First, you can send less data using lower-?¬?delity images, compressed textures,
or decimated models. Alternatively, you can distribute the graphics data transfers
over a longer time span. That is, you can spread the data you??™re downloading
among multiple frames of rendering. You sometimes see the results of
a technique like this as a progressive improvement in image ?¬?delity over several
frames rendered, as more and higher-?¬?delity models and textures are transferred
and rendered.
A third bus also has relevance for graphics programmers: the bus bandwidth
available on the actual graphics card on which you??™re rendering. Most modern
graphics hardware have much faster busses than either the graphics or memory
busses described thus far. Typically, these are approximately an order of
magnitude faster than their counterparts farther upstream. Current GPUs have
memory bandwidths in the 10s of gigabytes per second. In most applications,
you probably won??™t be bound by this bus, but, as always, it??™s possible to get
bottlenecked here as well. Never say never! It??™s vastly more common to see an
application have dif?¬?culties managing data computation, copies, and transfer
to the graphics card.
Video Memory: VRAM
Video memory on graphics hardware is a complex and opaque topic. Modern
graphics cards have lots of video memory??”amounts that for old-timers in the
computing industry were once large amounts to have on complete computing
30 Chapter 3: Mac Hardware Architecture
systems, let alone subsystems like graphics.
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