    displayZone.imageData = displayZone.ctx.createImageData(image.width, image.height);
        }

        function display() {
          // Unfortunatly, bitmap pixels are RGB and canvas expects RGBA.
          // So we have to convert pixel by pixel, and it is slow !
          // The canvas Alpha is set in createDisplayZone() with fill(0xff)
          // To improve performance, we should make the Web Assembly code
          // do the pixel conversion into an allocated RGBA pixels space.
          let len 