From f05ce5d5ff7d650a439b363993f8b38b817d3efe Mon Sep 17 00:00:00 2001
From: Miquel Sabaté Solà <mikisabate@gmail.com>
Date: Tue, 11 Mar 2025 16:39:44 +0100
Subject: scroll: Provide an example with sprite 0 collision
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Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
---
 scroll/include/all.s        |  1 +
 scroll/include/background.s | 17 ++++++++++++++---
 2 files changed, 15 insertions(+), 3 deletions(-)

(limited to 'scroll/include')

diff --git a/scroll/include/all.s b/scroll/include/all.s
index e9f059d..763feb4 100644
--- a/scroll/include/all.s
+++ b/scroll/include/all.s
@@ -6,6 +6,7 @@
 .include "ppu.s"
 .include "../../shared/asm.s"
 .include "../../shared/joypad.s"
+.include "../../shared/ppu.s"
 
 .include "reset.s"
 .include "palettes.s"
diff --git a/scroll/include/background.s b/scroll/include/background.s
index f392792..69725fd 100644
--- a/scroll/include/background.s
+++ b/scroll/include/background.s
@@ -27,6 +27,15 @@
     DISABLED_BACKGROUND_CYCLE = $FE
     BACKGROUND_CYCLE_MAX = $02
 
+    ;; The offset for metatile rows. That is, from where should the engine start
+    ;; counting rows of metatiles. By default it's 0, but it could be set to
+    ;; something else to reserve so top space for a HUD or something similar.
+    ;;
+    ;; NOTE: configurable.
+    .ifndef BACKGROUND_ROW_OFFSET
+        BACKGROUND_ROW_OFFSET = 0
+    .endif
+
     ;; Current column (or the next to be loaded by functions like
     ;; `load_column`).
     zp_cur_column = $75
@@ -51,9 +60,11 @@
         lda #$00
         sta zp_scroll
         sta Buffer::zp_vram_idx
-        sta zp_cur_row
         sta zp_cur_column
 
+        lda #BACKGROUND_ROW_OFFSET
+        sta zp_cur_row
+
         rts
     .endproc
 
@@ -112,7 +123,7 @@
 
         ;; Increase the column being used and loop if we are not at the end of
         ;; the screen yet.
-        lda #0
+        lda #BACKGROUND_ROW_OFFSET
         sta zp_cur_row
         inc zp_cur_column
         lda zp_cur_column
@@ -418,7 +429,7 @@
     ;; screen/level by poking the right flags.
     .proc prepare_next_column
         ;; The row index always has to be set to zero.
-        lda #0
+        lda #BACKGROUND_ROW_OFFSET
         sta zp_cur_row
 
         ;; Is this the last column? If not, just quit.
-- 
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