// Change
 dy = Math.abs(y1 - y0)
 // to
 dy = -Math.abs(y1 - y0)


 // Change
 err = dx - dy;
 // to
 err = dx + dy

 // Change
 if (e2 > -dy) {
   err -= dy;
   x0 += sx;
 }
 // to
 if (e2 > dy) {  // removes the - operation
   err += dy;
   x0 += sx;
 }

// just before the loop
var column = bitmap[x0];

// then in the loop you only need to index the row
if (column[y0] === 1) {
  return 1;
}
if (e2 > dy) {
  err += dy;
  x0 += sx;
  column = bitmap[x0]; // and update the column only when x0 changes.
}

// using bitmap as a flat array with width as the pixel width
// Assuming coords are ints
function collision(x0, y0, x1, y1, width) {
  const dx = Math.abs(x1 - x0);
  const dy = -Math.abs(y1 - y0);
  const sx = x0 < x1 ? 1 : -1;
  const sy = y0 < y1 ? width : -width;
  const endIndex = x1 + y1 * width;
  var e2, err = dx + dy;
  var index = x0 + y0 * width;

  // loop through line drawing
  while (index !== endIndex) {
    if (bitmap[index] === 1) { return 1 }
    e2 = err << 1;
    if (e2 > dy) {
      err += dy;
      index += sx;
    }
    if (e2 < dx) {
      err += dx;
      index += sy;
    }
  }
  return 0;
}