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.

1 files changed, 3 insertions, 4 deletions

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;