Problems with JPG

[mike_and_alex]

When posting pictures to this site I encountered the following

problem. If you look at the http://www.photo.net/photodb/photo?

photo_id=2214150 photo (Sunny day in Sintra in my Portugal folder),

you will see strange "dirty" effects on the sky along the line of the

roof. When I open the same photo with Internet Explorer on my PC, I

see no effects at all - an even blue color of the sky. I had this

effect before when saving photoes for web with low quality, but this

one is High... What can this be?<div></div>

[mike_and_alex]

It's getting more confusing - I don't see the effect in picture attached to the previous message, but I do see them when I view it in my gallery!

[michaelkh]

As I understand it, photo.net processes JPEGs to reduce their size, by dropping the quality. I seem to recall reading that this is less likely to happen to your images if they aren't saved as high quality.

[mattvardy]

http://www.photo.net/learn/jpeg/

 

See the above link to learn everything there is to know about JPEG's and PNet JPEG compression

[mike_and_alex]

Thank you for your comments, but sorry, I still do not understand what is happening :-)

The picture was saved in high quality. When I attach it to the message, as I did in this forum, the image on the web site does not differ from that on my computer. But when I upload THE SAME file to my gallery, I see some strange effects on the high-contrast margin between the sky and the roof instead of the veven color you can see above. The most confusing thing is that the same file uploaded to different parts of the site is shown differently.

[mottershead]

Images attached to forum posts are not recompressed. The recourse in the forums if images are too big is not to display them at all, but rather to give people a link. This is what happened with the version of the image that you posted here.

 

In the Gallery that approach does not make sense. All images are recompressed so that they are small enough to download in reasonable time. This recompression has different effects depending on the image. In most cases, the photographer is much more aware of artifacts than any viewer will be. The compression artifacts are rarely visible, and generally don't interfere with the ability of others to provide input on the image, which is the purpose of the Gallery. It is much more important that the majority of viewers not have to wait too long for the images to download. While the number of people with broadband connections and large monitors is increasing steadily, many viewers are still on dialups and the most common monitor size is still 800x600 pixels.

[gordonr]

I am the author of the article on <a href="http://www.photo.net/learn/jpeg/">Jpeg Compression</a> (http://www.photo.net/learn/jpeg/). The problem with this image (and another one last week) is that the chroma subsampling is 2X2 (should be 1X1).

<p>

Brian: Can you please check if the recent server upgrade has changed the default setting for Imagemagick!? If so there could be a lot more complaints about images with sky and edges...

[gordonr]

For the record - many images submitted recently seem to have the 2X2 chroma subsampling, although few show obvious artefacts.

 

This is what your image should look like when recompresed (45KB rather than 34KB).

[mike_and_alex]

Thank you very much for your comments - I do appreciate your attention. I must apologize for my lack of technical knowledge :-), but as far as I understand, the whole issue is a result of problems with the software settings you are going to fix. If so, I am glad that we found this problem before it destroyed other photos, much better than mine :-)

Thanks a lot again.

[gordonr]

I don't control anything, and I haven't heard from "the powers that be", so I'm not sure if it will be fixed...