[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Re: how to detect if a jpeg file is progressive or not

Wayne Topa wrote:

> Having never heard of "a progressive jpeg" I was interested in your
> query, and the answer you received.  
> I tried out the conversion to "progressive" on some of my large jpegs
> to see if it would help (as I have the same problem you have, dialup).
> I used the suggested convert command on a 77K jpeg and it was
> converted to 68K.  Not bad but I have been using a different convert
> option and getting much better results.

I also noticed the small reduction in file size in progressive jpeg
files. However, converting a jpeg to progressive is not really a file
compression in the usual sense. All it means is that the global image
information is sent first to the client with which the client can render
the low resolution image. As further detail is received by the client,
the image is re-rendered in ever increasing details. Thus the user
doesn't need to download the whole image before s/he can view it -- and
if the used decides not to view the image, s/he can stop the transfer.

But converting a jpeg to progressive somehow has the effect that the
progressive jpeg file is slightly smaller than the non-progressive one,
but the client then uses up more RAM to reconstruct the image -- not
that that will be a problem in most computers today.

> convert -quality 25 infile.jpg outfile
> That option got the 77K file down to 24K which is more manageable, for
> me anyway.  I see no difference in the pictures but the size.

This is a case of compression with a loss in quality. Note that you can
still have the new smaller image either as progressive or non-progressive.

> I would be interested in hearing your results with this option.

I am still playing around to decide what size (in pixels) images to
upload for others to view and how much quality to encode the jpegs with.
In any case, I am planning to upload the larger size images only
progressive jpegs no matter what quality I use them.


Reply to: