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Photoshop/Mac tech-talk


Sam P
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Ed,

 

At the risk of going slightly off topic, can you tell me what aspects of the hardware Photoshop tends to tax most? Obviously, you're getting this G5 with manic performance partly so you can do your work with greater ease.

 

I'm still undecided on my laptop, and I might still go the Powerbook route.

 

I do use Photoshop a fair bit, so it would be a bonus if it ran well on the 1 Ghz 12" Powerbook. Do you think it will be up for the job? It's only got 256MB of ram and 32MB of video ram.

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Sam, it depends on what size images you work on. The rule of thumb is that to get maximum performance out of Photoshop, you should have 5x more the RAM available than the size of your file. So if you open a 10MB file, you should have 50MB available for photoshop to use.*

 

If Photoshop has less than that, it uses your HD as a scratch disk, writing undo states and other stuff to the drive instead of to RAM, which obviously dramatically slows down your work. When working from RAM, though, the speed of the RAM and speed and width of the bus come into play, and this is where the G5 shows the most improvement..going from a 133Mhz bus to 1Ghz!

 

As for the G4, it should do fine unless you are working on hi-res images (print resolution). The AltiVec instruction set does wonders for many PS operations, making it a fast CPU even though it's hampered by relatively slow bus, etc.

 

*Note that this is the size of the file uncompressed, so if you open a 2MB JPG file that's compressed 10:1, the file will be 20MB while open and you should have 100MB RAM available.

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Ed,

 

Thanks! That was very helpful.

 

Also, does video ram ever come into play? What about if I am moving a relatively large image across the screen, say while scrolling it, will this be taxing the Video card's ability or is that still within the realm of the CPU?

 

Are there any other areas where the 32MB of video ram will not be enough?

 

Also, can you briefly explain what the AltiVec Instruction Set is?

 

Sorry for all the questions! :oops:

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Video RAM comes into play in displaying the graphics on-screen, but has nothing to do with running filters and operations on images. Scrolling is a video card task, and I suppose video RAM could come into play if you don't have enough...but 32MB is plenty.

 

If you want to drive a second monitor (like an external) then video RAM gets used (unless you're just mirroring the main display).

 

AltiVec is a set of instructions added to the PPC instruction set for accelerating certain kinds of mathematical operations. These operations are common in graphics programs, including video compression, certain kinds of transforms, etc. SSE is Intel's version, and is regarded as inferior to AltiVec, for reasons I don't understand fully.

 

AltiVec is actually a separate unit on the CPU, similar to the FPU and Integer units. Programs compiled to use AltiVec can gain a huge performance leap for operations that use the kinds of operations that AltiVec handles, in some cases more than 5x faster.

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