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04/05/2005 07:27:18 AM · #1 |
I have a feeling this is one of those basic questions, but being new to digital photography I have to admit, I know very little.
What are Jpeg artifacts? Where do they come from and how do you prevent them?
Thanks,
Barry
PS I'm sure more questions will follow this one. |
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04/05/2005 07:38:37 AM · #2 |
JPEG is a compression method used to reduce the file size of photos and pictures. It is lossy, meaning that one one hand it reduces the amount of data you have to store, up- and download, you will lose quality, every single time you save your picture.
If you compress your photos too much, small artifacts will become visible, small distortions and wrong colours.
Try to compress your submissions all little as possible whilst staying under the 150kb limit on DPC.
Edit: Eddy G's Tutorial might interest you.
Message edited by author 2005-04-05 07:43:45.
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04/05/2005 07:41:26 AM · #3 |
Uncompressed:
(Badly) Compressed:
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04/05/2005 08:25:35 AM · #4 |
Thanks for the reply "gloda".
Can cropping images cause artifacts? Or does cropping make the problem more visable?
Thanks
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Barry |
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04/05/2005 08:32:34 AM · #5 |
cropping doesn't 'create artifacts' -- cropping is just removing a portion of the image around the outside. Resizing might be what you're asking, but I don't know the answer if it can cause it. As far as I know resizing can cause your image to lose sharpness, but doesn't cause articafts...but I've been wrong before.
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04/05/2005 08:42:20 AM · #6 |
I don't know if this is related to the question that Barry asks, but cropping can make any noise in your photo more visible, similar to using digital zoom. If your original photo is 2272x1704 pixels and you crop it to 1500x1200, then resize the photo down to 640x512 (as you would for a DPC challenge entry) - the cropped photo at the final size will show more noise than the original non-cropped photo would at the same final size.
I hope I worded that correctly. ;^)
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04/05/2005 08:49:55 AM · #7 |
Barry,
Severe cropping (cropping cown to a small area in the image) can cause the image to look blocky, and increase the visibility of jpeg (and other) artifacts.
A good rule of thumb to follow is that if your final image size will be 640x480, for example, don't crop your original any smaller than twice that size (1280 x 960). Then resize down to 640x480.
When saving, try not to save below 60% quality, and use the highest quality setting that you can while keeping your desired file size. If you are using Photohop, you can use the "save for web" function, it is the most flexible.
The final thing to avoid is, as pointed out above, repeatedly opening, editing, saving, and closing the file. You'll lose a bit of qulity each time. Repeatedly saving an open file is fine, though.
If you want to experiment, start with an image wiht some detailed areas and some smoth areas of color (a landscape with sky works well). "Save as" different versions, each with different quality settings. Code the quality setting you used into the file name. Then open each, and look at the differences. You'll start to see what folks are talking about.
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04/05/2005 11:16:20 AM · #8 |
Thank yous go out to all who replied to my question. I have a bit to chew on now and will try to digest the information.
Some much to learn.............
Thanks again,
Barry
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