diff --git a/src/buttons_effects.c b/src/buttons_effects.c
index 7229d3ad..84dc8857 100644
--- a/src/buttons_effects.c
+++ b/src/buttons_effects.c
@@ -437,6 +437,11 @@ void Button_Constraint_mode(void)
     Verbose_message("Error!", "Emulation of Amstrad CPC's Mode5 can only be used on an image whose width is a multiple of 48.");
     return;
   }
+  if (Selected_Constraint_Mode == IMAGE_MODE_C64FLI && ((Main.image_width < 160) || (Main.image_height < 200)))
+  {
+    Verbose_message("Error!", "Emulation of Commodore 64 FLI Mode needs a 160x200 sized image.");
+    return;
+  }
 
   // now check the constraints on existing pixels
   switch (Selected_Constraint_Mode)
@@ -467,14 +472,54 @@ void Button_Constraint_mode(void)
     // switch to layer mode if needed
     if (Main.backups->Pages->Image_mode != IMAGE_MODE_LAYERED)
       Switch_layer_mode(IMAGE_MODE_LAYERED);
+    Main.backups->Pages->Transparent_color = 16;
     // auto-create extra layers
     while (Main.backups->Pages->Nb_layers < 3)
-      if (Add_layer(Main.backups, Main.backups->Pages->Nb_layers))
+      if (Add_layer(Main.backups, 0))
       {
         Verbose_message("Error!", "Failed to create the 3 layers needed by C64 Flexible Line Interpretation mode.");
         return;
       }
-    Main.backups->Pages->Transparent_color = 16;
+    {
+      word x, y;
+      byte bitmap[8000],screen_ram[1024*8],color_ram[1024];
+      byte background[200];
+
+      memset(bitmap, 0, sizeof(bitmap));
+      memset(screen_ram, 0, sizeof(screen_ram));
+      memset(color_ram, 0, sizeof(color_ram));
+      memset(background, 0, sizeof(background));
+
+      // give "hints" to the converter
+      for (y = 0; y < 200; y++)
+        background[y] = Read_pixel_from_layer(0, 0, y);
+      for (y = 0; y < 25; y++)
+      {
+        for (x = 0; x < 40; x++)
+          color_ram[x + y*40] = Read_pixel_from_layer(1, x*4, y*8);
+      }
+
+      if (C64_pixels_to_FLI(bitmap, screen_ram, color_ram, background, Main.backups->Pages->Image[2].Pixels, Main.image_width, 1) > 0)
+      {
+        // put errors in layer 4 if not already done
+        if (Main.backups->Pages->Nb_layers < 4)
+        {
+          Add_layer(Main.backups, Main.backups->Pages->Nb_layers);
+          C64_pixels_to_FLI(bitmap, screen_ram, color_ram, background, Main.backups->Pages->Image[2].Pixels, Main.image_width, 1);
+        }
+      }
+
+      // copy background to layer 1
+      // and color RAM to layer 2
+      for (y = 0; y < 200; y++)
+      {
+        for (x = 0; x < 160; x++)
+        {
+          Pixel_in_layer(0, x, y, background[y]);
+          Pixel_in_layer(1, x, y, color_ram[(x >> 2) + (y >> 3)*40]);
+        }
+      }
+    }
     break;
   case IMAGE_MODE_HGR:
   case IMAGE_MODE_DHGR:
diff --git a/src/miscfileformats.c b/src/miscfileformats.c
index 1596e533..4036e97e 100644
--- a/src/miscfileformats.c
+++ b/src/miscfileformats.c
@@ -3348,7 +3348,10 @@ int Save_C64_fli_monolayer(T_IO_Context *context, byte saveWhat, word loadAddr)
   memset(color_ram, 0, sizeof(color_ram));
   memset(background, 0, sizeof(background));
 
-  if (C64_pixels_to_FLI(bitmap, screen_ram, color_ram, background, context->Target_address, context->Pitch) > 0)
+  memset(color_ram, 0xff, 40*25); // no hint
+  memset(background, 0xff, 200);
+
+  if (C64_pixels_to_FLI(bitmap, screen_ram, color_ram, background, context->Target_address, context->Pitch, 0) > 0)
     return 1;
 
   file = Open_file_write(context);
diff --git a/src/oldies.c b/src/oldies.c
index 25021278..340b6009 100644
--- a/src/oldies.c
+++ b/src/oldies.c
@@ -38,6 +38,7 @@
 #include "pages.h"
 #include "windows.h"
 #include "layers.h"
+#include "graph.h"
 
 // I don't have round() in MSVC++ 2010 (_MSC_VER=1600)
 #if defined(_MSC_VER)
@@ -399,7 +400,8 @@ int C64_FLI(T_IO_Context * context, byte *bitmap, byte *screen_ram, byte *color_
 
 }
 
-int C64_pixels_to_FLI(byte *bitmap, byte *screen_ram, byte *color_ram, byte *background, const byte * pixels, long pitch)
+int C64_pixels_to_FLI(byte *bitmap, byte *screen_ram, byte *color_ram,
+                      byte *background, const byte * pixels, long pitch, int errmode)
 {
   int bx, by; // 4x8 block coordinates
   int cx, cy; // coordinates inside block
@@ -445,13 +447,22 @@ int C64_pixels_to_FLI(byte *bitmap, byte *screen_ram, byte *color_ram, byte *bac
       if (n_possible_backgrounds == 0)
       {
         //ERROR
-        Warning_with_format("No possible background color for line %d.\n4x1 pixel blocks using 4 different colors must share at least one color.", by*8+cy);
-        return 1;
+        if (errmode == 0)
+        {
+          Warning_with_format("No possible background color for line %d.\n4x1 pixel blocks using 4 different colors must share at least one color.", by*8+cy);
+          return 1;
+        }
+        error_count++;
+        GFX2_Log(GFX2_INFO, "C64_pixels_to_FLI() no possible background for line %u. Default to #0.\n",  by*8+cy);
+        // default to background color #0
+        if (background[by*8+cy] >= 16)
+          background[by*8+cy] = 0;
       }
       else
       {
-        // pick the first one
-        background[by*8+cy] = count_trailing_zeros(background_possible[cy]);
+        // if the caller gave us a "hint", check it is "possible"
+        if (background[by*8+cy] >= 16 || (background_possible[cy] & (1 << background[by*8+cy])) == 0)
+          background[by*8+cy] = count_trailing_zeros(background_possible[cy]); // pick the first possible
 #ifdef _DEBUG
         if (background[by*8+cy] != 0)
           GFX2_Log(GFX2_DEBUG, "  y=%d background_possible=$%04x (count=%d) background color=#%d\n",
@@ -488,10 +499,38 @@ int C64_pixels_to_FLI(byte *bitmap, byte *screen_ram, byte *color_ram, byte *bac
       // choose the color RAM values (default to #0)
       if (color_ram_possible[bx] == 0)
       {
-        Warning_with_format("No possible color RAM value for 4x8 block (%d,%d) coordinates (%d,%d)\nThe 8 4x1 blocks must share a color.", bx, by, bx*4, by*8);
-        return 1;
+        if (errmode == 0)
+        {
+          Warning_with_format("No possible color RAM value for 4x8 block (%d,%d) coordinates (%d,%d)\nThe 8 4x1 blocks must share a color.", bx, by, bx*4, by*8);
+          return 1;
+        }
+        else
+        {
+          // Mark errors in Layer 4
+          for(cy = 0; cy < 8; cy++)
+          {
+            word colors_used = 0;
+            for(cx = 0; cx < 4; cx++)
+            {
+              pixel = pixels[bx*4+cx + pitch*(by*8+cy)];
+              if (pixel < 16 && pixel != background[by*8+cy])
+                colors_used |= 1 << pixel;
+            }
+            if (count_set_bits(colors_used) >= 3)
+            {
+              error_count++;
+              GFX2_Log(GFX2_INFO, "C64_pixels_to_FLI() too much colors in block at (%u to %u, %u)\n", bx*4, bx*4+3, by*7+cy);
+              for(cx = 0; cx < 4; cx++)
+              {
+                pixel = pixels[bx*4+cx + pitch*(by*8+cy)];
+                if (pixel < 16 && pixel != background[by*8+cy] && Main.backups->Pages->Nb_layers >= 4)
+                  Pixel_in_layer(3, bx*4+cx, by*8+cy, 17);
+              }
+            }
+          }
+        }
       }
-      else
+      else if (color_ram[by*40+bx] >= 16 || ((1 << color_ram[by*40+bx]) & color_ram_possible[bx]) == 0)
       {
         word possible = color_ram_possible[bx];
         // avoid using same color as background
diff --git a/src/oldies.h b/src/oldies.h
index c4f9d46c..26f444f3 100644
--- a/src/oldies.h
+++ b/src/oldies.h
@@ -43,15 +43,22 @@ int C64_FLI(T_IO_Context * context, byte *bitmap, byte *screen_ram, byte *color_
 /**
  * Convert a (1 layer) picture to C64 FLI format
  *
+ * Some "hints" can be put in background and color_ram.
+ * (a color value >= 16 means no hint).
+ *
+ * Errors can be either outputed to the user with Warning messages,
+ * or put in layer 4. The layer 4 has to be created before.
+ *
  * @param bitmap a 8000 byte buffer to store bitmap data
  * @param screen_ram a 8192 byte buffer to store the 8 screen RAMs
  * @param color_ram a 1000 byte buffer to store the color RAM
  * @param background a 200 byte buffer to store the background colors
  * @param pixels source pixel buffer (at least 160x200)
  * @param pitch bytes per line of the pixel buffer
+ * @param errmode error reporting mode 0 = report, 1 = mark in layer 4
  * @return 0 the number of constraint errors
  */
-int C64_pixels_to_FLI(byte *bitmap, byte *screen_ram, byte *color_ram, byte *background, const byte * pixels, long pitch);
+int C64_pixels_to_FLI(byte *bitmap, byte *screen_ram, byte *color_ram, byte *background, const byte * pixels, long pitch, int errmode);
 
 #if 0
 /**