Canon XL 1 Manuel de l'utilisateur - Page 24

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Choosing whether to preprocess or to avoid rendering

The first question that you should ask is when to apply preprocessing (described in the

next section): during the rendering phase or later. Preprocessing during rendering

generally results in a smaller file, but adds more rendering time. It also means you

can't use an encoding tool for special operations, like inverse telecine.

Basically, to avoid rendering, you use the same codec and file format as your source

video. If you don't preprocess, you don't need to uncompress and then recompress

unmodified frames—instead, you copy them from the source into the output.

Different tools support this process in different ways—in Adobe Premiere Pro, you

turn off the Recompress option in the Video pane of the Export Movie Settings dialog

box. For this process to work, the video must truly be unchanged with the same frame

rate, frame size, pixel aspect ratio, and so on. The simplest way to ensure that no

changes were made to the video is to use the same project preset for export that was

used for import, which is the default behaviour in Adobe Premiere Pro.

When you preprocess, choose the highest frame rate and largest frame size that you

will use.

Choosing a file format for rendering

You can choose from a variety of file formats for rendering. The most common

formats are QuickTime and AVI; both offer good-quality intermediate codecs. You

can also use MPEG-2 at high data rates. Modern tools like After Effects and Adobe

Premiere Pro can export to Windows Media 9 format. It is essential to make sure that

the encoding tool supports the file format.

Choosing a codec for exporting

When you export an intermediate file, pick a codec and a data rate that doesn't

introduce compression artefacts. When you are not preprocessing, it's typical to use

the source codec or Motion JPEG. When you use Motion JPEG, use it in single field

when you export as progressive scan, or interlaced when you export as interlaced.

For AVI files on Windows, the Huffyuv codec is quite popular.

Deinterlacing video

The most basic and critical form of preprocessing for the web is deinterlacing. Most

video is produced and edited as interlaced (although, an increasing amount is done as

progressive scan). But almost all web video is encoded as progressive scan. If the

source frame of video is left with interlaced lines intact, the output shows the stacked

lines. The lines look bad, but worse, those lines have sharp edges and are difficult to

encode. So, at the same bit rate, deinterlaced video shows fewer artefacts than

interlaced video.

Cropping

Cropping is excluding areas of the source frame from inclusion in the output frame.

Content composed for video normally confines critical elements to what are called