iter())
+            .map(|(id, (_, w))| hmtx.advance(id.unwrap()).unwrap_or(0) as u32 * w)
+            .collect::<Vec<_>>();
+        (
+            "the weighted average of the widths of the latin lowercase glyphs in the font",
+            advances.iter().sum::<u32>() / advances.len() as u32,
+        )
+    };
+    let actual = os2.x_avg_char_width();
+    let difference = (expected as i16 - actual).abs();
+    Ok(match difference {
+        0|1 => Status::just_one_pass(),
+        2|3|4|5|6|7|8|9|20 => Status::just_one_info(
+            "xAvgCharWidth-close",
+            &format!("OS/2 xAvgCharWidth is {} but it should be {} which corresponds to {}. These are similar values, which may be a symptom of the slightly different calculation of the xAvgCharWidth value in font editors. There's further discussion on this at https://github.com/fonttools/fontbakery/issues/1622",
+                actual, expected, rule
+            )
+        ),
+        _ => Status::just_one_warn(
+            "xAvgCharWidth-wrong",
+            &format!("OS/2 xAvgCharWidth is {} but it should be {} which corresponds to {}. This may indicate a problem with the font editor or the font compiler.",
+                actual, expected, rule
+            )
+        )
+    }    )
+}

profile-universal/src/lib.rs

@@ -31,6 +31,7 @@ impl fontspector_checkapi::Plugin for Universal {
         cr.register_check(checks::layout::layout_valid_language_tags);
         cr.register_check(checks::layout::layout_valid_script_tags);
         cr.register_check(checks::name::check_name_no_copyright_on_description);
+        cr.register_check(checks::name::check_name_match_familyname_fullfont);
         cr.register_check(checks::post::post_table_version);
         cr.register_check(checks::post::underline_thickness);
         cr.register_check(checks::stat::stat_axis_record);
@@ -39,6 +40,7 @@ impl fontspector_checkapi::Plugin for Universal {
         cr.register_check(checks::os2::fsselection);
         cr.register_check(checks::os2::panose_familytype);
         cr.register_check(checks::os2::check_vendor_id);
+        cr.register_check(checks::os2::xavgcharwidth);
 
         let opentype_profile = Profile::from_toml(
             r#"
@@ -76,6 +78,8 @@ impl fontspector_checkapi::Plugin for Universal {
     "opentype/weight_class_fvar",
     "opentype/vendor_id",
     "opentype/name/match_familyname_fullfont",
+    "opentype/varfont/valid_default_instance_nameids",
+    "opentype/xavgcharwidth",
 
     # Checks left to port
     "opentype/cff2_call_depth",
@@ -120,8 +124,6 @@ impl fontspector_checkapi::Plugin for Universal {
     "opentype/name/postscript_vs_cff",
     "opentype/postscript_name",
     "opentype/slant_direction",
-    "opentype/varfont/valid_default_instance_nameids",
-    "opentype/xavgcharwidth",
 ]
 "#,
         )