diff --git a/src/brush.c b/src/brush.c
index bf4c0d39..6ed6addf 100644
--- a/src/brush.c
+++ b/src/brush.c
@@ -130,7 +130,7 @@ void Display_paintbrush(short x,short y,byte color,byte is_preview)
   if (is_preview && Mouse_K) // pas de curseur si on est en preview et 
     return;                  // en train de cliquer
     
-  if (Main_current_layer < 4)
+  if (Constraint_mode && Main_current_layer < 4)
   {
     if (is_preview)
       goto single_pixel;
@@ -2046,4 +2046,4 @@ void Brush_set_palette(T_Palette *palette)
   }
   
 }
-*/
\ No newline at end of file
+*/
diff --git a/src/buttons.c b/src/buttons.c
index d483ea53..8d424254 100644
--- a/src/buttons.c
+++ b/src/buttons.c
@@ -4465,23 +4465,27 @@ void Display_effect_state(short x, short y, char * label, byte state)
 
   Print_in_window(x,y,label,(state)?MC_White:MC_Black,MC_Light);
   if (state)
-    Print_in_window(x+56,y,":ON ",MC_White,MC_Light);
+    Window_select_normal_button(x-23, y-5, 16, 16);
   else
-    Print_in_window(x+56,y,":OFF",MC_Black,MC_Light);
+    Window_unselect_normal_button(x-23, y-5, 16, 16);
 }
 
+#define C2 92
+
 void Display_effect_states(void)
 {
-  Display_effect_state( 30, 24,"Shade"  ,Shade_mode);
-  Display_effect_state( 30, 43,"Q-shade",Quick_shade_mode);
-  Display_effect_state( 30, 62,"Transp.",Colorize_mode);
-  Display_effect_state( 30, 81,"Smooth" ,Smooth_mode);
-  Display_effect_state( 30,100,"Smear"  ,Smear_mode);
-  Display_effect_state(176, 24,"Stencil",Stencil_mode);
-  Display_effect_state(176, 43,"Mask"   ,Mask_mode);
-  Display_effect_state(176, 62,"Sieve"  ,Sieve_mode);
-  Display_effect_state(176, 81,"Grid"   ,Snap_mode);
-  Display_effect_state(176,100,"Tiling" ,Tiling_mode);
+  Display_effect_state( 30, 24,   "Shade"  ,Shade_mode);
+  Display_effect_state( 30, 43,   "Q-shade",Quick_shade_mode);
+  Display_effect_state( 30, 62,   "Transp.",Colorize_mode);
+  Display_effect_state( 30, 81,   "Smooth" ,Smooth_mode);
+  Display_effect_state( 30,100,   "Smear"  ,Smear_mode);
+  Display_effect_state(C2+23, 24, "Stencil",Stencil_mode);
+  Display_effect_state(C2+23, 43, "Mask"   ,Mask_mode);
+  Display_effect_state(C2+23, 62, "Sieve"  ,Sieve_mode);
+  Display_effect_state(C2+23, 81, "Grid"   ,Snap_mode);
+  Display_effect_state(C2+23,100, "Tiling" ,Tiling_mode);
+
+  Display_effect_state(177+23,24, "8 bit" ,Constraint_mode);
 }
 
 
@@ -4503,25 +4507,29 @@ void Button_Effects(void)
   Window_set_normal_button(  7, 57, 16,16,"",0,1,Config_Key[SPECIAL_COLORIZE_MODE][0]); // 3
   Window_set_normal_button(  7, 76, 16,16,"",0,1,Config_Key[SPECIAL_SMOOTH_MODE][0]); // 4
   Window_set_normal_button(  7, 95, 16,16,"",0,1,Config_Key[SPECIAL_SMEAR_MODE][0]); // 5
-  Window_set_normal_button(153, 19, 16,16,"",0,1,Config_Key[SPECIAL_STENCIL_MODE][0]); // 6
-  Window_set_normal_button(153, 38, 16,16,"",0,1,Config_Key[SPECIAL_MASK_MODE][0]); // 7
-  Window_set_normal_button(153, 57, 16,16,"",0,1,Config_Key[SPECIAL_SIEVE_MODE][0]); // 8
-  Window_set_normal_button(153, 76, 16,16,"",0,1,Config_Key[SPECIAL_GRID_MODE][0]); // 9
-  Window_set_normal_button(153, 95, 16,16,"",0,1,Config_Key[SPECIAL_TILING_MODE][0]); // 10
+  Window_set_normal_button(C2, 19, 16,16,"",0,1,Config_Key[SPECIAL_STENCIL_MODE][0]); // 6
+  Window_set_normal_button(C2, 38, 16,16,"",0,1,Config_Key[SPECIAL_MASK_MODE][0]); // 7
+  Window_set_normal_button(C2, 57, 16,16,"",0,1,Config_Key[SPECIAL_SIEVE_MODE][0]); // 8
+  Window_set_normal_button(C2, 76, 16,16,"",0,1,Config_Key[SPECIAL_GRID_MODE][0]); // 9
+  Window_set_normal_button(C2, 95, 16,16,"",0,1,Config_Key[SPECIAL_TILING_MODE][0]); // 10
+
   Window_set_normal_button(195,131, 68,14,"Close",0,1,SDLK_RETURN); // 11
   Window_set_normal_button(  7,131, 68,14,"All off",0,1,SDLK_DELETE); // 12
   Window_set_normal_button( 83,131,104,14,"Feedback:   ",1,1,SDLK_f); // 13
+
+  Window_set_normal_button(177, 19, 16,16,"",0,1,Config_Key[SPECIAL_FORMAT_CHECKER_MENU][0]); // 14
+
   Display_feedback_state();
   Display_effect_sprite(0,  8,20);
   Display_effect_sprite(0,  8,39);
   Display_effect_sprite(1,  8,58);
   Display_effect_sprite(2,  8,77);
   Display_effect_sprite(8,  8,96);
-  Display_effect_sprite(4,154,20);
-  Display_effect_sprite(7,154,39);
-  Display_effect_sprite(5,154,58);
-  Display_effect_sprite(6,154,77);
-  Display_effect_sprite(3,154,96);
+  Display_effect_sprite(4,C2+1,20);
+  Display_effect_sprite(7,C2+1,39);
+  Display_effect_sprite(5,C2+1,58);
+  Display_effect_sprite(6,C2+1,77);
+  Display_effect_sprite(3,C2+1,96);
   Display_effect_states();
 
   Print_in_window(12,117,"click: Left:Switch / Right:Edit",MC_Dark,MC_Light);
@@ -4658,7 +4666,7 @@ void Button_Effects(void)
         {
           Button_Stencil_mode();
           Hide_cursor();
-          Display_effect_state(176,24,"Stencil",Stencil_mode);
+          Display_effect_state(C2+23,24,"Stencil",Stencil_mode);
           Display_cursor();
         }
         else
@@ -4674,7 +4682,7 @@ void Button_Effects(void)
         {
           Button_Mask_mode();
           Hide_cursor();
-          Display_effect_state(176,43,"Mask",Mask_mode);
+          Display_effect_state(C2+23,43,"Mask",Mask_mode);
           Display_cursor();
         }
         else
@@ -4690,7 +4698,7 @@ void Button_Effects(void)
         {
           Button_Sieve_mode();
           Hide_cursor();
-          Display_effect_state(176,62,"Sieve",Sieve_mode);
+          Display_effect_state(C2+23,62,"Sieve",Sieve_mode);
           Display_cursor();
         }
         else
@@ -4706,7 +4714,7 @@ void Button_Effects(void)
         {
           Button_Snap_mode();
           Hide_cursor();
-          Display_effect_state(176,81,"Grid",Snap_mode);
+          Display_effect_state(C2+23,81,"Grid",Snap_mode);
           Display_cursor();
         }
         else
@@ -4749,6 +4757,22 @@ void Button_Effects(void)
         Display_feedback_state();
         Display_cursor();
         break;
+
+
+      case 14: // Constraint checker/enforcer
+        if (Window_attribute1==LEFT_SIDE)
+        {
+          Button_Constraint_mode();
+          Hide_cursor();
+          Display_effect_state(177+23,24, "8 bit" ,Constraint_mode);
+          Display_cursor();
+        } else {
+          Close_window();
+          Display_cursor();
+          // Contraint checker/enforcer menu
+          clicked_button = 11;
+        }
+        break;
     }
   }
   while (clicked_button!=11);
@@ -4758,12 +4782,14 @@ void Button_Effects(void)
   else
     Hide_cursor();
 
-  if (!(Shade_mode||Quick_shade_mode||Colorize_mode||Smooth_mode||Tiling_mode||Smear_mode||Stencil_mode||Mask_mode||Sieve_mode||Snap_mode))
+  if (!(Shade_mode||Quick_shade_mode||Colorize_mode||Smooth_mode||Tiling_mode||Smear_mode||Stencil_mode||Mask_mode||Sieve_mode||Snap_mode||Constraint_mode))
     Unselect_button(BUTTON_EFFECTS);
 
   Display_cursor();
 }
 
+#undef C2
+
 // Callback to display a font name in the list
 void Draw_one_font_name(word x, word y, word index, byte highlighted)
 {
diff --git a/src/buttons.h b/src/buttons.h
index eed881dc..4a755f5c 100644
--- a/src/buttons.h
+++ b/src/buttons.h
@@ -438,6 +438,9 @@ void Button_Tiling_mode(void);
 */
 void Button_Tiling_menu(void);
 
+void Button_Constraint_mode(void);
+void Button_Constraint_menu(void);
+
 /*!
     Callback for the command that turns off all drawaing effects.
 */
diff --git a/src/buttons_effects.c b/src/buttons_effects.c
index d95e1514..614be571 100644
--- a/src/buttons_effects.c
+++ b/src/buttons_effects.c
@@ -162,6 +162,19 @@ void Menu_tag_colors(char * window_title, byte * table, byte * mode, byte can_ca
 }
 
 
+// Constaint enforcer/checker ------------------------------------------------
+void Button_Constraint_mode(void)
+{
+  Constraint_mode=!Constraint_mode;
+}
+
+
+void Button_Constraint_menu(void)
+{
+
+}
+
+
 //--------------------------------- Stencil ----------------------------------
 void Button_Stencil_mode(void)
 {
@@ -547,7 +560,7 @@ void Button_Colorize_mode(void)
     switch(Colorize_current_mode)
     {
       case 0 :
-        Effect_function=Effect_layer_copy;
+        Effect_function=Effect_interpolated_colorize;
         break;
       case 1 :
         Effect_function=Effect_additive_colorize;
diff --git a/src/global.h b/src/global.h
index 3746126b..771e630e 100644
--- a/src/global.h
+++ b/src/global.h
@@ -726,6 +726,9 @@ GFX2_GLOBAL byte Mask_mode;
 /// Array of booleans. True if the indexed color is protected by the mask.
 GFX2_GLOBAL byte Mask_table[256];
 
+// -- Constraint enforcer
+GFX2_GLOBAL byte Constraint_mode;
+
 // -- Magnifier data
 
 #ifdef GLOBAL_VARIABLES
diff --git a/src/graph.c b/src/graph.c
index d2d09ae4..d263fc72 100644
--- a/src/graph.c
+++ b/src/graph.c
@@ -2977,7 +2977,24 @@ byte Read_pixel_from_current_screen  (word x,word y)
 void Pixel_in_current_screen      (word x,word y,byte color,int with_preview)
 {
   #ifndef NOLAYERS
-  if ( Main_current_layer == 4)
+  if (!Constraint_mode)
+  {
+    byte depth = *(Main_visible_image_depth_buffer.Image+x+y*Main_image_width);
+    *(Main_backups->Pages->Image[Main_current_layer] + x+y*Main_image_width)=color;
+    if ( depth <= Main_current_layer)
+    {
+      if (color == Main_backups->Pages->Transparent_color) // transparent color
+        // fetch pixel color from the topmost visible layer
+        color=*(Main_backups->Pages->Image[depth] + x+y*Main_image_width);
+      
+      *(x+y*Main_image_width+Main_screen)=color;
+      
+      if (with_preview)
+        Pixel_preview(x,y,color);
+    }
+    
+  }
+  else if ( Main_current_layer == 4)
   {
     if (color<4)
     {
diff --git a/src/pages.c b/src/pages.c
index 0669e963..b7901bb8 100644
--- a/src/pages.c
+++ b/src/pages.c
@@ -199,7 +199,7 @@ void Redraw_layered_image(void)
   // Re-construct the image with the visible layers
   byte layer=0;  
   // First layer
-  if (Main_layers_visible & (1<<4))
+  if (Constraint_mode && Main_layers_visible & (1<<4))
   {
     // The raster result layer is visible: start there
     // Copy it in Main_visible_image