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Progressive JPG is a image format, that contains low quality snapshots. These snapshots can be displayed during the transmission of the higher quality snapshot, which gives better end-user experience.

Progressive JPG is a image format, that can be progressively displayed in the web browser during its transmission.

Can end-user benefit form the progressive JPG, if the file is transferred over HTTPS? If the image is encrypted, snapshots would have to be separated...

user2449761
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  • In ye olde days of dial up modems, there was some advantage to using progressive jpeg incrementally. Now, unless the image is very large, there really is not much advantage in getting the fade in effect. One problem is to decide when to update the image. In theory you can break an image into over a hundred scans. Refreshing after each one would be time consuming. Plus the scans have to be ordered correctly to get the best effect. Progressive JPEG can provide better compression in many cases than sequential JPEG if you tune it right, so it's still a useful format. – user3344003 Apr 15 '15 at 18:51

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Progressive JPG is a image format, that contains low quality snapshots

No, that’s not true¹. It is only one image – but saved in a way, that some ”broad”, lower-detail information is encoded first, and then more detailed data is coming later on, so that rendering can start early in lower quality, and as more details become available, the rendering can become more refined.

And serving such an image via HTTPS does not change that.

¹ Images can contain additional thumbnail version(s) inside the EXIF meta data – but that is not what the “progressive” format is about. And that feature is seldom used on the web, because it would increase the overall amount of data to be transferred (and I am not even sure if common browsers support this, in that they would display such a thumbnail first and then the full image once it is done loading.)

CBroe
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  • so the packets are encrypted, not the whole file? – user2449761 Apr 15 '15 at 11:39
  • Strictly, HTTPS is not a protocol in itself, but only HTTP send over a connection that is encrypted via SSL resp. TLS. And usually the encryption algorithms used are _block cipher_ algorithms, meaning not the whole data is encrypted in one go, but only data blocks of a fixed length are encrypted individually. – CBroe Apr 15 '15 at 11:46