Detect DPI information for a PNG image to determine if it's retina quality or not. This can be done by reading optional physical pixel dimensions chunk which both macOS and Windows attach to the image data.

determineRetinaUpload.js

import extract from "png-chunks-extract";

/**
  * Detect if PNG image is in retina resolution.
  *
  * PNG Chunk documentation: https://www.w3.org/TR/PNG-Chunks.html
  */
const determineRetinaUpload = async (imageUrl) => {
  let retinaImage = false;
  const response = await fetch(imageUrl);
  if (response.ok && response.headers.get("content-type") === "image/png") {
    // Read image content into buffer and extract PNG chunks
    const buffer = await response.arrayBuffer();
    const chunks = extract(new Uint8Array(buffer));
    // Decode physical pixel dimensions chunk
    let pHYs;
    const pHYsChunk = chunks.find(chunk => chunk.name === "pHYs");
    if (pHYsChunk) {
      const metersToInchMultiplies = 39.3701;
      const buffer = Buffer.from(pHYsChunk.data);
      const xDpu = buffer.readUIntBE(0, 4);
      const yDpu = buffer.readUIntBE(4, 4);
      const xDpi = Math.round(xDpu / metersToInchMultiplies);
      const yDpi = Math.round(yDpu / metersToInchMultiplies);
      const unit = buffer.readUIntBE(8, 1) === 1 ? "meter" : undefined;
      pHYs = {
        xDpu,
        yDpu,
        xDpi,
        yDpi,
        unit
      };
    }

    if (pHYs) {
      retinaImage =
        pHYs.unit === "meter" && pHYs.xDpi >= 144 && pHYs.yDpi >= 144;
    }
  }

  return retinaImage;
};