Adjust text fade width and alpha

ui/gfx/render_text.cc

Index: ui/gfx/render_text.cc
diff --git a/ui/gfx/render_text.cc b/ui/gfx/render_text.cc
index 27c9e5ada81279d215723897c8cafe6b79820023..8507d34b547a035e3a3677da91bdaf2dc17ca48b 100644
--- a/ui/gfx/render_text.cc
+++ b/ui/gfx/render_text.cc
@@ -95,12 +95,12 @@ SkTypeface::Style ConvertFontStyleToSkiaTypefaceStyle(int font_style) {
 
 // Given |font| and |display_width|, returns the width of the fade gradient.
 int CalculateFadeGradientWidth(const FontList& font_list, int display_width) {
-  // Fade in/out about 2.5 characters of the beginning/end of the string.
+  // Fade in/out about 3 characters of the beginning/end of the string.
   // The .5 here is helpful if one of the characters is a space.
-  // Use a quarter of the display width if the display width is very short.
+  // Use a 1/3 of the display width if the display width is very short.
   const int average_character_width = font_list.GetExpectedTextWidth(1);

[Peter Kasting, 2015/12/03 05:29:25]

While here: Instead of getting the width of 1 char and multiplying by 3, get the
width of 3 chars.  This returns the same value save for being slightly less
prone to rounding error.

Also use this where you want 4 char widths below.

[Tomasz Moniuszko, 2015/12/03 11:18:42]

Done.

-  const double gradient_width = std::min(average_character_width * 2.5,
-                                         display_width / 4.0);
+  const double gradient_width =
+      std::min(average_character_width * 3.0, display_width / 3.0);
   DCHECK_GE(gradient_width, 0.0);
   return static_cast<int>(floor(gradient_width + 0.5));

[Peter Kasting, 2015/12/03 05:29:25]

Nit: For now, define a round() helper in this file and call that here and below
(see my comment there); I've separately sent a proposal to the C++11 arbiters to
allow std::round() from C++11.

[Tomasz Moniuszko, 2015/12/03 11:18:42]

Done.

 }
@@ -130,12 +130,21 @@ void AddFadeEffect(const Rect& text_rect,
 
 // Creates a SkShader to fade the text, with |left_part| specifying the left
 // fade effect, if any, and |right_part| specifying the right fade effect.
-skia::RefPtr<SkShader> CreateFadeShader(const Rect& text_rect,
+skia::RefPtr<SkShader> CreateFadeShader(const FontList& font_list,
+                                        const Rect& text_rect,
                                         const Rect& left_part,
                                         const Rect& right_part,
                                         SkColor color) {
-  // Fade alpha of 51/255 corresponds to a fade of 0.2 of the original color.
-  const SkColor fade_color = SkColorSetA(color, 51);
+  // Use 0 fade target alpha. But if text is narrow (less than 4 average
+  // characters) linearly increase fade target alpha up to 51/255 what
+  // corresponds to a fade of 0.2 of the original color.
+  const int average_character_width = font_list.GetExpectedTextWidth(1);
+  int alpha =
+      text_rect.width() > 4 * average_character_width
+          ? 0
+          : static_cast<int>(
+                51 * (1 - text_rect.width() / (4.0 * average_character_width)));
+  const SkColor fade_color = SkColorSetA(color, alpha);

[Peter Kasting, 2015/12/03 05:29:25]

How about this:

  // In general, fade down to 0 alpha.  But when the available width is less
  // than four characters, linearly ramp up the fade target alpha to as high as
  // 20% at zero width.  This allows the user to see the last faded characters a
  // little better when there are only a few characters shown.
  const float width_fraction =
      text_rect.width() / static_cast<float>(font_list.GetExpectedTextWidth(4));
  const SkAlpha kAlphaAtZeroWidth = 51;
  const SkAlpha alpha = (width_fraction < 1) ?
      static_cast<SkAlpha>(round((1 - width_fraction) * kAlphaAtZeroWidth)) : 0;
  const SkColor fade_color = SkColorSetA(color, alpha);

[Tomasz Moniuszko, 2015/12/03 11:18:42]

Done.

   std::vector<SkScalar> positions;
   std::vector<SkColor> colors;
 
@@ -1209,8 +1218,9 @@ void RenderText::ApplyFadeEffects(internal::SkiaTextRenderer* renderer) {
   text_rect.Inset(GetAlignmentOffset(0).x(), 0, 0, 0);
 
   // TODO(msw): Use the actual text colors corresponding to each faded part.
-  skia::RefPtr<SkShader> shader = CreateFadeShader(
-      text_rect, left_part, right_part, colors_.breaks().front().second);
+  skia::RefPtr<SkShader> shader =
+      CreateFadeShader(font_list(), text_rect, left_part, right_part,
+                       colors_.breaks().front().second);
   if (shader)
     renderer->SetShader(shader.get());
 }