mix-images-with-gpujs.js

const gpu = new GPU();
const kernel = gpu.createKernel(function (baseImage, overlayImages, overlayImagesCount, overlayImagesSizes, offsets) {
  const pixel = baseImage[this.thread.y][this.thread.x];
  for (let i = 0; i < overlayImagesCount; i++) {
    const offset = offsets[i];
    const size = overlayImagesSizes[i];
    if (this.thread.x >= offset[0] && this.thread.x <= size[0] - offset[0]) {
      if (this.thread.y >= offset[1] && this.thread.x <= size[1] - offset[1]) {
        const mixPixel = overlayImagesCount[i][this.thread.y - offset[1]][this.thread.x - offset[0]];
        //mix mixPixel with pixel
      }
    }
  }
  this.color(pixel[0], pixel[1], pixel[2], pixel[3]);
}, {
  graphical: true,
  output: [baseImage.width, baseImage.height]
});

// make canvas visible
document.body.appendChild(kernel.canvas);

const overlayImages = [overlay1, overlay2];
const overlayImagesSizes = overlayImages.map(image => [image.width, image.height]);
kernel(
  baseImage,
  overlayImages,
  overlayImages.length,
  overlayImagesSizes,
  [[10, 10],[50, 50]]
);

Actually, to use const offset = offsets[i]; in this way, you need to set the argument type to "Array1D(2)".