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Photography and Video Helping you take better photos and videos

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Old 09-05-2008, 05:32 PM
DanClark's Avatar
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Location: Bellevue, WA, USA
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Default Re: tF picture quality

Quote:
Originally Posted by RickChristopherson View Post
It happens because the algorithm used for the downsampling is not as powerful as a true editing system. The same is true for most non-photo editing software (like word processors, etc.).

If you down sample from 300dpi to 150dpi, the conversion isn't so bad because it just merges four pixels into one. However, if you were downsampling from 300dpi to 200dpi, then it gets more complicated because it is not an even division of pixels.
Rick is correct. Overall, you will almost always get better results from decent image processing software. For example, PhotoShop CS3 defaults to a bicubic algorithm for resizing. And it has several other algorithms available for image resizing. I've had the best results with standard Bicubic, followed by applying a little Unsharp Mask at between 30 and 50%.

Also remember that this is running on a server that is being shared with other processes. Any library must balance speed versus quality. On a server, speed will almost always win. (No one wants a slower forum.)

But I was not referring to investigating the cause. I was referring to our server setup...

Currently we are using the php GD library for image processing. There is another library called ImageMagick that is supported by vBulletin. However, it's a fair amount of work to set it up, users have reported several issues with it, and it's not clear that ImageMagick can be installed on our server with the necessary permissions. Finally, it's not clear whether we would get appreciably better results with ImageMagick.

Regarding image processing, my work flow is:

1) Crop image to remove extraneous parts and focus on the imporant elements.

2) Adjust lighting and color if necessary. (I typically just use "Levels" in PhotoShop. "Image --> Adjustments --> Levels..." in PhotoShop CS3 or "Enhance --> Adjust Lighting --> Levels..." in PhotoShop Elements 6.)

3) Resize image. ("Image --> Image Size..." in PS CS3 or "Image --> Resize --> Image Size..." in PSE 6.)

4) Sharpen lightly ("Filter --> Sharpen --> Unsharp Mask..." in PS CS3 or "Enhance --> Unsharp Mask..." in PSE 6.)

5) Export for the web. ("File --> Save for Web & Devices..." in PS CS3 or "File --> Save for Web..." in PSE 6.)

I hope this helps. To assist others, I'm adding this in the help forum and the FAQ.

Regards,

Dan.
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