img-resize-canvas.js

export function drawImageProp(ctx, img, x, y, w, h, offsetX, offsetY) {
  if (arguments.length === 2) {
    x = y = 0;
    w = ctx.canvas.width;
    h = ctx.canvas.height;
  }

  // default offset is center
  offsetX = typeof offsetX === 'number' ? offsetX : 0.5;
  offsetY = typeof offsetY === 'number' ? offsetY : 0.5;

  // keep bounds [0.0, 1.0]
  if (offsetX < 0) offsetX = 0;
  if (offsetY < 0) offsetY = 0;
  if (offsetX > 1) offsetX = 1;
  if (offsetY > 1) offsetY = 1;

  const iw = img.width;
  const ih = img.height;
  const r = Math.min(w / iw, h / ih);
  let nw = iw * r; // new prop. width
  let nh = ih * r; // new prop. height
  let cx; let cy; let cw; let ch; let
    ar = 1;

  // decide which gap to fill
  if (nw < w) ar = w / nw;
  if (Math.abs(ar - 1) < 1e-14 && nh < h) ar = h / nh; // updated
  nw *= ar;
  nh *= ar;

  // calc source rectangle
  cw = iw / (nw / w);
  ch = ih / (nh / h);

  cx = (iw - cw) * offsetX;
  cy = (ih - ch) * offsetY;

  // make sure source rectangle is valid
  if (cx < 0) cx = 0;
  if (cy < 0) cy = 0;
  if (cw > iw) cw = iw;
  if (ch > ih) ch = ih;

  // fill image in dest. rectangle
  ctx.drawImage(img, cx, cy, cw, ch, x, y, w, h);
}