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Old 21st August 2009
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jdal jdal is offline
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JPEG/File Size/Image Size/Image Quality

There have been a couple of threads where people have asked about JPEGS for the web, and in particular how you reduce the file sizes, so I thought I'd start a thread to get peoples experiences in one place.

My 2d's worth:

First off, as almost everyone will know by now, JPEG is a file compression technique and a one which will always lose quality in the image, but not necessarily so you'd notice. It's a very complex algorithm, but there are a few simple controllable factors which affect the final size.

1. Obviously if everything else is the same, the smaller an image is the smaller the file will be. So after any jiggery-pokery on the image is complete, I pick on a suitable size and resize to it.

2. The sharper an image is, the larger the resulting JPEG will be. This is because the contrasty edges are harder to compress. Because of this I sometimes select out the subject matter and sharpen only that bit, particularly if it's something like a building in amongst trees (see 3).

3. The more small detail there is in the image, the larger the JPEG will be. I find forest scenes, heather and grass to be particularly prone to this.

4. Unless you're using a sophisticated 'jpegger' you then for the final step you have only the "quality" to play with. The lower the quality, the higher the compression ration and the smaller the image. Look out for artifacts on high contrast areas, particularly where things meet the sky. You may end up having to make the image a little smaller.

My final point would be that it's worth investing in something like Advanced Jpeg Compressor if you're doing lots of web images, it allows you much more control over the compression.


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