Does ShortPixel Adaptive Images serve Retina images?
ShortPixel Adaptive Images can serve images for Retina displays when it finds images that have the proper dimensions. Meaning that if you have a Retina counterpart for your original image, then SPAI will be able to serve it. This makes ShortPixel AI compatible with all the plugins that generate Retina images.
It is important to note that SPAI doesn't generate Retina images. You need to use another plugin like WP Retina 2x so SPAI can serve them.
Example 1: If your original image is 600px wide and you have a Macbook with a Retina display (DPI 2x), SPAI will serve a 1200px image only if you previously generated a Retina image with any tool you could find.
Example 2: If your original image is 2048px wide, but the placeholder of the image is 551px wide, and you have a high-density display with a DPI of 1.5, SPAI will serve a 827px (551*1.5) version based on the original image.