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fix: implement sprite coord overflow behaviour

This commit is contained in:
Rekai Nyangadzayi Musuka 2022-03-12 03:46:41 -04:00
parent 0c50ef1e6d
commit cb10dfbdfd
1 changed files with 92 additions and 67 deletions
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159
src/ppu.zig
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@ -93,12 +93,19 @@ pub const Ppu = struct {
// Only consider enabled sprites
if (sprite.isDisabled()) continue;
// Determine sprite bounds
// We only care about the Y axis since that value remains constant
const start = sprite.y();
const end = start + sprite.height;
// When fetching sprites we only care about ones that could be rendered
// on this scanline
const iy = @bitCast(i8, y);
if (start <= y and y < end) {
const start = sprite.y();
const istart = @bitCast(i8, start);
const end = start +% sprite.height;
const iend = @bitCast(i8, end);
// Sprites are expected to be able to wraparound, we perform the same check
// for unsigned and signed values so that we handle all valid sprite positions
if ((start <= y and y < end) or (istart <= iy and iy < iend)) {
for (self.scanline_sprites) |*maybe_sprite| {
if (maybe_sprite.* == null) {
maybe_sprite.* = sprite;
@ -112,72 +119,90 @@ pub const Ppu = struct {
}
}
/// Draw all relevant sprites on a scanline
fn drawSprites(self: *Self, prio: u2) void {
// Object VRAM 3rd and 4th (0-indexed) charblocks
const char_base = 0x4000 * 4;
const scanline = self.vcount.scanline.read();
const y = @bitCast(i8, self.vcount.scanline.read());
var i: u32 = 0;
var i: u9 = 0;
while (i < width) : (i += 1) {
// Exit early if a pixel is already here
if (self.scanline_buf[i] != null) continue;
const x = i;
const y = scanline;
for (self.scanline_sprites) |maybe_sprite| {
if (maybe_sprite) |sprite| {
if (sprite.priority() != prio) continue;
const ix = @bitCast(i9, x);
const start = sprite.x();
const end = start + sprite.width;
const istart = @bitCast(i9, start);
if (start <= x and x < end) {
const end = start +% sprite.width;
const iend = @bitCast(i9, end);
// FIXME: We branch on this condition quite a lot
const is_8bpp = sprite.is_8bpp();
// Y and X coordinates within the context of a singular 8x8 tile
const tile_y: u16 = (y - sprite.y()) ^ if (sprite.v_flip()) (sprite.height - 1) else 0;
const tile_x = (x - sprite.x()) ^ if (sprite.h_flip()) (sprite.width - 1) else 0;
const tile_id: u32 = sprite.tile_id();
const tile_row_offset: u32 = if (is_8bpp) 8 else 4;
const tile_len: u32 = if (is_8bpp) 0x40 else 0x20;
const row = tile_y % 8;
const col = tile_x % 8;
const tile_base: u32 = char_base + (0x20 * tile_id) + (tile_row_offset * row) + if (is_8bpp) col else col / 2;
var tile_offset = (tile_x / 8) * tile_len;
if (self.dispcnt.obj_mapping.read()) {
// One Dimensional
tile_offset += (tile_y / 8) * tile_len * (sprite.width / 8);
} else {
// Two Dimensional
// TODO: This doesn't work
tile_offset += (tile_y / 8) * tile_len * 0x20;
}
const tile = self.vram.buf[tile_base + tile_offset];
const pal_id: u16 = if (!is_8bpp) blk: {
const nybble_tile = if (col & 1 == 1) tile >> 4 else tile & 0xF;
if (nybble_tile == 0) break :blk 0;
const pal_bank = @as(u8, sprite.pal_bank()) << 4;
break :blk pal_bank | nybble_tile;
} else tile;
// Sprite Palette starts at 0x0500_0200
if (pal_id != 0) self.scanline_buf[i] = self.palette.get16(0x200 + pal_id * 2);
// Sprites are expected to wrap, by performing the same check on both
// signed and unsigned values we ensure that sprites are properly displayed
// in all valid scenarios
if ((start <= x and x < end) or (istart <= ix and ix < iend)) {
self.drawSpritePixel(char_base, sprite, ix, y);
}
} else break;
}
}
}
/// Draw a Pixel of a Sprite Tile
fn drawSpritePixel(self: *Self, char_base: u32, sprite: Sprite, x: i9, y: i8) void {
// FIXME: We branch on this condition quite a lot
const is_8bpp = sprite.is_8bpp();
// std.math.absInt is branchless
const x_diff = @bitCast(u9, std.math.absInt(x - @bitCast(i9, sprite.x())) catch unreachable);
const y_diff = @bitCast(u8, std.math.absInt(y -% @bitCast(i8, sprite.y())) catch unreachable);
// Note that we flip the tile_pos not the (tile_pos % 8) like we do for
// Background Tiles. By doing this we mirror the entire sprite instead of
// just a specific tile (see how sprite.width and sprite.height are involved)
const tile_y = y_diff ^ if (sprite.v_flip()) (sprite.height - 1) else 0;
const tile_x = x_diff ^ if (sprite.h_flip()) (sprite.width - 1) else 0;
// Like in the background Tiles are 8x8 groups of pixels in 8bpp or 4bpp formats
const tile_id = sprite.tile_id();
const tile_row_offset: u32 = if (is_8bpp) 8 else 4;
const tile_len: u32 = if (is_8bpp) 0x40 else 0x20;
const row = tile_y % 8;
const col = tile_x % 8;
// When calcualting the inital address, the first entry is always 0x20 * tile_id, even if it is 8bpp
const tile_base = char_base + (0x20 * @as(u32, tile_id)) + (tile_row_offset * row) + if (is_8bpp) col else col / 2;
// TODO: Understand more
var tile_offset = (tile_x / 8) * tile_len;
if (self.dispcnt.obj_mapping.read()) {
tile_offset += (tile_y / 8) * tile_len * (sprite.width / 8); // 1D Mapping
} else {
tile_offset += (tile_y / 8) * tile_len * 0x20; // 2D Mapping
}
const tile = self.vram.buf[tile_base + tile_offset];
const pal_id: u16 = if (!is_8bpp) blk: {
const nybble_tile = if (col & 1 == 1) tile >> 4 else tile & 0xF;
if (nybble_tile == 0) break :blk 0;
const pal_bank = @as(u8, sprite.pal_bank()) << 4;
break :blk pal_bank | nybble_tile;
} else tile;
// Sprite Palette starts at 0x0500_0200
if (pal_id != 0) self.scanline_buf[@bitCast(u9, x)] = self.palette.get16(0x200 + pal_id * 2);
}
fn drawBackround(self: *Self, comptime n: u3) void {
// A Tile in a charblock is a byte, while a Screen Entry is a halfword
const charblock_len: u32 = 0x4000;
@ -228,11 +253,11 @@ pub const Ppu = struct {
// If we're in 8bpp, then the tile value is an index into the palette,
// If we're in 4bpp, we have to account for a pal bank value in the Screen entry
// and then we can index the palette
const pal_id = if (!is_8bpp) blk: {
const pal_id: u16 = if (!is_8bpp) blk: {
const nybble_tile = if (col & 1 == 1) tile >> 4 else tile & 0xF;
if (nybble_tile == 0) break :blk 0;
const pal_bank: u16 = @as(u8, entry.palette_bank.read()) << 4;
const pal_bank = @as(u8, entry.pal_bank.read()) << 4;
break :blk pal_bank | nybble_tile;
} else tile;
@ -483,7 +508,7 @@ const ScreenEntry = extern union {
tile_id: Bitfield(u16, 0, 10),
h_flip: Bit(u16, 10),
v_flip: Bit(u16, 11),
palette_bank: Bitfield(u16, 12, 4),
pal_bank: Bitfield(u16, 12, 4),
raw: u16,
};
@ -494,8 +519,8 @@ const Sprite = struct {
attr1: Attr1,
attr2: Attr2,
width: u16,
height: u16,
width: u8,
height: u8,
fn init(attr0: Attr0, attr1: Attr1, attr2: Attr2) Self {
const d = spriteDimensions(attr0.shape.read(), attr1.size.read());
@ -509,7 +534,7 @@ const Sprite = struct {
};
}
inline fn x(self: *const Self) u16 {
inline fn x(self: *const Self) u9 {
return self.attr1.x.read();
}
@ -579,28 +604,28 @@ const Attr2 = extern union {
pal_bank: Bitfield(u16, 12, 4),
};
fn spriteDimensions(shape: u2, size: u2) [2]u16 {
fn spriteDimensions(shape: u2, size: u2) [2]u8 {
@setRuntimeSafety(false);
return switch (shape) {
0b00 => switch (size) {
// Square
0b00 => [_]u16{ 8, 8 },
0b01 => [_]u16{ 16, 16 },
0b10 => [_]u16{ 32, 32 },
0b11 => [_]u16{ 64, 64 },
0b00 => [_]u8{ 8, 8 },
0b01 => [_]u8{ 16, 16 },
0b10 => [_]u8{ 32, 32 },
0b11 => [_]u8{ 64, 64 },
},
0b01 => switch (size) {
0b00 => [_]u16{ 16, 8 },
0b01 => [_]u16{ 32, 8 },
0b10 => [_]u16{ 32, 16 },
0b11 => [_]u16{ 64, 32 },
0b00 => [_]u8{ 16, 8 },
0b01 => [_]u8{ 32, 8 },
0b10 => [_]u8{ 32, 16 },
0b11 => [_]u8{ 64, 32 },
},
0b10 => switch (size) {
0b00 => [_]u16{ 8, 16 },
0b01 => [_]u16{ 8, 32 },
0b10 => [_]u16{ 16, 32 },
0b11 => [_]u16{ 32, 64 },
0b00 => [_]u8{ 8, 16 },
0b01 => [_]u8{ 8, 32 },
0b10 => [_]u8{ 16, 32 },
0b11 => [_]u8{ 32, 64 },
},
else => std.debug.panic("{} is an invalid sprite shape", .{shape}),
};