diff --git a/src/buttons_effects.c b/src/buttons_effects.c
index 4ac4a88c..77aac882 100644
--- a/src/buttons_effects.c
+++ b/src/buttons_effects.c
@@ -352,6 +352,68 @@ static void Convert_to_hgr(void)
   }
 }
 
+/// convert a picture to the DHGR mode
+///
+/// Recognize monochrome pictures.
+static void Convert_to_dhgr(void)
+{
+  int i;
+  word count, x, y;
+  dword usage[256];
+
+  count = Count_used_colors(usage);
+  if (count <= 1) // blank picture, nothing to do :)
+    return;
+  if (count == 2) // monochrome !
+  {
+    byte bg, fg;
+    i = 0;
+    while (usage[i] == 0 && i < 256)
+      i++;
+    bg = (byte)i;
+    i++;
+    while (usage[i] == 0 && i < 256)
+      i++;
+    fg = (byte)i;
+    GFX2_Log(GFX2_DEBUG, "Convert_to_dhgr() monochrome bg=%u fg=%u\n", bg, fg);
+    if (!(bg == 0 && fg == 3) && !(bg == 4 && fg == 7))
+    {
+      // convert to B&W
+      for (y = 0; y < Main.image_height; y++)
+      {
+        for (x = 0; x < Main.image_width; x++)
+        {
+          byte c = Read_pixel_from_layer(0, x, y);
+          Pixel_in_layer(0, x, y, (c == fg) ? 15 : 0);
+        }
+      }
+    }
+  }
+  else
+  {
+    // "convert" color picture to B&W
+    for (y = 0; y < Main.image_height; y++)
+    {
+      for (x = 0; x < Main.image_width; x += 4)
+      {
+        byte c = Read_pixel_from_layer(0, x, y);
+        Pixel_in_layer(0, x, y, c & 0x18);
+        Pixel_in_layer(0, x + 1, y, c & 0x14);
+        Pixel_in_layer(0, x + 2, y, c & 0x12);
+        Pixel_in_layer(0, x + 3, y, c & 0x11);
+      }
+    }
+  }
+  // update color layer
+  for (y = 0; y < Main.image_height; y++)
+  {
+    for (x = 0; x < Main.image_width; x += 4)
+    {
+      Update_color_dhgr_pixel(x, y, 0);
+    }
+  }
+}
+
 
 /// Constaint enforcer/checker
 ///
@@ -415,7 +477,8 @@ void Button_Constraint_mode(void)
       }
     if (Selected_Constraint_Mode == IMAGE_MODE_HGR)
       Convert_to_hgr();
-    /// @todo conversion to DHGR
+    else
+      Convert_to_dhgr();
     break;
   default:
     Check_block_constraints(Selected_Constraint_Mode);
@@ -454,7 +517,7 @@ void Button_Constraint_menu(void)
     {IMAGE_MODE_C64MULTI,"C64 Multicolor","4 colors per 4x1 block", 1}, // 160x200
     //{IMAGE_MODE_C64FLI,  "C64 FLI",       "improved multicolor   ", 1}, // 160x200
     {IMAGE_MODE_HGR,     "Apple II HGR",  "6 colors              ", 1},  // 280x192
-    //{IMAGE_MODE_DHGR,    "Apple II DHGR", "16 colors             ", 1},  // 560x192
+    {IMAGE_MODE_DHGR,    "Apple II DHGR", "\"Le Chat Mauve\" mode3 ", 1},  // 560x192
   };
 
   Open_window(194,95+36,"8-bit constraints");
@@ -607,10 +670,13 @@ void Button_Constraint_menu(void)
             Snap_height = 999;  // maximum value (3 digits)
             break;
           case IMAGE_MODE_HGR:
-          case IMAGE_MODE_DHGR:
             Snap_width = 7;
             Snap_height = 999;  // maximum value (3 digits)
             break;
+          case IMAGE_MODE_DHGR:
+            Snap_width = 4;
+            Snap_height = 999;  // maximum value (3 digits)
+            break;
           default:
             set_grid = 0;
         }
diff --git a/src/graph.c b/src/graph.c
index 8508fd56..ee350c4b 100644
--- a/src/graph.c
+++ b/src/graph.c
@@ -3668,6 +3668,100 @@ static void Pixel_in_screen_hgr_color_with_opt_preview(word x,word y,byte color,
   }
 }
 
+/// Update the color layer of DHGR according to the monochrome layer
+///
+/// Emulate \"Le Chat Mauve\" mode 3 (mixed mode).
+void Update_color_dhgr_pixel(word x, word y, int preview)
+{
+  byte b3, b2, b1, b0, color;
+
+  x &= ~3;
+  // read monochrome pixels
+  b3 = Read_pixel_from_layer(0, x, y);
+  b2 = Read_pixel_from_layer(0, x + 1, y);
+  b1 = Read_pixel_from_layer(0, x + 2, y);
+  b0 = Read_pixel_from_layer(0, x + 3, y);
+  if (b3 & 16)
+  {
+    // monochrome pixel
+    Pixel_in_layer_with_opt_preview(1, x, y, b3, preview);
+    Pixel_in_layer_with_opt_preview(1, x + 1, y, b2, preview);
+    Pixel_in_layer_with_opt_preview(1, x + 2, y, b1, preview);
+    Pixel_in_layer_with_opt_preview(1, x + 3, y, b0, preview);
+  }
+  else
+  {
+    // color pixel
+    color = (b3 & 8) | (b2 & 4) | (b1 & 2) | (b0 & 1);
+    Pixel_in_layer_with_opt_preview(1, x, y, color, preview);
+    Pixel_in_layer_with_opt_preview(1, x + 1, y, color, preview);
+    Pixel_in_layer_with_opt_preview(1, x + 2, y, color, preview);
+    Pixel_in_layer_with_opt_preview(1, x + 3, y, color, preview);
+  }
+}
+
+
+/// Paint in the monochrome layer of DHGR
+///
+/// also update the color pixels.
+static void Pixel_in_screen_dhgr_mono_with_opt_preview(word x,word y,byte color,int preview)
+{
+  byte oldcolor;
+
+  if (color >= 32)
+    return;
+  if ((color & 15) != 0)
+    color |= 15; // force black or white.
+
+  // put pixel
+  oldcolor = Read_pixel_from_layer(0, x, y);
+  if (color == oldcolor)
+    return; // nothing to do !
+  Pixel_in_layer_with_opt_preview(0, x, y, color, preview);
+  Update_color_dhgr_pixel(x, y, preview);
+
+  // change bit7 if needed.
+  if ((color & 16) != (oldcolor & 16))
+  {
+    int i;
+    x -= (x % 7);
+    for (i = 0; i < 7; i++)
+    {
+      oldcolor = Read_pixel_from_layer(0, x, y);
+      if ((oldcolor & 16) != (color & 16))
+      {
+        Pixel_in_layer_with_opt_preview(0, x, y, (color & 16) | (oldcolor & 15), preview);
+        Update_color_dhgr_pixel(x, y, preview);
+      }
+      x++;
+    }
+  }
+}
+
+/// Paint in the color layer of DHGR
+///
+/// use of color 16-31 forces the cell to be monochrome.
+static void Pixel_in_screen_dhgr_color_with_opt_preview(word x,word y,byte color,int preview)
+{
+  if (color & 16)
+  {
+    // monochrome pixel
+    Pixel_in_screen_dhgr_mono_with_opt_preview(x, y, color, preview);
+    // force monochrome for this cell
+    if ((x & 3) != 0)
+      Pixel_in_screen_dhgr_mono_with_opt_preview(x & ~3, y, Read_pixel_from_layer(0, x & ~3, y) | 16, preview);
+  }
+  else
+  {
+    // color pixel
+    x &= ~3;
+    Pixel_in_screen_dhgr_mono_with_opt_preview(x, y, color & 8, preview); // also set this cell in color mode
+    Pixel_in_screen_dhgr_mono_with_opt_preview(x + 1, y, color & 4, preview);
+    Pixel_in_screen_dhgr_mono_with_opt_preview(x + 2, y, color & 2, preview);
+    Pixel_in_screen_dhgr_mono_with_opt_preview(x + 3, y, color & 1, preview);
+  }
+}
+
 // end of constraints group
 /// @}
 
@@ -3723,7 +3817,6 @@ void Update_pixel_renderer(void)
     // direct
     Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_direct_with_opt_preview;
     break;
-  case IMAGE_MODE_DHGR: // TODO
   case IMAGE_MODE_LAYERED:
     // layered
     Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_layered_with_opt_preview;
@@ -3763,5 +3856,12 @@ void Update_pixel_renderer(void)
     else
       Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_layered_with_opt_preview;
     break;
+  case IMAGE_MODE_DHGR:
+    if (Main.current_layer == 0)  // monochrome layer
+      Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_dhgr_mono_with_opt_preview;
+    else if (Main.current_layer == 1)  // color layer
+      Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_dhgr_color_with_opt_preview;
+    else
+      Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_layered_with_opt_preview;
   }
 }
diff --git a/src/graph.h b/src/graph.h
index ebbf597b..7513ec14 100644
--- a/src/graph.h
+++ b/src/graph.h
@@ -154,5 +154,6 @@ extern Func_pixel_opt_preview Pixel_in_current_screen_with_opt_preview;
 void Update_pixel_renderer(void);
 
 void Update_color_hgr_pixel(word x, word y, int preview);
+void Update_color_dhgr_pixel(word x, word y, int preview);
 
 #endif