4

Understanding Pixel Differences in Graphics and Video
The term square pixel actually refers to the horizontal and vertical distance from a pixel
to its neighbors. With a square pixel, the distance is the same in both directions. This
distance is a function of the sampling rates, both horizontal and vertical, and the aspect
ratio the graphic will be displayed in. Given the right aspect ratio, virtually any combination
of horizontal and vertical sampling rates could produce square pixels.
Computer graphics programs are optimized to work with square pixels: When you draw
a square, it has the same number of pixels in all four of its sides and looks like a square
on the display.
Unfortunately, SD video is different.
Video Pixels
Standard definition video uses a 4:3 aspect ratio. No matter what the size of the display,
the height is always 75 percent of the width. This is true for both NTSC, which has 525
lines in each frame (480 of these active with DVDs), and PAL, which has 625 lines per
frame (576 active). Both standards also have 720 pixels per line when converted to MPEG.
To fit the 4:3 aspect ratio perfectly using square pixels and 720 pixels per line, there would
have to be 540 lines in each frame. So for NTSC, which has 480 lines, the lines must be
spaced slightly apart to fill the same area, resulting in rectangular pixels. For PAL, with
576 lines, they must be squeezed together slightly to fit.
The Settings pop-up menu in the DVD Studio Pro Menu Editor has a square/rectangular
pixel selection that allows you to choose whether to view menu graphics as rectangular
pixels (forcing a 4:3 aspect ratio) or as square pixels while creating menus. The Viewer
tab also has the same settings. The Simulator always shows rectangular pixels, simulating
what the viewer will see.
The real problem arises when you create your menu graphics.
Square Pixels in Graphics
Graphics programs use square pixels. If you set the dimensions of a graphic to 720 x 480
pixels (the NTSC frame size), you will notice that the drawing area of the graphic in the
graphics program is not a 4:3 aspect ratio (it is a bit short). If you set a graphic to PAL
dimensions (720 x 576 pixels), the drawing area you see in your graphics program is taller
than 4:3.
Anything you put in the graphic will be distorted (either vertically stretched or compressed,
depending on your video standard) when viewed on a video monitor. If you draw a
square, it will look like a rectangle. In some cases, the distortion is not great enough to
worry about, but in most cases you should build your graphics with the distortion in
mind.
99Chapter 6 Preparing Menu Assets