From b65f833b284ee5ab301cd0e3b39fa6812c616e11 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Sat, 26 Feb 2022 18:33:16 -0600
Subject: [PATCH] feat(ppu): implement transparency + backdrop in mode 0

---
 src/ppu.zig | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/ppu.zig b/src/ppu.zig
index f842781..be7ddd9 100644
--- a/src/ppu.zig
+++ b/src/ppu.zig
@@ -115,7 +115,11 @@ pub const Ppu = struct {
             const pal_id = if (!is_8bpp) blk: {
                 tile = if (col & 1 == 1) tile >> 4 else tile & 0xF;
                 const pal_bank: u16 = @as(u8, entry.palette_bank.read()) << 4;
-                break :blk pal_bank | tile;
+
+                // If Tile is 0 then we need to access the Backdrop
+                // The backdrop is the first colour in Palette RAM
+                // AFAIK there is no special logic needed for doing this in 8bpp
+                break :blk if (tile == 0) 0 else pal_bank | tile;
             } else tile;
 
             std.mem.copy(u8, scanline_buf[i * 2 ..][0..2], self.palette.buf[pal_id * 2 ..][0..2]);