Imaging - Lossy File Formats

[Kov-Ice]

I know JPG files are lossy, meaning that when they are opened and closed they lose file info. Simply moving the file from place to place doesn't change the file compression in any way, tho, right?

What about video files? I think I read MPEG is a lossy format as well. Is that true of AVI files?

[doctorgonzo]

AVI is apparently a container format, not an actual compression format. AVI files can contain video encoded in many different ways, lossy and lossless. MPEG is lossy.

Moving JPGs won't affect quality, nor will simply opening them. Editing may degrade quality, depending on the program you use to edit them. I've heard that the effect of "bit rot" on JPG files is exaggerated in real life.

[RJ]

Yep, AVI is only a container. The video inside can be encoded using many different codecs.

For example Huffyuv, it's a lossless codec. MJPEG is also lossy. . you can compare it to JPEG. Every frame in the MJPEG is a JPEG, hence the name.
DV is also a lossy codec.
And DivX, XVid etc. derive all from MPEG-4, they're all lossy codecs as well.

Though DV and MJPEG are lossy, they are far less lossy than a typical MPEG, for example.

RJ

[Kov-Ice]

On JPGs, I was looking through a Photoshop book I got as a gift, and the author alludes to this problem. Opening, editing, and closing multiple times means multiple compressions. I guess that leads to loss of image quality and pixelation. He suggests converting to TIFF files and editing from there, as they are lossless.

Here's a Photoshop sidenote... When I save as JPG in Photoshop, the Save As box shows up with a compression slide bar. They can be save from 1-12 on the scale, with 12 being Maximum Quality. Saving at 10 keeps the file size very close to the original. However, when I save a 2.5MB pic on 11 or 12 on the scale, my file size becomes larger than the original. Why is that, and what's the better choice for best quality?

[doctorgonzo]

Saving it in a lossless format is a good idea if you are going to be doing a lot of editing. I store all my pictures in PNG format.

I'm not sure why the file would be larger than the original when you save as an 11 or 12. The best choice for quality is up to you, but 9 or 10 sounds right.

[mairving]

I usually save as PSD's for another reason. That is it doesn't flatten the layers which makes for easy editing.

[Statica]

While on the subject of Photoshop, I wish the JPEG 2000 file format would get more popular; it's a very good format that has the ability to be lossless or lossy.

[Kov-Ice]

I normally save it at the 10 level, as I just felt like adding to the size of the file couldn't possibly improve quality.

Preserving the layers with PSD files is fine while editing, but as for storing photos long-term, I figure JPG is the choice. It's as standard as file types get, I suppose.

HERE is a neat little JPEG 2000 comparison, btw.