[ygra]

Since progressive JPEGs are displayed while downloading and the connection could just be closed at any moment anyway ... I don't think that'd be a problem. Whether that's more efficient than an extra thumbnail is probably the more interesting question.

[TapamN]

If you have time and space to pre-generate thumbnails, it's probably not a significant win, but I think it could work well for displaying local thumbnails of JPEGs, like from a camera.

If you're browsing a directory of hundreds of large (e.g. 10+ MB) JPEG photographs, generating the thumbnails by fully decompressing all of them would take while. "Progressive thumbnails" that only decompress the first ~100 KB would be much faster.

[mkl]

You can do that even with non-progressive JPEGs, as you can use just the low frequency terms from the discrete cosine transform (the same data that comes first with progressive ordering).

Epeg and libjpeg-turbo can do this.

[TapamN]

You would still have to read the entire JPEG in though, wouldn't you?

I'm not an expert on JPEG, but I think that if you want the macro blocks at the bottom of the image, you still need to un-Huffman the all the blocks before it to find where the macro blocks start (since AFAIK there isn't a table indicating where each block starts). That means you have to read the entire JPEG from storage, only to through away the vast majority of it.

Even if there was a way to magically predict where the low frequency values of the image are stored, you'd still have to do tens of thousands of random reads to just get to them. Reading the whole file would be faster.

So if you have 500 photos and you want to go though them and need some thumbnails, for non-progressive image thumbnail generation, you have to read 10 MB x 500 images = 5 GB of data, but with a progressive thumbnail you only need the first 100 KB x 500 images = 50 MB of data.

[mceachen]

As an aside, if you're just wanting thumbnails, most digital cameras encode small (120x160, ish) thumbnails in the EXIF header that can be quickly extracted by exiftool.

[mkl]

I don't know. These libraries claim to make thumbnails that way, and claim it's way faster.