What causes this?

[tfa8rva]

Whenever I edit a jpg in Photoshop 7 and export it, I get some sort of artifact or something in the solid colored areas. In the photo below, look in the sky and you will see curved lines or smears. What causes that? In the original its solid blue.

http://farm3.static.flickr.com/2027/2305612716_58d8cf749f_o.jpg

[BBPhoto]

I have seen this happen a lot in jpg compression. I've heard of one or two cases where the compression algorithm became corrupt and a reinstall of the software cured it but often it is just how the software works. You could try some different settings.

[TheWengler]

I've had this happen to me when I was outputting a low quality jpeg. When I increased the quality it either removed the problem or greatly reduced it.

[Dread Pirate Roberts]

I don't know how much you know so appologise if I only tell you what you already knew.

Looks like jpg artifacts most likely from overcompressing. They can also be caused by editing if the original file is also a jpg in which case after performing any operations on the file in PS look at the histogram for evidence of clipping. If the histogram still looks fine just before you save it and the saved version looks like this then it's definately a save problem.

When you're saving as a jpg experiment a little to find a setting that still leaves a file size of no less than 700KB - 1MB per image. On CS2 this is a setting of around "quality 8" or better. Only go lower quality for web posts.

Compression varies a lot from image to image too, large areas of solid colour are where you see it and photos with lots of solid colour compress to smaller files.