"Warp" <war### [at] tagpovrayorg> wrote in message
news:3d7341da@news.povray.org...
> Pandora <pan### [at] pandora-softwarecom> wrote:
> > that hit will be negligible.
>
>   Nope. Accessing the disk, even when cached by the OS, is always very
very
> much slower than accessing a memory location (given that it's not swapped
to
> disk, of course). Of course the disk cache makes the operations a lot
faster
> in the average (compared to the case where each disk read really reads
data
> physically from the disk), but they are still very much slower than a
direct
> memory access.
>


    I know. But if the buffering is written intelligently you can minimise
the hit - a bit like the buffering currently used in POV, but with the added
ability to be able to read & write to any part of the image at any time -
sucking a whole 1Mb off disk in one go, adjusting (i.e. reading the data
that's there an updating it or writing new data based on what's there) half
of it and then writing the whole 1Mb back in one go is far faster than
reading/writing 1/2Mb in single byte steps...
    Of course, direct memory access is quicker, but if you're trying to
anti-alias an image with a width of say 400,000,000 pixels (that's over 1GB
per line!) when you've only got 1/2GB of physical memory is going to fail
each and every time, unless you have read-write buffering... (yes, this is
an _extreme_ example, but it illustrates my point quite nicely - the same
applies to much smaller images once you take into account the memory used
for the scene description, program & O/S)

--
Pandora/Scott Hill/[::O:M:C::]Scorpion
Software Engineer.
http://www.pandora-software.com

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