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Sizing Your Images

Now that we’ve got our frame in, say, Photoshop it might look like FIGURE 4.8. This was a circle created in a Final Cut Pro DV sequence. DV, and uncompressed standard definition video, don’t use the square pixels that computers use, but instead they use rectangular pixels; specifically, narrow, tall pixels for NTSC 4:3 and wide, short pixels for PAL.

Figure 4.8

Figure 4.8 Displaying a video image on a computer monitor illustrates the differences in aspect ratio between video (rectangular pixels) and computer (square pixels). For instance, this is a circle in Final Cut, but an oval when displayed in Photoshop.

Square pixels are described as having an aspect ratio of 1, meaning 1:1. CCIR-601 (NTSC 4:3) digital video pixels are described as having an aspect ratio 0.9; that is 1 unit high and 0.906 units wide. When your rectangular NTSC pixel image is displayed on a computer screen, it looks like the one shown in Figure 4.8.

Recent versions of Photoshop have an option under the Image menu to set the pixel aspect ratio. This is similar to the pixel aspect ratio adjustment that’s normally on in the Final Cut Viewer and Canvas. Fixing that adjustment makes the image look right, but don’t be fooled. The image is still in the wrong aspect. If you print it out, or put it on the web, or take it to some other application that doesn’t understand pixel aspect ratio adjustment you’ll still see the pretty oval in Figure 4.8, not the circle you should get.

So how do we adjust this? The simplest way is to change the image size of the your picture. In Photoshop, you can do this by choosing Image > Image Size (Cmd+Option+I). For a DV NTSC 4:3 image such as this one, you would deselect Constraint Proportions and set Resample Image to Bicubic Sharper (see FIGURE 4.9). Set the width of Pixel Dimensions to 640.

Figure 4.9

Figure 4.9 To convert a still frame from rectangular to square pixels, select Image > Image Size, turn off Constrain Proportions, and set the Width to 640.

There is a long debate about whether you should resize to 640 × 480 by changing the horizontal dimension or to scale to 720 × 540 by changing the vertical dimension. I prefer the former. If you use the latter, set Resample Image to Bicubic Smoother. Either way you end up with a square pixel 4:3 image.

PAL uses a different aspect ratio pixel, 1.07. (1 unit high by 1.07 units wide.) PAL video images should be resized in Photoshop to 768 × 576. See TABLE 4.1 for a resizing chart of different aspect ratios.

Table 4.1. Resizing chart for a variety of still image formats.

Footage

Source

Scale to

Function

DV NTSC

720 × 480

640 × 480

720 × 540

SD NTSC

720 × 486

640 × 486

720 × 546

DV NTSC Anamorphic

720 × 480

864 × 480

SD NTSC Anamorphic

720 × 486

864 × 486

PAL

720 × 576

768 × 576

PAL Anamorphic

720 × 576

1024 × 576

All 1080 HD formats

Varies

1920 × 1080

All 720 HD formats

Varies

1280 × 720

What about anamorphic material? Anamorphic NTSC DV uses yet another pixel aspect ratio of 1.25. For this material, I would resize horizontally to 863 × 480 using Bicubic Smoother to create a widescreen image. PAL anamorphic should be resized to 1024 × 576 to bring it back to widescreen square pixel aspect.

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