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Here are the most noteworthy changes in CS4.

1. GPU acceleration and 64-bit. Depending on your view, this new feature may be exciting or disappointing. The bad news is that only canvas rotating and zooming support GPU acceleration. Adobe said it will make additional features available, such as the “Pixel Bender” component, which did not make it into the final version. The good news is that we finally see GPGPU acceleration in a commercial application and see more horsepower to deal with those huge image files. Combine this feature with 64-bit support and lots of RAM and you could see dramatic performance improvements beyond a factor of 10. Adobe, by the way, said that both AMD/ATI and Nvidia graphic cards will provide (OpenGL-based) acceleration, as long as a graphics card has at least 128 MB of memory.

Our review was limited to the 32-bit version, but the performance jump was obvious. We used an 80 megapixel (22.300 x 3600 pixels) picture with a file size of 720 MB and found that zooming worked as smooth as it does in a 3 megapixel image, while it was painful to do the same in CS3. Canvas rotation saw some delay, usually two to three seconds. However, this performance translates into a 6x improvement over CS3. We were told that a GPGPU-accelerated filter called Pixel Bender with GPGPU support will be offered as a free download from Adobe Labs for CS4 in the near future.

So far, GPGPU acceleration doesn’t go far, but the potential is clear. Adobe noted that CS4 is a first step into that direction and additional features that support GPGPUs are likely to be added down the road.

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The upgrade price of $199 ($699 for the full version) is a tough pill to swallow, but I feel comfortable recommending the upgrade to CS4 from any previous version.

According to Adobe, Photoshop CS4 will be shipping sometime in October.