Posterize to 8 colors when outer terminal does not support 256 colors

Fixes #58 github issue.

src/terminal/terminaldisplay.h

@@ -54,9 +54,13 @@ namespace Terminal {
 
     bool has_bce; /* erases result in cell filled with background color */
 
+    int posterize_colors; /* downsample input colors >8 to [0..7] */
+
     void put_cell( bool initialized, FrameState &frame, const Framebuffer &f ) const;
 
   public:
+    void downgrade( Framebuffer &f ) const { if ( posterize_colors ) { f.posterize(); } }
+
     std::string new_frame( bool initialized, const Framebuffer &last, const Framebuffer &f ) const;
 
     Display( bool use_environment );

src/terminal/terminaldisplayinit.cc

@@ -29,7 +29,7 @@
 using namespace Terminal;
 
 Display::Display( bool use_environment )
-  : has_ech( true ), has_bce( true )
+  : has_ech( true ), has_bce( true ), posterize_colors( false )
 {
   if ( use_environment ) {
     int errret = -2;
@@ -57,8 +57,7 @@ Display::Display( bool use_environment )
     char *val = tigetstr( ech_name );
     if ( val == (char *)-1 ) {
       throw std::string( "Invalid terminfo string capability " ) + ech_name;
-    }
-    if ( val == 0 ) {
+    } else if ( val == 0 ) {
       has_ech = false;
     }
 
@@ -67,9 +66,17 @@ Display::Display( bool use_environment )
     int bce_val = tigetflag( bce_name );
     if ( bce_val == -1 ) {
       throw std::string( "Invalid terminfo boolean capability " ) + bce_name;
-    }
-    if ( bce_val == 0 ) {
+    } else if ( bce_val == 0 ) {
       has_bce = false;
     }
+
+    /* check colors */
+    char colors_name[] = "colors";
+    int color_val = tigetnum( colors_name );
+    if ( color_val == -2 ) {
+      throw std::string( "Invalid terminfo numeric capability " ) + colors_name;
+    } else if ( color_val < 256 ) {
+      posterize_colors = true;
+    }
   }
 }

src/terminal/terminalframebuffer.cc

@@ -320,6 +320,15 @@ void Framebuffer::soft_reset( void )
   ds.clear_saved_cursor();
 }
 
+void Framebuffer::posterize( void )
+{
+  for ( BOOST_AUTO( i, rows.begin() ); i != rows.end(); i++ ) {
+    for ( BOOST_AUTO( j, i->cells.begin() ); j != i->cells.end(); j++ ) {
+      j->renditions.posterize();
+    }
+  }
+}
+
 void Framebuffer::resize( int s_width, int s_height )
 {
   assert( s_width > 0 );
@@ -453,6 +462,64 @@ std::string Renditions::sgr( void ) const
   return ret;
 }
 
+/* Reduce 256 "standard" colors to the 8 ANSI colors. */
+
+/* We could do something fancy like find the nearest system color in
+   the deltaE(2000) sense, but for "business graphics," the colorimetric
+   intent is probably less important than just having separate colors
+   be separate as much as possible. */
+
+/* Terminal emulators generally agree on the (R',G',B') values of the
+   "standard" 256-color pallette beyond #15, but for the first 16
+   colors there is disagreement. Most terminal emulators are roughly
+   self-consistent, except on Ubuntu's gnome-terminal where "ANSI
+   blue" (#4) has been replaced with the aubergine system-wide
+   color. See
+   https://lists.ubuntu.com/archives/ubuntu-devel/2011-March/032726.html
+
+   For purposes of this routine, we just treat the eight colors as
+   taking each possible assignment of 0 or 255 to their components.
+
+   Terminal emulators that advertise "xterm" are inconsistent on the
+   handling of initc to change the contents of a cell in the color
+   pallette. On RIS (reset to initial state) or choosing reset from
+   the user interface, xterm resets all entries, but gnome-terminal
+   only resets entries beyond 16. (rxvt doesn't reset any entries,
+   and Terminal.app ignores initc.) On initc, xterm applies changes
+   immediately (but slowly), but gnome-terminal's changes are only
+   prospective unless the user resizes the terminal.
+
+   mosh ignores initc for now, despite advertising xterm-256color. */
+
+static const char standard_posterization[] = {
+  0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7,
+  0, 0, 4, 4, 4, 4, 0, 0, 4, 4, 4, 4, 2, 2, 6, 6,
+  6, 6, 2, 2, 6, 6, 6, 6, 2, 2, 6, 6, 6, 6, 2, 2,
+  6, 6, 6, 6, 0, 0, 4, 4, 4, 4, 0, 0, 4, 4, 4, 4,
+  2, 2, 6, 6, 6, 6, 2, 2, 6, 6, 6, 6, 2, 2, 6, 6,
+  6, 6, 2, 2, 6, 6, 6, 6, 1, 1, 5, 5, 5, 5, 1, 1,
+  5, 5, 5, 5, 3, 3, 7, 7, 7, 7, 3, 3, 7, 7, 7, 7,
+  3, 3, 7, 7, 7, 7, 3, 3, 7, 7, 7, 7, 1, 1, 5, 5,
+  5, 5, 1, 1, 5, 5, 5, 5, 3, 3, 7, 7, 7, 7, 3, 3,
+  7, 7, 7, 7, 3, 3, 7, 7, 7, 7, 3, 3, 7, 7, 7, 7,
+  1, 1, 5, 5, 5, 5, 1, 1, 5, 5, 5, 5, 3, 3, 7, 7,
+  7, 7, 3, 3, 7, 7, 7, 7, 3, 3, 7, 7, 7, 7, 3, 3,
+  7, 7, 7, 7, 1, 1, 5, 5, 5, 5, 1, 1, 5, 5, 5, 5,
+  3, 3, 7, 7, 7, 7, 3, 3, 7, 7, 7, 7, 3, 3, 7, 7,
+  7, 7, 3, 3, 7, 7, 7, 7, 0, 0, 0, 0, 0, 0, 0, 0,
+  0, 0, 0, 0, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7 };
+
+void Renditions::posterize( void )
+{
+  if ( foreground_color ) {
+    foreground_color = 30 + standard_posterization[ foreground_color - 30 ];
+  }
+
+  if ( background_color ) {
+    background_color = 40 + standard_posterization[ background_color - 40 ];
+  }
+}
+
 void Row::reset( int background_color )
 {
   for ( std::vector<Cell>::iterator i = cells.begin();

src/terminal/terminalframebuffer.h

@@ -40,6 +40,8 @@ namespace Terminal {
     void set_rendition( int num );
     std::string sgr( void ) const;
 
+    void posterize( void );
+
     bool operator==( const Renditions &x ) const
     {
       return (bold == x.bold) && (underlined == x.underlined)
@@ -285,6 +287,8 @@ namespace Terminal {
     void reset_cell( Cell *c ) { c->reset( ds.get_background_rendition() ); }
     void reset_row( Row *r ) { r->reset( ds.get_background_rendition() ); }
 
+    void posterize( void );
+
     void ring_bell( void ) { bell_count++; }
     unsigned int get_bell_count( void ) const { return bell_count; }