From 681ba9adf948ebdc909b08979c76251053af7a9e Mon Sep 17 00:00:00 2001
From: "Juan J. Martinez" <jjm@usebox.net>
Date: Thu, 11 May 2023 20:57:55 +0100
Subject: Add starfield effect

---
 game/entities.asm  |  2 +-
 game/main.asm      | 54 ++++++++++++++++++++++++++++++++++++++---
 game/starfield.asm | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 122 insertions(+), 4 deletions(-)
 create mode 100644 game/starfield.asm

diff --git a/game/entities.asm b/game/entities.asm
index 6116624..b3e1eea 100644
--- a/game/entities.asm
+++ b/game/entities.asm
@@ -265,7 +265,7 @@ entity_draw_frame_done:
 
     ; blit
     dec b
-    ; writw, transparent
+    ; write, transparent
     ld y, 3
     port b, y
 
diff --git a/game/main.asm b/game/main.asm
index 68d1267..e779a95 100644
--- a/game/main.asm
+++ b/game/main.asm
@@ -8,6 +8,7 @@ game_loop:
     halt
 
     call entities_erase
+    call stars_update
     call entities_update
     call entities_draw
 
@@ -29,14 +30,13 @@ init:
 
     halt
 
-    ; pattern for the BG
-    ld b, 15
+    ; erase the screen
+    ld b, 0
     ld a, 0xbf
     ld x, 0
     ld y, 0x40
 fill_loop:
     ld [a : x], b
-    xor b, 5
     inc x
     bno
     jmp fill_loop
@@ -52,6 +52,7 @@ fill_loop:
 int_handler:
     iret
 
+.include "starfield.asm"
 .include "entities.asm"
 .include "player.asm"
 
@@ -75,3 +76,50 @@ entities:
 player_sprite:
     .incpng "assets/player.png"
 
+; starfield data
+stars:
+    ; video addr, speed, color
+    .dw 0xc361
+    .db 1, 15
+    .dw 0xc204
+    .db 1, 6
+    .dw 0xcdf9
+    .db 1, 15
+    .dw 0xdb87
+    .db 1, 3
+    .dw 0xd242
+    .db 1, 15
+    .dw 0xdeea
+    .db 1, 6
+    .dw 0xe7af
+    .db 1, 15
+    .dw 0xed68
+    .db 1, 3
+    .dw 0xf812
+    .db 1, 15
+    .dw 0xfbca
+    .db 1, 13
+    .dw 0xc361
+    .db 2, 8
+    .dw 0xc204
+    .db 2, 9
+    .dw 0xcdf9
+    .db 3, 7
+    .dw 0xdb87
+    .db 3, 4
+    .dw 0xd242
+    .db 2, 8
+    .dw 0xdeea
+    .db 3, 9
+    .dw 0xe7af
+    .db 3, 6
+    .dw 0xed68
+    .db 2, 9
+    .dw 0xf812
+    .db 3, 14
+    .dw 0xfbca
+    .db 2, 3
+    ; end of list
+    .dw 0xffff
+
+
diff --git a/game/starfield.asm b/game/starfield.asm
new file mode 100644
index 0000000..e8b8ed1
--- /dev/null
+++ b/game/starfield.asm
@@ -0,0 +1,70 @@
+;
+; Simple starfield effect
+;
+stars_update:
+    ld x, <stars
+    ld a, >stars
+
+stars_update_loop:
+    ld y, [a : x]
+    inc x
+    bo
+    inc a
+    ld b, [a : x]
+    ; is the end of list?
+    cmp b, 0xff
+    bz
+    ret
+
+    push a
+    push x
+
+    ; erase old
+    ld x, 0
+    ld [b : y], x
+    ld x, [sp + 0]
+
+    ; update address
+    inc x
+    bo
+    inc a
+    push x
+    ld x, [a : x]
+stars_update_addr:
+    add y, 128
+    bo
+    inc b
+    cmp b, 0xfe
+    bnz
+    jmp stars_no_wrap
+    cmp y, 0x80
+    bc
+    ld b, 0xbf
+stars_no_wrap:
+    dec x
+    bnz
+    jmp stars_update_addr
+    pop x
+
+    ; draw new: color
+    inc x
+    bo
+    inc a
+    ld x, [a : x]
+    ld [b : y], x
+
+    pop x
+    pop a
+
+    ; save new address
+    ld [a : x], b
+    dec x
+    bo
+    dec a
+    ld [a : x], y
+
+    ; next star
+    add x, 4
+    bo
+    inc a
+    jmp stars_update_loop
-- 
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