Pyro Industries API-1394PCI Manual de usuario descargar pdf (Pagina 216)

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■ STILL GRAPHICS

Pixel Aspect

Television signals have long had accepted standards. The idea of standards when creat-

ing media is that the picture can have consistent quality no matter where it is produced

or who produces it. The current standard used for SDTV is called ITU-601. This stan-

dard determines the dimension and pixel sizes of both PAL and NTSC SDTV images.

It also dictates other specifications. For example, the 601 standard determines the size

and shape of the pixels used in broadcast images.

Surprisingly enough, the pixel defined by the ITU-601 standard is not a square

pixel. If you think of an electron gun moving at a constant speed across a phosphorous

surface, you see it would be difficult to maintain a square shape. In fact, the shape of

the pixel is a little rectangular. This situation makes file imports a little more challeng-

ing because almost all computer graphic applications contain square pixels, although

many adapt for NTSC and PAL signals.

You can now understand the challenge: If you mapped a square-pixeled image

into a rectangular-pixeled image, it would appear to be horizontally stretched-out. To

solve the problem, we need to squeeze horizontal pixels in computer graphic applica-

tions, so that when the image is mapped to Xpress Pro and Free DV, the resulting

image isn’t horizontally stretched-out. Adobe’s Photoshop CS application has the capa-

bility to create and view files for this problem by previewing the corrected pixel aspect.

Other applications do this as well.

When you import graphics into Free DV or Xpress Pro, you’re given four options

for correction of pixel aspect ratios. Avid assumes nothing about what preparation has

been done prior to import, so you can choose between these options (see Figure 6.3):

• 601, Non-Square

• Maintain, Non-square

• Maintain, Square

• Maintain and Resize, Square

Figure 6.3 Import options

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Pyro Industries API-1394PCI Manual de usuario Pagina 216