I was skimming over Deke McClelland's, Photoshop CS Bible: Professional Edition, and I stumbled over a point worthy of note. Photoshop's scratch disk functionality allows us to edit enormous images, even images larger than our available RAM might permit.

Lots of filters in Photoshop use beaucoup memory. Sure, the simple advice is to add RAM. After all, RAM is cheap. However, the BIOS can place a practical limit of 1 GB, even less. If a filter runs out of RAM, that's it. It stops dead in its tracks.

Deke mentions a workaround . . . Try applying the same filter to each color channel separately. An RGB or L*a*b image would require 1/3 the RAM, a CMYK image image would require 1/4 the RAM.

Deke's technique will not work with a small handful of filters. Lens Flare, Lighting Effects, and NTSC Colors require a full-color composite image. You'll know when a filter cannot work on an independent channel. It's menu option will gray.

Oh, about Deke's book. It's an excellent book! Worthy of every digital photographer's bookshelf.

Photoshop CS Bible: Professional Edition at Amazon.com

Cheers!