Use the MSB of the index to mark a transparent color

Because we use 1 byte to store 4bpp, we don't need a mask and we can
encode a "transparent" color  in the bits we don't use.

4 files changed, 16 insertions, 6 deletions

diff --git a/README.md b/README.md
index a6d17e3..c7a7340 100644
--- a/README.md
+++ b/README.md
@@ -236,8 +236,10 @@ Read the file and add the content to the output at current position.
 
 .incpng "filename"  
 Read the PNG image and add the content to the output at current position. The
-image colours will be matched with the TR8 palette and any non-matchin color
-will be condiered transparent and the image will be included with a mask.
+image colours will be matched with the TR8 palette and any non-matching color
+will be condiered transparent and the index `128` will be used.
+
+The blitter won't draw any pixel with index larger than 15.
 
 #### Load and Store
 
diff --git a/assets/icon.png b/assets/icon.png
index 0890b92..e216044 100644
--- a/assets/icon.png
+++ b/assets/icon.png
diff --git a/example.asm b/example.asm
index 73aa9a5..2ab0ee7 100644
--- a/example.asm
+++ b/example.asm
@@ -75,7 +75,16 @@ draw_sprite:
 line:
     push x
     ld x, [a : x]
+
+    ; any color with index > 15
+    ; is transparent
+    cmp x, 16
+    bnc
+    jmp transparent
+
     ld [b : y], x
+
+transparent:
     pop x
 
     inc x
diff --git a/tr8as.c b/tr8as.c
index 1a83b97..38b327d 100644
--- a/tr8as.c
+++ b/tr8as.c
@@ -495,12 +495,11 @@ static uint8_t parse_incpng(As *as, char **c)
         goto exit_parseinc;
     }
 
-    /* TODO: mask */
-
     uint16_t addr = as->addr;
     uint8_t index;
 
-    /* map RGB values to the palette indexes */
+    /* map RGB values to the palette indexes;
+     * any index over 15 is "transparent" with index 128 */
     for (int i = 0; i < x * y * 3; i += 3)
     {
         for (index = 0; index < 16; index++)
@@ -512,7 +511,7 @@ static uint8_t parse_incpng(As *as, char **c)
                 break;
             }
         if (index == 16)
-            as->out[addr++] = 0;
+            as->out[addr++] = 128;
     }
 
     as->addr = addr;