feat: hacky scrolling (horiz only)

Author: nathanleiby

src/main.rs

@@ -83,7 +83,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         move |ppu: &Ppu, gamepad1: &mut GamepadRegister, _gamepad2: &mut GamepadRegister| {
             // compute the screen's content from the PPU
             let mut frame = Frame::new();
-            ppu.draw_background(&mut frame);
+            ppu.draw_scrollable_background(&mut frame);
             ppu.draw_sprites(&mut frame);
 
             // redraw the screen

src/ppu.rs

@@ -182,7 +182,60 @@ impl Ppu {
                 let tile_n = self.vram[self.mirror_vram_addr((nt_start + offset) as u16) as usize];
                 let bgp_idx = self.palette_for_bg_tile((x, y), nt_start);
                 let palette = self.lookup_palette(bgp_idx);
-                frame.draw_bg_tile(pattern_table, tile_n as usize, (x, y), palette);
+                frame.draw_bg_tile(pattern_table, tile_n as usize, (x, y), 0, palette);
+            }
+        }
+    }
+
+    /// Draws the entire possible background of 2 frames, but it gets filtered within, which you can then filter by applying a windowed viewport
+    pub fn draw_scrollable_background(&self, frame: &mut Frame) {
+        let bank = self.get_background_pattern_bank();
+        let pattern_table =
+            &self.chr_rom[bank * PATTERN_TABLE_SIZE..(bank + 1) * PATTERN_TABLE_SIZE];
+
+        // In horizontal scrolling, this is the nametable at the LEFT of the screen
+        let which_nametable = self
+            .registers
+            .control
+            .intersection(ControlRegister::NAMETABLE)
+            .bits() as usize;
+        assert!(which_nametable <= 3);
+        let nt_start = which_nametable * 0x400;
+
+        // self.registers.control.
+
+        // Determine which nametable is on the left
+        let nts = if nt_start == 0 {
+            [0, 0x400]
+        } else {
+            [0x400, 0x800]
+        };
+
+        let x_scroll: usize = self.registers.scroll.x_scroll as usize;
+        let y_scroll: usize = self.registers.scroll.y_scroll as usize;
+        // TODO
+        // if self.registers.control.contains(ControlRegister::X_SCROLL) {
+        //     x_scroll += 256;
+        // }
+
+        let rows = 30;
+        let cols = 32;
+        for tile_y in 0..rows {
+            for (nt_idx, &nt_start) in nts.iter().enumerate() {
+                for tile_x in 0..cols {
+                    let offset = tile_y * cols + tile_x;
+                    let tile_n =
+                        self.vram[self.mirror_vram_addr((nt_start + offset) as u16) as usize];
+                    let bgp_idx = self.palette_for_bg_tile((tile_x, tile_y), nt_start);
+                    let palette = self.lookup_palette(bgp_idx);
+                    frame.draw_bg_tile(
+                        pattern_table,
+                        tile_n as usize,
+                        (tile_x + cols * (nt_idx), tile_y),
+                        x_scroll,
+                        palette,
+                    );
+                }
             }
         }
     }
@@ -477,8 +530,7 @@ impl Ppu {
         let sprite0 = self.parse_sprite_from_oam_data(&self.oam_data[0..4]);
         let (scanline_y, scanline_x) = self.get_tick_status();
 
-        scanline_y >= sprite0.y as usize
-            && (scanline_x % CYCLES_PER_SCANLINE) >= sprite0.x as usize
+        scanline_y >= sprite0.y as usize && (scanline_x % CYCLES_PER_SCANLINE) >= sprite0.x as usize
     }
 }
 

src/render.rs

@@ -48,6 +48,7 @@ impl Frame {
 
     pub fn new() -> Self {
         Self {
+            // TODO: Consider a 'double-sized' frame with a viewport() function
             data: vec![0; Frame::WIDTH * Frame::HEIGHT * 3],
         }
     }
@@ -68,18 +69,24 @@ impl Frame {
         pattern_table: &[u8],
         tile_n: usize,
         pos: (usize, usize),
+        x_scroll: usize,
+        // y_scroll: usize,
         palette: [u8; 4],
     ) {
         let tile = get_tile(pattern_table, tile_n);
         let (tile_x, tile_y) = pos;
         for (row, row_data) in tile.iter().enumerate() {
             for (col, &palette_idx) in row_data.iter().enumerate() {
                 let color = SYSTEM_PALETTE[palette[palette_idx as usize] as usize];
-                self.set_pixel(
-                    (tile_x * TILE_SIZE_PIXELS) + col,
-                    (tile_y * TILE_SIZE_PIXELS) + row,
-                    color,
-                );
+                let left_x = (tile_x * TILE_SIZE_PIXELS) + col;
+                // only draw pixels within the screen width
+                if x_scroll <= left_x && left_x - x_scroll <= 256 {
+                    self.set_pixel(
+                        (tile_x * TILE_SIZE_PIXELS) + col - x_scroll,
+                        (tile_y * TILE_SIZE_PIXELS) + row,
+                        color,
+                    );
+                }
             }
         }
     }
@@ -156,7 +163,7 @@ mod test {
 
         let mut f = Frame::new();
         assert_eq!(f.data[0], 0);
-        f.draw_bg_tile(&pattern_table, tile_n, pos, palette);
+        f.draw_bg_tile(&pattern_table, tile_n, pos, 0, palette);
         assert_eq!(f.data[0], 128);
         assert_eq!(f.data[1], 128);
         assert_eq!(f.data[2], 128);
@@ -175,7 +182,7 @@ mod test {
         let palette = [65, 65, 65, 3]; // 65 should crash if read, since it's OOB the palette with 64 colors
 
         let mut f = Frame::new();
-        f.draw_bg_tile(&pattern_table, tile_n, pos, palette);
+        f.draw_bg_tile(&pattern_table, tile_n, pos, 0, palette);
 
         // verify we drew the entire tile in the one selected color
         for row in 0..8 {