Fix sprite priority (visible in SMB3 powerups and Piranha Plants)

Makefile

@@ -4,7 +4,7 @@ PFLAGS = -g
 #PFLAGS = -O3
 #PFLAGS += -DDEBUG_MAPPER
 #PFLAGS += -DDEBUG_RENDER
-#PFLAGS += -DDEBUG_PPU -DDEBUG_VRAM
+#PFLAGS += -DDEBUG_PPU -DDEBUG_VRAM -DDEBUG_OAM
 #PFLAGS += -DE6502_DEBUG
 CFLAGS = $(PFLAGS) -Wall -Werror -Wshadow -Wunused -I..
 LDFLAGS = $(PFLAGS)

src/sdl_render.c

@@ -195,13 +195,18 @@ static void sdl_render_done(nes_Renderer* rend) {
 
 static inline void render_sprite_line(
         const nes_ppu* ppu, int index, int y, const uint8_t* pal,
-        uint8_t* dst, int start, int end) {
+        uint8_t* dst, int start, int end, const uint8_t* back) {
     uint8_t* sprite = chr_mem(ppu, index * 16U);
     uint8_t lo = sprite[0U + y] << start;
     uint8_t hi = sprite[8U + y] << start;
     for (int x = start; x < end; ++x) {
-        int pal_idx = (((hi & 0x80) >> 6) | ((lo & 0x80) >> 7));
-        if (pal_idx) *dst = pal[pal_idx];
+        if (back && *back != 0xFFU) {
+            *dst = *back;
+        } else {
+            int pal_idx = (((hi & 0x80) >> 6) | ((lo & 0x80) >> 7));
+            if (pal_idx) *dst = pal[pal_idx];
+        }
+        if (back) ++back;
         dst++;
         hi <<= 1;
         lo <<= 1;
@@ -210,13 +215,18 @@ static inline void render_sprite_line(
 
 static inline void render_sprite_line_flip(
         const nes_ppu* ppu, int index, int y, const uint8_t* pal,
-        uint8_t* dst, int start, int end) {
+        uint8_t* dst, int start, int end, const uint8_t* back) {
     uint8_t* sprite = chr_mem(ppu, index * 16U);
     uint8_t lo = sprite[0U + y] >> start;
     uint8_t hi = sprite[8U + y] >> start;
     for (int x = start; x < end; ++x) {
-        int pal_idx = (((hi & 1) << 1) | (lo & 1));
-        if (pal_idx) *dst = pal[pal_idx];
+        if (back && *back != 0xFFU) {
+            *dst = *back;
+        } else {
+            int pal_idx = (((hi & 1) << 1) | (lo & 1));
+            if (pal_idx) *dst = pal[pal_idx];
+        }
+        if (back) ++back;
         dst++;
         hi >>= 1;
         lo >>= 1;
@@ -413,16 +423,12 @@ static void render_sprite(nes_ppu* ppu, int index,
 */
 
 static void render_line_sprites(nes_ppu* ppu, uint8_t* dst_line,
-                                int scanline, int background,
+                                int scanline, const uint8_t* back,
                                 const oam_sprite* sprites,
                                 int n_sprites) {
     for (int i_sprite = n_sprites - 1; i_sprite >= 0; --i_sprite) {
         const oam_sprite* sprite = &sprites[i_sprite];
 
-        if ((sprite->attr & oam_Attr_Background) ^ background) {
-            continue;
-        }
-
         int index = sprite->index;
         int bank = (ppu->control & ppu_Control_Sprite_Bank) ?
                    0x100 : 0;
@@ -455,13 +461,19 @@ static void render_line_sprites(nes_ppu* ppu, uint8_t* dst_line,
             start = 8 - sprite->x;
         }
         if (sprite->attr & oam_Attr_Flip_X) {
-            render_sprite_line_flip(ppu, index, y, pal,
-                                    dst_line + sprite->x + start,
-                                    start, end);
+            render_sprite_line_flip(
+                    ppu, index, y, pal,
+                    dst_line + sprite->x + start, start, end,
+                    (sprite->attr & oam_Attr_Background) ?
+                        (back + sprite->x + start) : NULL
+                );
         } else {
-            render_sprite_line(ppu, index, y, pal,
-                               dst_line + sprite->x + start,
-                               start, end);
+            render_sprite_line(
+                    ppu, index, y, pal,
+                    dst_line + sprite->x + start, start, end,
+                    (sprite->attr & oam_Attr_Background) ?
+                        (back + sprite->x + start) : NULL
+                );
         }
     }
 }
@@ -734,6 +746,16 @@ static int select_line_sprites(const nes_ppu* ppu, int scanline,
     return n_sprites;
 }
 
+static void dump_line_sprites(const nes_ppu* ppu, int line,
+                              const oam_sprite* sprites, int n) {
+    while (n-- > 0) {
+        PPU_LOG("PPU: Line %3d: Sprite $%02x @ %3d, %3d (%02x)\n",
+                line, sprites->index, sprites->x, sprites->y,
+                sprites->attr);
+        ++sprites;
+    }
+}
+
 static void render_scanline(nes_ppu* ppu, int line,
                             sdl_render_data* data) {
 
@@ -769,27 +791,18 @@ static void render_scanline(nes_ppu* ppu, int line,
     uint8_t* foreground = data->foreground->pixels;
     uint8_t* background = data->background->pixels;
 
-
     // We check for hits if EITHER layer is enabled.
     render_bg_scanline(ppu, background);
     if (ppu->hit_line <= 0) {
         update_scanline_hit(ppu, background, line);
     }
 
-
     if (line >= 0) {
         oam_sprite line_sprites[8] = {0};
         int n_sprites = select_line_sprites(ppu, line,
                                             line_sprites, 8);
 
-        if (ppu->mask & ppu_Mask_Sprite) {
-            memset(foreground, 0xFFU, nes_ppu_render_w);
-            render_line_sprites(ppu, foreground, line,
-                                oam_Attr_Background,
-                                line_sprites, n_sprites);
-            SDL_BlitSurface(data->foreground, &src_rect,
-                            data->target, &dst_rect);
-        }
+        dump_line_sprites(ppu, line, line_sprites, n_sprites);
 
         if (ppu->mask & ppu_Mask_Back) {
             SDL_BlitSurface(data->background, &src_rect,
@@ -798,7 +811,7 @@ static void render_scanline(nes_ppu* ppu, int line,
 
         if (ppu->mask & ppu_Mask_Sprite) {
             memset(foreground, 0xFFU, nes_ppu_render_w);
-            render_line_sprites(ppu, foreground, line, 0,
+            render_line_sprites(ppu, foreground, line, background,
                                 line_sprites, n_sprites);
             SDL_BlitSurface(data->foreground, &src_rect,
                             data->target, &dst_rect);