Be consistent about how the algorithm we use to modulate colors.

webrender/res/cs_border_segment.glsl

@@ -102,22 +102,41 @@ vec2 get_outer_corner_scale(int segment) {
     return p;
 }
 
-vec4 mod_color(vec4 color, float f) {
-    return vec4(clamp(color.rgb * f, vec3(0.0), vec3(color.a)), color.a);
+// NOTE(emilio): If you change this algorithm, do the same change
+// in border.rs
+vec4 mod_color(vec4 color, bool is_black, bool lighter) {
+    const float light_black = 0.7;
+    const float dark_black = 0.3;
+
+    const float dark_scale = 0.66666666;
+    const float light_scale = 1.0;
+
+    if (is_black) {
+        if (lighter) {
+            return vec4(vec3(light_black), color.a);
+        }
+        return vec4(vec3(dark_black), color.a);
+    }
+
+    if (lighter) {
+        return vec4(color.rgb * light_scale, color.a);
+    }
+    return vec4(color.rgb * dark_scale, color.a);
 }
 
 vec4[2] get_colors_for_side(vec4 color, int style) {
     vec4 result[2];
-    const vec2 f = vec2(1.3, 0.7);
+
+    bool is_black = color.rgb == vec3(0.0, 0.0, 0.0);
 
     switch (style) {
         case BORDER_STYLE_GROOVE:
-            result[0] = mod_color(color, f.x);
-            result[1] = mod_color(color, f.y);
+            result[0] = mod_color(color, is_black, true);
+            result[1] = mod_color(color, is_black, false);
             break;
         case BORDER_STYLE_RIDGE:
-            result[0] = mod_color(color, f.y);
-            result[1] = mod_color(color, f.x);
+            result[0] = mod_color(color, is_black, false);
+            result[1] = mod_color(color, is_black, true);
             break;
         default:
             result[0] = color;

webrender/src/border.rs

@@ -201,40 +201,34 @@ impl<'a> DisplayListFlattener<'a> {
 }
 
 pub trait BorderSideHelpers {
-    fn border_color(
-        &self,
-        scale_factor_0: f32,
-        scale_factor_1: f32,
-        black_color_0: f32,
-        black_color_1: f32,
-    ) -> ColorF;
+    fn border_color(&self, is_inner_border: bool) -> ColorF;
 }
 
 impl BorderSideHelpers for BorderSide {
-    fn border_color(
-        &self,
-        scale_factor_0: f32,
-        scale_factor_1: f32,
-        black_color_0: f32,
-        black_color_1: f32,
-    ) -> ColorF {
-        match self.style {
-            BorderStyle::Inset => {
-                if self.color.r != 0.0 || self.color.g != 0.0 || self.color.b != 0.0 {
-                    self.color.scale_rgb(scale_factor_1)
-                } else {
-                    ColorF::new(black_color_0, black_color_0, black_color_0, self.color.a)
-                }
-            }
-            BorderStyle::Outset => {
-                if self.color.r != 0.0 || self.color.g != 0.0 || self.color.b != 0.0 {
-                    self.color.scale_rgb(scale_factor_0)
-                } else {
-                    ColorF::new(black_color_1, black_color_1, black_color_1, self.color.a)
-                }
-            }
-            _ => self.color,
+    fn border_color(&self, is_inner_border: bool) -> ColorF {
+        let lighter = match self.style {
+            BorderStyle::Inset => is_inner_border,
+            BorderStyle::Outset => !is_inner_border,
+            _ => return self.color,
+        };
+
+        // The modulate colors below are not part of the specification. They are
+        // derived from the Gecko source code and experimentation, and used to
+        // modulate the colors in order to generate colors for the inset/outset
+        // and groove/ridge border styles.
+        //
+        // NOTE(emilio): Gecko at least takes the background color into
+        // account, should we do the same? Looks a bit annoying for this.
+        //
+        // NOTE(emilio): If you change this algorithm, do the same change on
+        // get_colors_for_side in cs_border_segment.glsl.
+        if self.color.r != 0.0 || self.color.g != 0.0 || self.color.b != 0.0 {
+            let scale = if lighter { 1.0 } else { 2.0 / 3.0 };
+            return self.color.scale_rgb(scale)
         }
+
+        let black = if lighter { 0.7 } else { 0.3 };
+        ColorF::new(black, black, black, self.color.a)
     }
 }
 
@@ -912,21 +906,8 @@ impl BorderRenderTaskInfo {
                 side1.style
             };
 
-            // These modulate colors are not part of the specification. They
-            // are derived from the Gecko source code and experimentation, and
-            // used to modulate the colors in order to generate colors for
-            // the inset/outset and groove/ridge border styles.
-            let color0 = if flip0 {
-                side0.border_color(2.0 / 3.0, 1.0, 0.7, 0.3)
-            } else {
-                side0.border_color(1.0, 2.0 / 3.0, 0.3, 0.7)
-            };
-
-            let color1 = if flip1 {
-                side1.border_color(2.0 / 3.0, 1.0, 0.7, 0.3)
-            } else {
-                side1.border_color(1.0, 2.0 / 3.0, 0.3, 0.7)
-            };
+            let color0 = side0.border_color(flip0);
+            let color1 = side1.border_color(flip1);
 
             add_segment(
                 info.task_rect,

webrender/src/prim_store.rs

@@ -428,7 +428,10 @@ impl BrushKind {
 
     // Construct a brush that is a border with `border` style and `widths`
     // dimensions.
-    pub fn new_border(border: NormalBorder, widths: BorderWidths) -> BrushKind {
+    pub fn new_border(mut border: NormalBorder, widths: BorderWidths) -> BrushKind {
+        // FIXME(emilio): Is this the best place to do this?
+        border.normalize(&widths);
+
         let cache_key = BorderCacheKey::new(&border, &widths);
         BrushKind::Border {
             source: BorderSource::Border {

webrender_api/src/display_item.rs

@@ -273,6 +273,33 @@ impl NormalBorder {
         b.bottom.color = color;
         b
     }
+
+    /// Normalizes a border so that we don't render disallowed stuff, like inset
+    /// borders that are less than two pixels wide.
+    #[inline]
+    pub fn normalize(&mut self, widths: &BorderWidths) {
+        #[inline]
+        fn renders_small_border_solid(style: BorderStyle) -> bool {
+            match style {
+                BorderStyle::Groove |
+                BorderStyle::Ridge |
+                BorderStyle::Inset |
+                BorderStyle::Outset => true,
+                _ => false,
+            }
+        }
+
+        let normalize_side = |side: &mut BorderSide, width: f32| {
+            if renders_small_border_solid(side.style) && width < 2. {
+                side.style = BorderStyle::Solid;
+            }
+        };
+
+        normalize_side(&mut self.left, widths.left);
+        normalize_side(&mut self.right, widths.right);
+        normalize_side(&mut self.top, widths.top);
+        normalize_side(&mut self.bottom, widths.bottom);
+    }
 }
 
 #[repr(u32)]

wrench/reftests/border/border-groove-simple-ref.yaml

@@ -9,7 +9,7 @@ root:
           width: [ 6, 6, 6, 6 ]
           border-type: normal
           style: [ solid, solid, solid, solid ]
-          color: [ 0 0 178 1.0, 0 0 255 1.0, 0 0 255 1.0, 0 0 178 1.0 ]
+          color: [ 0 0 170 1.0, 0 0 255 1.0, 0 0 255 1.0, 0 0 170 1.0 ]
     - type: stacking-context
       bounds: [6, 6, 38, 38]
       items:
@@ -18,4 +18,4 @@ root:
           width: [ 6, 6, 6, 6 ]
           border-type: normal
           style: [ solid, solid, solid, solid ]
-          color: [ 0 0 255 1.0, 0 0 178 1.0, 0 0 178 1.0, 0 0 255 1.0 ]
+          color: [ 0 0 255 1.0, 0 0 170 1.0, 0 0 170 1.0, 0 0 255 1.0 ]

wrench/reftests/border/border-ridge-simple-ref.yaml

@@ -9,7 +9,7 @@ root:
           width: [ 6, 6, 6, 6 ]
           border-type: normal
           style: [ solid, solid, solid, solid ]
-          color: [ 0 0 255 1.0, 0 0 178 1.0, 0 0 178 1.0, 0 0 255 1.0 ]
+          color: [ 0 0 255 1.0, 0 0 170 1.0, 0 0 170 1.0, 0 0 255 1.0 ]
     - type: stacking-context
       bounds: [6, 6, 38, 38]
       items:
@@ -18,4 +18,4 @@ root:
           width: [ 6, 6, 6, 6 ]
           border-type: normal
           style: [ solid, solid, solid, solid ]
-          color: [ 0 0 178 1.0, 0 0 255 1.0, 0 0 255 1.0, 0 0 178 1.0 ]
+          color: [ 0 0 170 1.0, 0 0 255 1.0, 0 0 255 1.0, 0 0 170 1.0 ]