WIP render one background to TV (no color yet)

snes-core/src/ppu/interface.rs

@@ -1,4 +1,4 @@
-use super::registers::{PPURegisters, MAX_TV_HEIGHT, MAX_TV_WIDTH};
+use super::registers::{PPURegisters, Background, MAX_TV_HEIGHT, MAX_TV_WIDTH};
 
 const FRAMEBUFFER_SIZE: usize = MAX_TV_HEIGHT * MAX_TV_WIDTH * 4;
 
@@ -12,13 +12,24 @@ pub struct PPU {
 impl PPU {
     pub fn new() -> Self {
         Self {
-            framebuffer: vec![0xFF; FRAMEBUFFER_SIZE],
+            framebuffer: Self::initialize_tv_framebuffer(),
             registers: PPURegisters::new(),
             was_vblank_nmi_set: false,
             is_irq_set: false,
         }
     }
 
+    fn initialize_tv_framebuffer() -> Vec<u8> {
+        let mut fb = vec![0x00; FRAMEBUFFER_SIZE];
+        // 0x00 also makes the alpha channel transparent, we need to make it opaque
+        let mut i = 3; // start at the first alpha channel
+        while i < FRAMEBUFFER_SIZE {
+            fb[i] = 0xFF;
+            i += 4;
+        }
+        fb
+    }
+
     pub fn tick(&mut self, cpu_cycles: usize) {
         for _ in 0..(cpu_cycles * 2) {
             self.dot_cycle();
@@ -56,10 +67,102 @@ impl PPU {
     }
 
     fn compute_pixel(&self) -> (u8, u8, u8) {
-        if self.registers.is_hblanking() || self.registers.is_hblanking() {
-            return (0x00, 0x00, 0x00)
+        // Objectives:
+        // 1. first try to render one of the backgrounds. Background 0 for now.
+        // 2. render the rest of the backgrounds
+        // 3. render sprites
+        // 4. figure out the priorities of each background
+        self.compute_background_pixel(Background::Bg1)
+        // (0xFF, 0x00, 0xFF)
+    }
+
+    // TODO: wrte tests for this function
+    fn compute_background_pixel(&self, background: Background) -> (u8, u8, u8) {
+        // 0. detect video mode?
+        // 1. get base tileset vram address
+        // 2. get base charset vram address
+        // 3. know the height and width of background
+        // 4. calculate which is the current tile: (x / tile width) + ((y / tile height) * background width)
+        // 4.1: TODO: consider scroll values of the background for this calculation (x += x scroll, y += y scroll)
+        // 5. get the tile information from vram
+        // 6. get character index for the tile
+        // 7. calculate the vram address of the character
+        // TODO: consider that each tile can be mirrored either vertically or horizontally. Keep this in mind when fetching the character information from vram
+        // 8. look up color palette
+        // ----
+        // possible optimizations:
+        // - Fetch all of the necessary data before starting to render the scanline
+        let tileset_vram_base_address = self.registers.get_bg_tile_base_address(background) as usize;
+        let charset_vram_base_address = self.registers.get_bg_char_base_address(background) as usize;
+        let (bg_size_width, _) = self.registers.get_bg_size(background).to_usize();
+        let (tile_size_width, tile_size_height) = self.registers.get_bg_tile_size(background).to_usize();
+        let vram = self.registers.vram();
+        let x = (self.registers.h_count as usize) - 22; // H count ranges from 0 to 339. Pixels become visible at 22.
+        let y = (self.registers.v_count as usize) - 1; // V count ranges from 0 to 261 (depending on the region). Pixels become visible at 1.
+
+        let current_tile = (x / tile_size_width) + ((y / tile_size_height) * bg_size_width);
+        let tile_byte = vram[tileset_vram_base_address + current_tile];
+        let char_index = tile_byte & 0b11_11111111;
+        let current_char_column = x.rem_euclid(tile_size_width);
+        let current_char_row = y.rem_euclid(tile_size_height);
+
+        let effective_vram_address =
+            charset_vram_base_address +
+            ((char_index as usize) * tile_size_width) +
+            current_char_row;
+
+        let vram_word = vram[effective_vram_address];
+        let lsb_bitplane= vram_word as u8;
+        let msb_bitplane= (vram_word >> 8) as u8;
+
+        let pixels = Self::mix_pixel_bitplanes(lsb_bitplane, msb_bitplane);
+        let effective_pixel = pixels[current_char_column];
+
+        return match effective_pixel {
+            0b00 => (0xFF, 0xFF, 0xFF),
+            0b01 => (0xAC, 0xAC, 0xAC),
+            0b10 => (0x56, 0x56, 0x56),
+            0b11 => (0x00, 0x00, 0x00),
+            _ => unreachable!(),
         }
-        (0xFF, 0x00, 0xFF)
+    }
+
+    // TODO: write tests
+    fn mix_pixel_bitplanes(lsb_bitplane: u8, msb_bitplane: u8) -> [u8; 8] {
+        [
+            (
+                (lsb_bitplane >> 7) |
+                ((msb_bitplane >> 7) << 1)
+            ),
+            (
+                ((lsb_bitplane >> 6) & 1) |
+                (((msb_bitplane >> 6) & 1) << 1)
+            ),
+            (
+                ((lsb_bitplane >> 5) & 1) |
+                (((msb_bitplane >> 5) & 1) << 1)
+            ),
+            (
+                ((lsb_bitplane >> 4) & 1) |
+                (((msb_bitplane >> 4) & 1) << 1)
+            ),
+            (
+                ((lsb_bitplane >> 3) & 1) |
+                (((msb_bitplane >> 3) & 1) << 1)
+            ),
+            (
+                ((lsb_bitplane >> 2) & 1) |
+                (((msb_bitplane >> 2) & 1) << 1)
+            ),
+            (
+                ((lsb_bitplane >> 1) & 1) |
+                (((msb_bitplane >> 1) & 1) << 1)
+            ),
+            (
+                (lsb_bitplane & 1) |
+                ((msb_bitplane & 1) << 1)
+            ),
+        ]
     }
 
     fn put_pixel(&mut self, pixel: (u8, u8, u8)) {