Add support for alpha channel in colors

This makes it easier to define faces that lighten/darken whatever
they apply on.

doc/pages/faces.asciidoc

@@ -6,9 +6,9 @@ A 'face' refers how the specified text is displayed, it has a foreground
 color, a background color, and some attributes. The value of a face has the
 following format:
 
----------------------------------------
+-----------------------------------------
 [fg_color][,bg_color][+attributes][@base]
----------------------------------------
+-----------------------------------------
 
 'fg_color', 'bg_color'::
     a color whose value can be expressed in the following formats:
@@ -20,7 +20,11 @@ following format:
             keep the existing color
         *rgb:RRGGBB*:::
             hexadecimal value
+        *rgba:RRGGBBAA*:::
+            hexadecimal value
     if unspecified, *default* is used.
+    Alpha values are used to blend the face onto either its base or the existing
+    color. For technical reasons alpha values must be >16 if specified.
 
 'attributes'::
     string whose individual letters set an attribute:

src/color.cc

@@ -34,6 +34,13 @@ bool is_color_name(StringView color)
     return contains(color_names, color);
 }
 
+void Color::validate_alpha()
+{
+    static_assert(RGB == 17);
+    if (a < RGB)
+        throw runtime_error("Colors alpha must be > 16");
+}
+
 Color str_to_color(StringView color)
 {
     auto it = find_if(color_names, [&](const char* c){ return color == c; });
@@ -55,6 +62,11 @@ Color str_to_color(StringView color)
         return { (unsigned char)(hval(color[4]) * 16 + hval(color[5])),
                  (unsigned char)(hval(color[6]) * 16 + hval(color[7])),
                  (unsigned char)(hval(color[8]) * 16 + hval(color[9])) };
+    if (color.length() == 13 and color.substr(0_byte, 5_byte) == "rgba:")
+        return { (unsigned char)(hval(color[5])  * 16 + hval(color[6])),
+                 (unsigned char)(hval(color[7])  * 16 + hval(color[8])),
+                 (unsigned char)(hval(color[9])  * 16 + hval(color[10])),
+                 (unsigned char)(hval(color[11]) * 16 + hval(color[12])) };
 
     throw runtime_error(format("unable to parse color: '{}'", color));
     return Color::Default;
@@ -62,10 +74,13 @@ Color str_to_color(StringView color)
 
 String to_string(Color color)
 {
-    if (color.color == Color::RGB)
+    if (color.isRGB())
     {
-        char buffer[11];
+        char buffer[14];
+        if (color.a == 255)
             sprintf(buffer, "rgb:%02x%02x%02x", color.r, color.g, color.b);
+        else
+            sprintf(buffer, "rgba:%02x%02x%02x%02x", color.r, color.g, color.b, color.a);
         return buffer;
     }
     else

src/color.hh

@@ -3,6 +3,7 @@
 
 #include "hash.hh"
 #include "meta.hh"
+#include "assert.hh"
 
 namespace Kakoune
 {
@@ -12,7 +13,7 @@ class StringView;
 
 struct Color
 {
-    enum NamedColor : char
+    enum NamedColor : unsigned char
     {
         Default,
         Black,
@@ -34,15 +35,27 @@ struct Color
         RGB,
     };
 
+    union
+    {
         NamedColor color;
+        unsigned char a;
+    };
     unsigned char r = 0;
     unsigned char g = 0;
     unsigned char b = 0;
 
+    constexpr bool isRGB() const { return a >= RGB; }
+
     constexpr Color() : Color{Default} {}
     constexpr Color(NamedColor c) : color{c} {}
-    constexpr Color(unsigned char r, unsigned char g, unsigned char b)
-        : color{RGB}, r{r}, g{g}, b{b} {}
+    constexpr Color(unsigned char r, unsigned char g, unsigned char b, unsigned char a = 255)
+        : a{a}, r{r}, g{g}, b{b}
+    {
+        validate_alpha();
+    }
+
+private:
+    void validate_alpha();
 };
 
 constexpr bool operator==(Color lhs, Color rhs)
@@ -66,8 +79,8 @@ bool is_color_name(StringView color);
 
 constexpr size_t hash_value(const Color& val)
 {
-    return val.color == Color::RGB ?
-        hash_values(val.color, val.r, val.g, val.b)
+    return val.isRGB() ?
+        hash_values(val.a, val.r, val.g, val.b)
       : hash_value(val.color);
 }
 

src/commands.cc

@@ -2328,7 +2328,7 @@ const CommandDesc set_face_cmd = {
     "set-face <scope> <name> <facespec>: set face <name> to refer to <facespec> in <scope>\n"
     "\n"
     "facespec format is <fg color>[,<bg color>][+<attributes>][@<base>]\n"
-    "colors are either a color name, or rgb:###### values.\n"
+    "colors are either a color name, rgb:######, or rgba:######## values.\n"
     "attributes is a combination of:\n"
     "    u: underline, i: italic, b: bold, r: reverse,\n"
     "    B: blink, d: dim,\n"

src/face.hh

@@ -50,6 +50,14 @@ struct Face
 
 inline Face merge_faces(const Face& base, const Face& face)
 {
+    auto alpha_blend = [](Color base, Color color) {
+        auto blend = [&](unsigned char Color::*field) {
+            int blended = (base.*field * (255 - color.a) + color.*field * color.a) / 255;
+            return static_cast<unsigned char>(blended <= 255 ? blended : 255);
+        };
+        return Color{blend(&Color::r), blend(&Color::g), blend(&Color::b), base.a};
+    };
+
     auto choose = [&](Color Face::*color, Attribute final_attr) {
         if (face.attributes & final_attr)
             return face.*color;
@@ -57,6 +65,8 @@ inline Face merge_faces(const Face& base, const Face& face)
             return base.*color;
         if (face.*color == Color::Default)
             return base.*color;
+        if ((base.*color).isRGB() and (face.*color).isRGB() and (face.*color).a != 255)
+            return alpha_blend(base.*color, face.*color);
         return face.*color;
     };
 

src/ncurses_ui.cc

@@ -252,7 +252,7 @@ int NCursesUI::Palette::get_color(Color color)
         return it->value;
     else if (m_change_colors and can_change_color() and COLORS > 16)
     {
-        kak_assert(color.color == Color::RGB);
+        kak_assert(color.isRGB());
         if (m_next_color > COLORS)
             m_next_color = 16;
         init_color(m_next_color,
@@ -264,7 +264,7 @@ int NCursesUI::Palette::get_color(Color color)
     }
     else
     {
-        kak_assert(color.color == Color::RGB);
+        kak_assert(color.isRGB());
         int lowestDist = INT_MAX;
         int closestCol = -1;
         for (int i = 0; i < std::min(256, COLORS); ++i)

src/remote.cc

@@ -127,7 +127,7 @@ private:
     void write_field(Color color)
     {
         write_field(color.color);
-        if (color.color == Color::RGB)
+        if (color.isRGB())
         {
             write_field(color.r);
             write_field(color.g);
@@ -305,7 +305,7 @@ struct MsgReader::Reader<Color> {
     {
         Color res;
         res.color = Reader<Color::NamedColor>::read(reader);
-        if (res.color == Color::RGB)
+        if (res.isRGB())
         {
             res.r = Reader<unsigned char>::read(reader);
             res.g = Reader<unsigned char>::read(reader);