D.5 Canvas Graphics Intrinsics

The WASM backend provides intrinsics for canvas-based graphics with animation support:

Intrinsic	Args	Description
`__canvas_init__(width height)`	2	Initialize canvas and return RGBA pixel buffer pointer
`__canvas_width__()`	0	Get current canvas width
`__canvas_height__()`	0	Get current canvas height
`__canvas_present__()`	0	Copy pixel buffer to canvas (display frame)
`__canvas_set_aspect_lock__(enabled)`	1	Enable or disable aspect-ratio locking using a udewy `bool`
`__frame_count__()`	0	Get current animation frame number
`__frame_time__()`	0	Get milliseconds since canvas initialization
`__window_width__()`	0	Get browser window inner width
`__window_height__()`	0	Get browser window inner height

Usage:

Call __canvas_init__(width height) to create a canvas and get a pointer to the pixel buffer
Optionally call __canvas_set_aspect_lock__(true) to keep the displayed canvas centered at its current aspect ratio as the browser window resizes
Call __canvas_set_aspect_lock__(false) later if you want to return to unrestricted fullscreen scaling
Write RGBA pixels (4 bytes per pixel) to the buffer: [R, G, B, A, R, G, B, A, ...]
Call __canvas_present__() to display the frame
The runtime automatically calls main() each animation frame when canvas mode is active

The pixel buffer lies past the memory the module asked for (its data, static reservations and stack), so it never overlaps program storage. Calling __canvas_init__ again with a size that fits keeps the buffer; a larger size moves it, so use the returned pointer.

A Dewy program's top-level code runs on the first call to main only; later calls (one per animation frame) run just the program's main function, so module state persists between frames.

__canvas_set_aspect_lock__(enabled) expects a udewy boolean value, normally passed as the true or false literals. Internally, any non-zero value enables the lock and 0 disables it.

When aspect lock is enabled, the runtime uses the canvas's current backing dimensions, typically the width and height passed to __canvas_init__(), as the aspect ratio to preserve.

Example:

let buffer:int = 0
let width:int = 320
let height:int = 240

let set_pixel = (x:int y:int r:int g:int b:int):>int => {
    let offset:int = ((y * width) + x) * 4
    let addr:int = buffer + offset
    __store_u8__(r addr)
    __store_u8__(g addr + 1)
    __store_u8__(b addr + 2)
    __store_u8__(255 addr + 3)
    return 0
}

let main = ():>int => {
    buffer = __canvas_init__(width height)
    let t:int = __frame_time__()
    
    # Draw something based on time...
    
    __canvas_present__()
    return 0
}

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D.5 Canvas Graphics Intrinsics