Fix excessive boldness on thin text

When the distance field curve is at 0.5, the fragment actually
contains the edge of the glyph, so we shouldn't fill the pixel
completely.

The smooth range should rather be given by the area spanned
by the fragment (approximated by the fwidth() function) and
centered on the edge.

The result was that glyphs would look bolder than they should,
especially visible at low resolutions with thin outlines (thus
it could be worked around by enabling SSAA, which would render
the glyphs at a higher resolution).

This is a fix for the boldness part of QT3DS-4175. For the
glyph corruption, this is actually due to features of the
glyphs being thinner than a pixel, in which case both fragments
may sample outside the outline, and we do not get valid
values back from fwidth(), which is a known draw-back of
this antialiasing strategy.

Task-number: QT3DS-4175
Change-Id: Ide8e190eed766d04fdccf203c4ecfedbb93c6247
Reviewed-by: Janne Koskinen <janne.p.koskinen@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/res/effectlib/distancefieldtext.frag b/res/effectlib/distancefieldtext.frag
index 1d83490..2236f9c 100644
--- a/res/effectlib/distancefieldtext.frag
+++ b/res/effectlib/distancefieldtext.frag
@@ -20,8 +20,8 @@ void main()
 #ifdef use_fallback
     highp float alpha = smoothstep(alphas.x, alphas.y, distance);
 #else
-    highp float f = fwidth(distance);
-    highp float alpha = smoothstep(0.5 - f, 0.5, distance);
+    highp float f = fwidth(distance) * 0.5;
+    highp float alpha = smoothstep(0.5 - f, 0.5 + f, distance);
 #endif
 
     gl_FragColor = color * alpha;