render objs on top of scrolled area instead of into 256x256 area

src/dmg.c

@@ -181,23 +181,6 @@ static void render_background(struct dmg *dmg, int lcdc)
     }
 }
 
-static void scroll(struct dmg *dmg)
-{
-    int scroll_y = lcd_read(dmg->lcd, REG_SCY);
-    int scroll_x = lcd_read(dmg->lcd, REG_SCX);
-
-    int lines;
-    for (lines = 0; lines < 144; lines++) {
-        int src_y = (scroll_y + lines) & 0xff;
-        int cols;
-        for (cols = 0; cols < 160; cols++) {
-            int src_off = (src_y << 8) + ((scroll_x + cols) & 0xff);
-
-            dmg->lcd->pixels[lines * 160 + cols] = dmg->lcd->buf[src_off];
-        }
-    }
-}
-
 struct oam_entry {
     u8 pos_y;
     u8 pos_x;
@@ -221,18 +204,22 @@ static void render_objs(struct dmg *dmg)
         lcd_x = oam->pos_x - 8;
         lcd_y = oam->pos_y - 16;
 
-        off = 256 * lcd_y + lcd_x;
+        off = 160 * lcd_y + lcd_x;
         int eff_addr = 0x8000 + 16 * oam->tile;
         int b, i;
         for (b = 0; b < 16; b += 2) {
             int data1 = dmg_read(dmg, eff_addr + b);
             int data2 = dmg_read(dmg, eff_addr + b + 1);
             for (i = 7; i >= 0; i--) {
-                dmg->lcd->buf[off] = ((data1 & (1 << i)) ? 1 : 0) << 1;
-                dmg->lcd->buf[off] |= (data2 & (1 << i)) ? 1 : 0;
+                if (off < 0 || off >= 160 * 144) {
+                    // terrible clipping. need to not have an if per-pixel
+                    continue;
+                }
+                dmg->lcd->pixels[off] = ((data1 & (1 << i)) ? 1 : 0) << 1;
+                dmg->lcd->pixels[off] |= (data2 & (1 << i)) ? 1 : 0;
                 off++;
             }
-            off += 248;
+            off += 152;
         }
     }
 }
@@ -284,14 +271,12 @@ void dmg_step(void *_dmg)
                 // printf("window\n");
             }
 
+            lcd_apply_scroll(dmg->lcd);
+
             if (lcdc & LCDC_ENABLE_OBJ) {
                 render_objs(dmg);
             }
 
-            scroll(dmg);
-
-            // now copy 256x256 buf to 160x144 based on window registers
-            lcd_copy(dmg->lcd);
             lcd_draw(dmg->lcd);
         }
     } else {

src/lcd.c

@@ -66,10 +66,20 @@ void lcd_put_pixel(struct lcd *lcd, u8 x, u8 y, u8 value)
     lcd->pixels[y * LCD_WIDTH + x] = value;
 }
 
-void lcd_copy(struct lcd *lcd)
+void lcd_apply_scroll(struct lcd *lcd)
 {
-    // use all the registers to compute the pixel data
-    
+    int scroll_y = lcd_read(lcd, REG_SCY);
+    int scroll_x = lcd_read(lcd, REG_SCX);
+
+    int lines;
+    for (lines = 0; lines < 144; lines++) {
+        int src_y = (scroll_y + lines) & 0xff;
+        int cols;
+        for (cols = 0; cols < 160; cols++) {
+            int src_off = (src_y << 8) + ((scroll_x + cols) & 0xff);
+            lcd->pixels[lines * 160 + cols] = lcd->buf[src_off];
+        }
+    }
 }
 
 int lcd_step(struct lcd *lcd)

src/lcd.h

@@ -58,9 +58,8 @@ void lcd_clear_bit(struct lcd *lcd, u16 addr, u8 bit);
 int lcd_isset(struct lcd *lcd, u16 addr, u8 bit);
 void lcd_set_mode(struct lcd *lcd, int mode);
 
-// i feel like i'm going to need to call this every cycle and update regs
 int lcd_step(struct lcd *lcd);
-void lcd_copy(struct lcd *lcd);
+void lcd_apply_scroll(struct lcd *lcd);
 
 // output the pixels to the screen
 void lcd_draw(struct lcd *lcd);