lib/tty: Add compatibility translation from 256 colors to true colors

If the terminal does not support 256 colors but does support true
color, allow the use of 256-color skins by adding a translation layer
that computes direct color values from 256-color palette.

Signed-off-by: Jakub Horký <[email protected]>

Author: Jakub Horký

doc/FAQ

@@ -432,12 +432,12 @@ Frequently Asked Questions
 
    If your terminal supports 256 colors, the correct entry name may be
    appended with -256color. If your terminal supports true color, it
-   should be appended with -direct, -direct16, or -direct256. The best
-   backwards compatibility option is -direct256 variant, which provides
-   both 256-color and true color support simultaneously. The -direct16
-   variant supports bright versions of the basic colors along with true
-   color support, in contrast to the -direct variant, which supports
-   only basic 8 colors together with true color when used in a skin.
+   should be appended with -direct, -direct16, or -direct256. All three
+   variants provide both 256-color and true color support simultaneously.
+   The latter two variants make no difference in mc, and they support
+   basic colors including their bright versions along with true color
+   support, in contrast to the -direct variant, which supports only basic
+   8 colors together with true color when used in a skin.
 
    If there is no 256-color or true-color terminfo variant for your
    terminal, even though your terminal supports it, send an e-mail to

lib/tty/color-internal.c

@@ -241,3 +241,37 @@ tty_attr_get_bits (const char *attrs)
 }
 
 /* --------------------------------------------------------------------------------------------- */
+
+int
+convert_256color_to_truecolor (int color)
+{
+    int r, g, b;
+
+    // Invalid color
+    if (color > 255)
+        return 0;
+
+    if (color >= 232)  // Gray scale
+        r = g = b = (color - 231) * 10 + 8;
+    else if (color >= 16)  // 6x6x6 color cube
+    {
+        color -= 16;
+
+        r = (color / (6 * 6) % 6);
+        r = r > 0 ? r * 40 + 55 : 0;
+
+        g = (color / 6 % 6);
+        g = g > 0 ? g * 40 + 55 : 0;
+
+        b = (color % 6);
+        b = b > 0 ? b * 40 + 55 : 0;
+    }
+    else  // We don't convert basic 16 colors as they are terminal-dependent and user-configurable
+        return color;
+
+    color = FLAG_TRUECOLOR | (r << 16) | (g << 8) | b;
+
+    return color;
+}
+
+/* --------------------------------------------------------------------------------------------- */

lib/tty/color-internal.h

@@ -47,12 +47,14 @@ typedef struct
 
 extern gboolean use_colors;
 extern gboolean mc_tty_color_disable;
+extern gboolean need_convert_256color;
 
 /*** declarations of public functions ************************************************************/
 
 const char *tty_color_get_name_by_index (int idx);
 int tty_color_get_index_by_name (const char *color_name);
 int tty_attr_get_bits (const char *attrs);
+int convert_256color_to_truecolor (int color);
 
 void tty_color_init_lib (gboolean disable, gboolean force);
 void tty_color_deinit_lib (void);

lib/tty/color-ncurses.c

@@ -129,6 +129,10 @@ tty_color_init_lib (gboolean disable, gboolean force)
         use_colors = TRUE;
         start_color ();
         use_default_colors ();
+
+        // Extended color mode detection routines must first be called before loading any skin
+        tty_use_256colors (NULL);
+        tty_use_truecolors (NULL);
     }
 
     mc_tty_color_color_pair_attrs = g_hash_table_new_full (
@@ -256,17 +260,21 @@ tty_use_256colors (GError **error)
 
     overlay_colors = tty_tigetnum ("CO", NULL);
 
-    if (COLORS != 256 && !(COLORS > 256 && overlay_colors == 256))
+    if (COLORS == 256 || (COLORS > 256 && overlay_colors == 256))
+        return TRUE;
+
+    if (tty_use_truecolors (NULL))
     {
-        g_set_error (error, MC_ERROR, -1,
-                     _ ("\nIf your terminal supports 256 colors, you need to set your TERM\n"
-                        "environment variable to match your terminal, perhaps using\n"
-                        "a *-256color or *-direct256 variant. Use the 'toe -a'\n"
-                        "command to list all available variants on your system.\n"));
-        return FALSE;
+        need_convert_256color = TRUE;
+        return TRUE;
     }
 
-    return TRUE;
+    g_set_error (error, MC_ERROR, -1,
+                 _ ("\nIf your terminal supports 256 colors, you need to set your TERM\n"
+                    "environment variable to match your terminal, perhaps using\n"
+                    "a *-256color or *-direct256 variant. Use the 'toe -a'\n"
+                    "command to list all available variants on your system.\n"));
+    return FALSE;
 }
 
 /* --------------------------------------------------------------------------------------------- */

lib/tty/color-slang.c

@@ -136,6 +136,10 @@ tty_color_init_lib (gboolean disable, gboolean force)
     if (has_colors (disable, force) && !disable)
     {
         use_colors = TRUE;
+
+        // Extended color mode detection routines must first be called before loading any skin
+        tty_use_256colors (NULL);
+        tty_use_truecolors (NULL);
     }
 }
 
@@ -218,22 +222,27 @@ tty_set_normal_attrs (void)
 gboolean
 tty_use_256colors (GError **error)
 {
-    gboolean ret;
     int colors, overlay_colors;
 
     colors = tty_tigetnum ("colors", "Co");
     overlay_colors = tty_tigetnum ("CO", NULL);
 
-    ret = (SLtt_Use_Ansi_Colors && (colors == 256 || (colors > 256 && overlay_colors == 256)));
+    if (SLtt_Use_Ansi_Colors && (colors == 256 || (colors > 256 && overlay_colors == 256)))
+        return TRUE;
 
-    if (!ret)
-        g_set_error (error, MC_ERROR, -1,
-                     _ ("\nIf your terminal supports 256 colors, you need to set your TERM\n"
-                        "environment variable to match your terminal, perhaps using\n"
-                        "a *-256color or *-direct256 variant. Use the 'toe -a'\n"
-                        "command to list all available variants on your system.\n"));
+    if (tty_use_truecolors (NULL))
+    {
+        need_convert_256color = TRUE;
+        return TRUE;
+    }
+
+    g_set_error (error, MC_ERROR, -1,
+                 _ ("\nIf your terminal supports 256 colors, you need to set your TERM\n"
+                    "environment variable to match your terminal, perhaps using\n"
+                    "a *-256color or *-direct256 variant. Use the 'toe -a'\n"
+                    "command to list all available variants on your system.\n"));
 
-    return ret;
+    return FALSE;
 }
 
 /* --------------------------------------------------------------------------------------------- */

lib/tty/color.c

@@ -53,6 +53,8 @@ static tty_color_pair_t tty_color_defaults = {
 /* Set if we are actually using colors */
 gboolean use_colors = FALSE;
 
+gboolean need_convert_256color = FALSE;
+
 /*** file scope macro definitions ****************************************************************/
 
 /*** file scope type declarations ****************************************************************/
@@ -175,6 +177,15 @@ tty_try_alloc_color_pair (const tty_color_pair_t *color, gboolean is_temp)
         return mc_color_pair->pair_index;
     }
 
+    if (need_convert_256color)
+    {
+        if ((ifg & FLAG_TRUECOLOR) == 0)
+            ifg = convert_256color_to_truecolor (ifg);
+
+        if ((ibg & FLAG_TRUECOLOR) == 0)
+            ibg = convert_256color_to_truecolor (ibg);
+    }
+
     mc_color_pair = g_try_new0 (tty_color_lib_pair_t, 1);
     if (mc_color_pair == NULL)
     {

misc/skins/seasons-spring16M.ini

@@ -5,8 +5,8 @@
 # True colors are referred to by the standard #rrggbb or #rgb notation, the
 # short form is interpreted by doubling each hex digit.
 #
-# In addition to these, you can use the basic 8 (non-light) colors and
-# the attributes. However, you can't use the 256 indexed colors.
+# In addition to these the 256 palette colors are still available, and
+# obviously so are the attributes. See the sand256.ini skin file for details.
 
 [skin]
     # Blooming.

misc/skins/seasons-summer16M.ini

@@ -5,8 +5,8 @@
 # True colors are referred to by the standard #rrggbb or #rgb notation, the
 # short form is interpreted by doubling each hex digit.
 #
-# In addition to these, you can use the basic 8 (non-light) colors and
-# the attributes. However, you can't use the 256 indexed colors.
+# In addition to these the 256 palette colors are still available, and
+# obviously so are the attributes. See the sand256.ini skin file for details.
 
 [skin]
     # Sunshine, ocean, beach, beachball, sunshades, ice cream.

misc/skins/seasons-winter16M.ini

@@ -5,8 +5,8 @@
 # True colors are referred to by the standard #rrggbb or #rgb notation, the
 # short form is interpreted by doubling each hex digit.
 #
-# In addition to these, you can use the basic 8 (non-light) colors and
-# the attributes. However, you can't use the 256 indexed colors.
+# In addition to these the 256 palette colors are still available, and
+# obviously so are the attributes. See the sand256.ini skin file for details.
 
 [skin]
     # The sun is shining and the grass is green,