From 8ef947d54a4b70a2cf2436cc0d44a77bbc54745b Mon Sep 17 00:00:00 2001
From: Alex Janka <alex@alexjanka.com>
Date: Wed, 8 Feb 2023 11:12:08 +1100
Subject: [PATCH] draw objects behind bg/window

---
 src/processor/gpu.rs | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/processor/gpu.rs b/src/processor/gpu.rs
index ce9bb3e..91324d5 100644
--- a/src/processor/gpu.rs
+++ b/src/processor/gpu.rs
@@ -304,11 +304,11 @@ impl CPU {
     }
 
     fn render_scanline(&mut self, scanline: u8, lcdc: &LCDC) {
+        let bg_palette = byte_to_palette(self.memory.get(0xFF47));
         if lcdc.bg_window_enable {
-            let palette = byte_to_palette(self.memory.get(0xFF47));
-            self.render_scanline_bg(scanline, lcdc, palette);
+            self.render_scanline_bg(scanline, lcdc, bg_palette);
             if lcdc.window_enable {
-                self.render_scanline_window(scanline, lcdc, palette);
+                self.render_scanline_window(scanline, lcdc, bg_palette);
             }
         } else {
             for x in 0..WIDTH {
@@ -317,7 +317,7 @@ impl CPU {
             }
         }
         if lcdc.obj_enable {
-            self.render_scanline_obj(scanline, lcdc);
+            self.render_scanline_obj(scanline, lcdc, bg_palette);
         }
     }
 
@@ -350,10 +350,10 @@ impl CPU {
         }
     }
 
-    fn render_scanline_obj(&mut self, scanline: u8, lcdc: &LCDC) {
+    fn render_scanline_obj(&mut self, scanline: u8, lcdc: &LCDC, bg_palette: Palette) {
         let objs = self.parse_oam(scanline, &lcdc.obj_size);
         for object in objs {
-            self.render_object(scanline, object, &lcdc.obj_size);
+            self.render_object(scanline, object, &lcdc.obj_size, bg_palette);
         }
     }
 
@@ -386,7 +386,13 @@ impl CPU {
         objs
     }
 
-    fn render_object(&mut self, scanline: u8, object: Object, obj_size: &ObjSize) {
+    fn render_object(
+        &mut self,
+        scanline: u8,
+        object: Object,
+        obj_size: &ObjSize,
+        bg_palette: Palette,
+    ) {
         let mut object_row = scanline - object.y;
         if object.flags.y_flip {
             object_row = obj_size.get_height() - (object_row + 1);
@@ -408,7 +414,12 @@ impl CPU {
             }
             let x_coord = x_coord_uncorrected - 8;
             if x_coord < WIDTH {
-                self.gpu.buffer[(scanline as usize * WIDTH) + x_coord] = colour.to_rgb();
+                let buffer_index = (scanline as usize * WIDTH) + x_coord;
+                if !object.flags.behind_bg_and_window
+                    || self.gpu.buffer[buffer_index] == bg_palette.zero.to_rgb()
+                {
+                    self.gpu.buffer[buffer_index] = colour.to_rgb();
+                }
             }
         }
     }