Apple Motion Manual do Utilizador descarregar pdf (Página 18)

System Requirements xvii

Each frame of 8-bit NTSC video contains 720 pixels horizontally and 486 pixels vertically

(480 for DV). When you add the memory required to store every one of those pixels in

the computer’s memory, it works out to about 1.3 MB (including an alpha channel). A full

raster 8-bit 1920 x 1080 HD video requires about 8.3 MB for each frame.

So if you want Motion to generate a real-time preview of your three-layer composite that

lasts for 120 frames (about four seconds), you need to multiply the memory requirement

for a single frame by the number of objects onscreen at the same time, and then multiply

that by the number of frames you want to play in real time: 1.3 x 3 x 120 = 468 MB. For

HD video, the memory requirement would be about 3 GB.

To adjust the three clips in your hypothetical composite in real time, you need at least

468 MB of free RAM, or 3 GB for HD. And that’s beyond the RAM used by the operating

system, Motion, and other background applications. So for this scenario to work well, you

need at least 1 GB of system RAM for NTSC, or 4 GB for HD. (PAL-format video requires

essentially the same amount of RAM as NTSC video: Although the images are 720 x 576,

there are only 25 frames each second.)

But all you really need to know is that if you have more system RAM, you can play more

objects in real time and watch a longer preview of your composite than you can with

minimal RAM. However, this is only part of the story.

Video Card Memory (VRAM)

In addition to your system RAM, your Macintosh also has memory on the graphics card,

known as VRAM (video RAM). It’s used by the graphics card while performing calcula-

tions to draw an image to your computer monitor. Your graphics card also has its own

processor, called a GPU (graphics processing unit), that calculates how images should be

drawn.

Every time Motion draws a frame of your composite on the monitor, it sends one frame

of each object in your composite to the VRAM of the graphics card, along with a set of

instructions telling the processor on the graphics card what it’s supposed to do with each

image. The processor might be told to scale down one image, blur another, or color-cor-

rect still another before combining them into a single image. This is where the real-time

aspect of Motion takes control.

Because the graphics card’s processor can render only what’s put into its VRAM, the

number of layers that can be processed in real time is limited by how much VRAM the

card has.

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Apple Motion Manual do Utilizador Página 18