Drawing Basics

The `PALETTE` Register#

WASM-4 can only display 4 colors on screen at a time. This palette's RGB values are stored in the PALETTE memory register, and can be modified.

For example, to change the palette to Ice Cream GB:

(i32.store (global.get $PALETTE0) (i32.const 0xfff6d3))(i32.store (global.get $PALETTE1) (i32.const 0xf9a875))(i32.store (global.get $PALETTE2) (i32.const 0xeb6b6f))(i32.store (global.get $PALETTE3) (i32.const 0x7c3f58))

The palette colors are considered to be numbered 1-4, even though they may be accessed with indices 0-3.

The default Gameboy-ish palette looks like this:

Color 1

Color 2

Color 3

Color 4

The first color in the palette register is used as the screen background color.

The `DRAW_COLORS` Register#

DRAW_COLORS is a set of 4 indexes into PALETTE. Drawing functions use these indexes to decide which colors to use, and what to use them for.

DRAW_COLORS is a 16 bit value that holds 4 indexes. Bits 0-3 (the least significant bits) hold the first draw color, bits 4-7 hold the second draw color, and so on.

Setting a draw color to 1 means use PALETTE color 1 for that draw color. The same applies when setting a draw color to 2, 3, or 4.

For example, rect() uses the first draw color for the fill color, and the second draw color as the outline color. To draw a light-green (palette color 2) rectangle with a black (palette color 4) outline:

;; Set DRAW_COLORS to 0x42.(i32.store16 (global.get $DRAW_COLORS) (i32.const 0x42))
;; Draw a rectangle at (10, 10) with size (32, 32).(call $rect (i32.const 10) (i32.const 10) (i32.const 32) (i32.const 32))

However, setting a draw color to 0 will make it transparent. For example, to draw a black outlined rectangle with no fill, set DRAW_COLORS to 0x40.

Other Shapes#

For info on other shape drawing functions like line() and oval(), see the Functions reference.

Direct Framebuffer Access#

The FRAMEBUFFER memory region contains the framebuffer, with each byte containing 4 pixels (2 bits per pixel). In the framebuffer, the palette colors 1-4 are represented numerically as 0-3.

For example, to clear the entire screen to palette color 4, we write 3 into each position:

{    var i = 0;    var value = 3 | (3 << 2) | (3 << 4) | (3 << 6);    {        if i == |FRAMEBUFFER|            goto end;        FRAMEBUFFER[i] = value;        i = i + 1;        loop;    }    end:}

;; i = 0;(local $i i32)
(loop $loop  ;; FRAMEBUFFER[i] = 0xff;  (i32.store8 offset=0xa0 (local.get $i) (i32.const 0xff))
  ;; i = i + 1;  (local.set $i (i32.add (local.get $i) (i32.const 1)))
  ;; loop while i < 160*160/4  (br_if $loop (i32.lt_u (local.get $i) (i32.const 6400))))

Advanced users can implement their own drawing functions by carefully manipulating the framebuffer. For example, to implement a pixel() function that draws a single pixel:

function pixel (x: i32, y: i32): void {    // The byte index into the framebuffer that contains (x, y)    const idx = (y*160 + x) >> 2;
    // Calculate the bits within the byte that corresponds to our position    const shift = u8((x & 0b11) << 1);    const mask = u8(0b11 << shift);
    // Use the first DRAW_COLOR as the pixel color.    const palette_color = u8(load<u16>(w4.DRAW_COLORS) & 0b1111);    if (palette_color == 0) {        // Transparent        return;    }    const color = (palette_color - 1) & 0b11;
    // Write to the framebuffer    store<u8>(w4.FRAMEBUFFER + idx, (color << shift) | (load<u8>(w4.FRAMEBUFFER + idx) & ~mask));}

void pixel (int x, int y) {    // The byte index into the framebuffer that contains (x, y)    int idx = (y*160 + x) >> 2;
    // Calculate the bits within the byte that corresponds to our position    int shift = (x & 0b11) << 1;    int mask = 0b11 << shift;
    // Use the first DRAW_COLOR as the pixel color    int palette_color = *DRAW_COLORS & 0b1111;    if (palette_color == 0) {        // Transparent        return;    }    int color = (palette_color - 1) & 0b11;
    // Write to the framebuffer    FRAMEBUFFER[idx] = (color << shift) | (FRAMEBUFFER[idx] & ~mask);}

fn void pixel(int x, int y) {    // The byte index into the framebuffer that contains (x, y)    int idx = (y * 160 + x) >> 2;
    // Calculate the bits within the byte that corresponds to our position    int shift = (x & 0b11) << 1;    int mask = 0b11 << shift;
    // Use the first DRAW_COLOR as the pixel color    int palette_color = *w4::DRAW_COLORS & 0b1111;    if (palette_color == 0)     {        // Transparent        return;    }    int color = (palette_color - 1) & 0b11;
    // Write to the framebuffer    w4::FRAMEBUFFER[idx] = (color << shift) | (w4::FRAMEBUFFER[idx] & ~mask);}

void pixel(int x, int y) {    // The byte index into the framebuffer that contains (x, y)    int idx = (y * w4.screenSize + x) >> 2;
    // Calculate the bits within the byte that corresponds to our position    int shift = (x & 0b11) << 1;    int mask = 0b11 << shift;
    // Use the first draw color as the pixel color    int palette_color = *w4.drawColors & 0b1111;    if (palette_color == 0) {        // Transparent        return;    }    int color = (palette_color - 1) & 0b11;
    // Write to the framebuffer    w4.framebuffer[idx] =        cast(ubyte)((color << shift) | (w4.framebuffer[idx] & ~mask));}

func pixel (x int, y int) {    // The byte index into the framebuffer that contains (x, y)    var idx = (y*160 + x) >> 2
    // Calculate the bits within the byte that corresponds to our position    var shift = uint8((x & 0b11) << 1)    var mask = uint8(0b11 << shift)
    // Use the first DRAW_COLOR as the pixel color    var palette_color = uint8(*w4.DRAW_COLORS & 0b1111)    if palette_color == 0 {        // Transparent        return    }    var color = uint8((palette_color - 1) & 0b11)
    // Write to the framebuffer    w4.FRAMEBUFFER[idx] = (color << shift) | (w4.FRAMEBUFFER[idx] &^ mask)}

local function pixel(x: integer, y: integer)    -- The byte index into the framebuffer that contains (x, y)    local idx = (y*160 + x) >> 2
    -- Calculate the bits within the byte that corresponds to our position    local shift = (x & 0b11) << 1    local mask = 0b11 << shift
    -- Use the first DRAW_COLOR as the pixel color    local palette_color = $DRAW_COLORS & 0b1111    if (palette_color == 0) then        -- Transparent        return    end        local color = (palette_color - 1) & 0b11;
    -- Write to the framebuffer    FRAMEBUFFER[idx] = (color << shift) | (FRAMEBUFFER[idx] & ~mask)end

proc pixel(x, y: int32) =  # The byte index into the framebuffer that contains (x, y)  let idx = (y * SCREEN_SIZE + x) shr 2
  # Calculate the bits within the byte that corresponds to our position  let shift = (x and 0b11) shl 1  let mask = uint8(0b11 shl shift)
  # Use the first DRAW_COLOR as the pixel color  let palette_color = uint8(DRAW_COLORS[] and 0b1111)  if (palette_color == 0) {      // Transparent      return;  }  let color = (palette_color - 1) & 0b11
  # Write to the framebuffer  FRAMEBUFFER[idx] = uint8((color shl shift) or (FRAMEBUFFER[idx] and not mask))

pixel :: proc "c" (x : int, y : int) {    // The byte index into the framebuffer that contains (x, y)    idx := (y*160 + x) >> 2
    // Calculate the bits within the byte that corresponds to our position    shift := u8((x & 0b11) << 1)    mask := u8(0b11 << shift)
    // Use the first DRAW_COLOR as the pixel color    palette_color := u8(w4.DRAW_COLORS^ & 0b1111)    if (palette_color == 0) {        // Transparent        return    }    color := (palette_color - 1) & 0b11;
    // Write to the framebuffer    w4.FRAMEBUFFER[idx] = (color << shift) | (w4.FRAMEBUFFER[idx] &~ mask)}

fn pixel(x: i32, y: i32){    // The byte index into the framebuffer that contains (x, y).    var idx = ((y as u32 * SCREEN_SIZE + x as u32) >> 2) as usize;
    // Calculate the bits within the byte that corresponds to our position.    var shift = (x as u8 & 0b11) << 1;    var mask = 0b11 << shift;
    // Use the first DRAW_COLOR as the pixel color.    var palette_color = (DRAW_COLORS & 0b1111) as u8;    if palette_color == 0    {        // Transparent        goto end;    }    var color = (palette_color - 1) & 0b11;
    // Write to the framebuffer.    FRAMEBUFFER[idx] = (color << shift) | (FRAMEBUFFER[idx] & !mask);
    end:}

proc pixel(x: i32, y: i32) {   // The byte index into the framebuffer that contains (x, y)   let idx = (y transmute usize * 160 + x transmute usize) >> 2;
   // Calculate the bits within the byte that corresponds to our position   let shift = (x truncate u8 & 0b11) << 1;   let mask = 0b11 << shift;
   let palette_color: u8 = (DRAW_COLORS~ & 0xf) truncate u8;   if (palette_color == 0) {      // Transparent      return;   }   let color = (palette_color - 1) & 0b11;
   // Write to the framebuffer   FRAMEBUFFER~[idx] = (color << shift) | (FRAMEBUFFER~[idx] & !mask);}

fn pixel(x: i32, y: i32) {    // The byte index into the framebuffer that contains (x, y)    let idx = (y as usize * 160 + x as usize) >> 2;
    // Calculate the bits within the byte that corresponds to our position    let shift = (x as u8 & 0b11) << 1;    let mask = 0b11 << shift;
    unsafe {        let palette_color: u8 = (*DRAW_COLORS & 0xf) as u8;        if (palette_color == 0) {            // Transparent            return;        }        let color = (palette_color - 1) & 0b11;
        let framebuffer = &mut *FRAMEBUFFER;
        framebuffer[idx] = (color << shift) | (framebuffer[idx] & !mask);    }}

(func $pixel (param $x i32) (param $y i32)  (local $idx i32)  (local $shift i32)  (local $mask i32)  (local $color i32)
  ;; The byte index into the framebuffer that contains (x, y)  ;;  idx = (y*160 + x) >> 2;  (local.set $idx    (i32.shr_u      (i32.add        (i32.mul          (local.get $y)          (i32.const 160))        (local.get $x))      (i32.const 2)))
  ;; Calculate the bits within the byte that corresponds to our position  ;; shift = (x & 0b11) << 1;  (local.set $shift    (i32.mul      (i32.and        (local.get $x)        (i32.const 3))      (i32.const 2)))
  ;; mask = 0b11 << shift;  (local.set $mask    (i32.shl      (i32.const 3)      (local.get $shift)))
  ;; Use the first DRAW_COLOR as the pixel color  ;; color = *DRAW_COLORS & 0b1111;  (local.set $color    (i32.and      (i32.load16_u (global.get $DRAW_COLORS))      (i32.const 15)))  ;; return if $color is zero, then subtract 1 and mask.  (if (i32.eqz (local.get $color)) (then (return)))  (local.set $color (i32.and                      (i32.const 3)                      (i32.sub (local.get $color) (i32.const 1))))
  ;; Write to the framebuffer:  ;; FRAMEBUFFER[idx] = (color << shift) | (FRAMEBUFFER[idx] & ~mask);  ;;  ;; Note that WebAssembly doesn't have a bitwise not instruction, so  ;; `~n` becomes `n ^ ~0` (where -1 is used for ~0 below).  (i32.store8 offset=0xa0    (local.get $idx)    (i32.or      (i32.shl        (local.get $color)        (local.get $shift))      (i32.and        (i32.load8_u offset=0xa0 (local.get $idx))        (i32.xor          (local.get $mask)          (i32.const -1))))))

fn pixel(x: i32, y: i32) void {    // The byte index into the framebuffer that contains (x, y)    const idx = (@intCast(usize, y) * 160 + @intCast(usize, x)) >> 2;
    // Calculate the bits within the byte that corresponds to our position    const shift = @intCast(u3, (x & 0b11) * 2);    const mask = @as(u8, 0b11) << shift;
    // Use the first DRAW_COLOR as the pixel color    const palette_color = @intCast(u8, w4.DRAW_COLORS.* & 0b1111);    if (palette_color == 0) {        // Transparent        return;    }    const color = (palette_color - 1) & 0b11;
    // Write to the framebuffer    w4.FRAMEBUFFER[idx] = (color << shift) | (w4.FRAMEBUFFER[idx] & ~mask);}

The PALETTE Register#

The DRAW_COLORS Register#

Other Shapes#

Direct Framebuffer Access#

The `PALETTE` Register#

The `DRAW_COLORS` Register#