There are two type of JPEG picture file.
- Regular JPEG, or baseline JPEG.
- Progressive JPEG, or Interlace JPEG.
It is very common, same quality picture, progressive jpeg is smaller than regular jpeg. And also progressive jpeg can show the content when the picture is download. This kind of show is to shown the whole picture with low quality then clear and clear, until whole picture is downloaded.
So, of course, if it is possible, we alwasy want to use progressive JPEG. Some old files need to be convert to progressive. But, before that, we have to identify which JPEG file is regular, which one is progressive already.
There is no simply way to check if a JPEG file is progressive on Windows.
I have CentOS server. It has ImageMagick.
I would like to introduce how to check if JPEG is progressive with ImageMagick.
identify -verbose file.jpg | grep Interlace
If the result is Interlace:JPEG it is progressive. If it say Interlace:None, then it is a baseline.