Draw into a canvas

"use typeshade";

/* @example
{
  "title": "Host: draw into a canvas",
  "blurb": "A full-screen `@fragment` entry and the host that draws it. The host imports `fs` and calls `fs(canvas, { frame })` once a frame, with the time in the uniform: one full-screen triangle, on WebGPU, then WebGL2, then the CPU tier, with no pipeline, buffer or bind group in the host's code (Rule 8.24).",
  "renderable": false,
  "reason": "no entry point"
}
*/

// What the host sets each frame: the time, and the canvas's size in pixels.
class Frame {
  time: f32;
  size: vec2;
}

declare const frame: uniform<Frame>;

// A full-screen fragment entry reads no builtin but `position`, and writes one colour, so a host
// draws it into a canvas with no vertex entry of its own.
@fragment
export function fs(@builtin("position") p: vec4): vec4 {
  const uv = p.xy / frame.size;
  const wave = 0.5 + 0.5 * sin(6.2831853 * uv.x + frame.time);
  return vec4(uv.x, uv.y, wave, 1.);
}

// Draw a full-screen fragment entry into a canvas (surface §67, Rule 8.24).
//
// The import is the shader module itself: the Vite plugin (`typeshade/vite`) turns `fs` into a
// function that draws one frame. The first draw into a canvas picks its tier: WebGPU, then
// WebGL2, then the CPU tier, which paints the frame pixel by pixel.
import { fs } from './host-draw.shade.ts';

/** Draw one frame at `time` seconds. The promise resolves when the frame is submitted. */
export function draw(canvas: HTMLCanvasElement | OffscreenCanvas, time: number): Promise<void> {
  return fs(canvas, { frame: { time, size: [canvas.width, canvas.height] } });
}

/** Animate `canvas`: a new uniform value each frame. A frame loop need not wait for a draw. */
export function animate(canvas: HTMLCanvasElement): void {
  const frame = (ms: number): void => {
    void draw(canvas, ms / 1000);
    requestAnimationFrame(frame);
  };
  requestAnimationFrame(frame);
}

/** Draw the frames at `times`, one after another. The canvas then shows the last one. */
export async function run(
  canvas: HTMLCanvasElement | OffscreenCanvas,
  times: readonly number[] = [0, 0.5, 1.25],
): Promise<void> {
  for (const time of times) await draw(canvas, time);
}