jpg question

thebaz

Can someone clarify does a jpg only loose quality , when you actually save the file -- can you mess about with layers and flatten , without any damage , i.e. its only when file is actually saved that quality is compromised.

cheers

Fajitas!

Yep!

thebaz

thanks Al - wasn't sure if flattening actually damaged the jpg

Karoma

When you flatten: You've most likely changed the value of pixels (information) in the JPEG. It's going to remove some information, and add some...
Work with the histogram open to get some idea of what's going on.

thebaz

Now , i'm confused -- who is right -- Karoma or Al ?

does flattening degrade a jpg ?

cheers

5uspect

They're both right. Since your JPEG is only open in memory and you don't save it the file is not written to. You only degrade the jpeg if you save it. Doing stuff in PS will add new information to the existing file. Think of it like opening a Word Document and adding another paragraph of text. (fortunately doc files aren't lossy!)

Timpa Bylon

thebaz wrote: »

Can someone clarify does a jpg only loose quality , when you actually save the file -- can you mess about with layers and flatten , without any damage , i.e. its only when file is actually saved that quality is compromised.

cheers

When you open JPEG file and add a layer to it (as in Photoshop), you cannot save it as a JPEG with layers. It will automatically flatten (reduce the layers to 1) before saving.
The other property of JPEG files is that every time you save them there is a certain amount of compression even at the highest quality setting which affects the quality of the image. If you keep editing and saving the same image, it will eventually begin to degrade noticeably.
Your best bet is to save the image as a Photoshop file (or PaintShop Pro or whatever you use) which is NOT compressed and regard that as your master file, then flatten the image and save as a JPEG for distribution to others by e-mail or whatever it is you do with them and that is your copy file. You then have the uncompressed image to edit again if you so wish.

Any better?

thebaz

thanks Timpa -- you can flatten without saving -- i just wondered if this flattening in itself degraded jpg quality - i know that changing then saving does reduce jpg quality

sheesh

wouldn't think so. ps will not change the values assigned to each pixel the reason you get artifacts is that to save to a jpeg it will carry out an algorithm on the image data changing the colours of pixels so that they match the surrounding pixels thereby making the file smaller. this leads to a loss of the more subtle colour differences in an image and larger more visible artifacts in a file that is repeatedly saved.

If that makes any sense!:)