A case of erroneous glyph placement by the swash scaler

[Mohammad AlSaleh]

Font

Kawkab Mono v0.501

Glyphs Affected

Bold ع and غ

Analysis

Applying this patch first:

diff --git a/src/swash.rs b/src/swash.rs
index 7c35ce9f5..ee704ce27 100644
--- a/src/swash.rs
+++ b/src/swash.rs
@@ -37,7 +37,7 @@ fn swash_image(
     let offset = Vector::new(cache_key.x_bin.as_float(), cache_key.y_bin.as_float());
 
     // Select our source order
-    Render::new(&[
+    let mut ret = Render::new(&[
         // Color outline with the first palette
         Source::ColorOutline(0),
         // Color bitmap with best fit selection mode
@@ -58,7 +58,8 @@ fn swash_image(
         None
     })
     // Render the image
-    .render(&mut scaler, cache_key.glyph_id)
+    .render(&mut scaler, cache_key.glyph_id);
+    dbg!(ret.as_ref().map(|img| img.placement));
 }
 
 fn swash_outline_commands(

We can see this erroneous placement:

[src/swash.rs:62:5] ret.as_ref().map(|img| img.placement) = Some(
    Placement {
        left: 1,
        top: -2412,
        width: 48,
        height: 89,
    },
)

As you can see, the top value is way off.

I added this temporary workaround to make sure that glyphs are at least in the general vicinity of where they're supposed to be:

ret.as_mut().map(|img| if img.placement.top < img.placement.height as i32 * -1 {
     img.placement.top = (img.placement.height / 2) as i32;
});

Hopefully a proper fix will materialize.

[Mohammad AlSaleh]

Bug appears to be in swash's CFF scaler.

[Mohammad AlSaleh]

With swash's new-cff branch:

[src/swash.rs:62:5] ret.as_ref().map(|img| img.placement) = Some(
    Placement {
        left: 1,
        top: -364,
        width: 48,
        height: 89,
    },
)

So still wrong, but less so.

[Chad Brokaw]

Thanks for reporting. I’ll take a look at this.

[Chad Brokaw]

Nice catch! This was a bug in CFF hinting (interestingly, the bug was different but in the same exact code for both the old and new CFF scalers). I pushed a fix, merged the new CFF scaler and released swash 0.1.9.

Please let me know if this fixes things on your end.

[Mohammad AlSaleh]

Please let me know if this fixes things on your end.

It does. Thank you for your timely fix.