| Author | Thread |
|
|
07/21/2005 05:13:45 PM · #1 |
When compressing an image for the web in Photoshop (or any other image editing program) is there a way to specify, say by using selection masks, that certain areas receive a higher or lower degree of compression than the remainder of the image? For instance, if you compress an image to 90% of the original, and things look good, but you must compress to 75% to meet the size requirements, you would select areas and specify that they be compressed no more than 90%, then the remainder of the image would be compressed to meet the size requirement. I have noticed that often the majority of an image may look fine after compression, where certain areas may look jagged or just plain bad.
So, anybody know of anything like this in Photoshop or elsewhere?
|
|
|
|
07/21/2005 05:16:20 PM · #2 |
|
|
|
07/21/2005 05:33:20 PM · #3 |
In a way, all compression algorithms do this already as part of the basic procedure. Areas of smooth color can be expressed with a smaller amount of data than a detailed image.
Imagine a shot of the sky with a leafy tree on the right. A given row of the image might be expressed as:
(224pixels of Blue XXXXXX)ΓΆ€ΒΆ(2pixels of Green XXXXXX)ΓΆ€ΒΆ(3pixels of Green XXXXYY)ΓΆ€ΒΆ(2pixels of Green XXXXYX)ΓΆ€ΒΆ etc....
These are not (of course) actual codes, but a general description of how compression works. It's the amount of detail and color variation which determines how much compression's needed to reach a certain file size.
Hint: If you sharpen your images, try applying about half the usual sharpening and save a copy, then revert and save another copy sharpened normally for you. Compare the file sizes when saved at the same JPEG quality level.
I've often found that a little less sharpening, or reducing/removing a small specular reflection can make a difference of 20-50kb in final file size when saved at the same JPEG quality. |
|
|
|
07/21/2005 06:09:11 PM · #4 |
| I understand the basics and basis of compression, but I'm questioning more if there is a program or something built into Photoshop that allows you to control the amount of compression for a particular area of an image to prevent degredation in that area (or conversly, have an extreme amount of compression in a less important area). With the fairly stringent file size requirements here and elsewhere on the web...hell, basically the web in general...it seems that this type of compression would be welcome and useful. |
|
|
|
07/21/2005 07:08:53 PM · #5 |
| This is one of the differences between JPG and JPG2000. In the latter, you can specify regions of different compression levels. Don't know of any way to do this in JPG, which unfortunately is still the standard on the web. |
|
|
|
07/21/2005 07:19:07 PM · #6 |
| ...JPG2000? I've never heard of that, what is it? |
|
|
|
07/21/2005 07:52:06 PM · #7 |
Originally posted by singe:
...JPG2000? I've never heard of that, what is it? |
JPEG2000 is a major update to the JPEG file specification. It has the extension .JP2. Here is some info about it. Note that a JP2 file can use either lossy (like JPG) or non-lossy (like TIFF) compression. The JP2 compression algorithm is much diffferent than JPG, JPO2 is based on wavelets, and is somewhat more efficient while haveing lower levels of artifacts.
|
|
|
|
07/21/2005 07:54:48 PM · #8 |
| If anyone's interested, there's a Photoshop plugin for .jp2 files here. |
|
Home -
Challenges -
Community -
League -
Photos -
Cameras -
Lenses -
Learn -
Help -
Terms of Use -
Privacy -
Top ^
DPChallenge, and website content and design, Copyright © 2001-2025 Challenging Technologies, LLC.
All digital photo copyrights belong to the photographers and may not be used without permission.
Current Server Time: 09/17/2025 05:08:48 PM EDT.
