Pixel Aspect Ratio, as described on Wikipedia and other sources, is the ratio of the width of a pixel to its height. It's a measure of how "stretched" or "squashed" a pixel is. This is particularly important in video, where different display standards have different pixel shapes. It contrasts with display aspect ratio, which is the overall width-to-height ratio of the displayed image.
Understanding Pixel Aspect Ratio (PAR)
Pixel Aspect Ratio (PAR) addresses the fact that pixels aren't always square. A square pixel has a PAR of 1:1. If a pixel is wider than it is tall, its PAR will be greater than 1:1 (e.g., 1.25:1). If it's taller than it is wide, its PAR will be less than 1:1 (e.g., 0.9:1).
Why is Pixel Aspect Ratio Important?
It's crucial for ensuring that images and videos appear correctly on different displays. If the PAR is not properly accounted for, circles might appear as ovals, and squares might appear as rectangles. This is because some video formats store images with non-square pixels, relying on the display device to stretch or squeeze the image accordingly.
Pixel Aspect Ratio vs. Display Aspect Ratio
- Pixel Aspect Ratio (PAR): The shape of a single pixel.
- Display Aspect Ratio (DAR): The overall shape of the image as it appears on the screen.
The display aspect ratio is determined by the pixel aspect ratio and the number of pixels in the horizontal and vertical dimensions (resolution). The relationship is:
Display Aspect Ratio = (Horizontal Resolution * Pixel Width) : (Vertical Resolution * Pixel Height)
Common Pixel Aspect Ratios
- 1:1 (Square): The most common PAR, used in computer displays and many modern video formats.
- NTSC DV (0.9091:1): Used in standard-definition NTSC digital video.
- PAL DV (1.0667:1): Used in standard-definition PAL digital video.
These non-square pixel aspect ratios were common in older video formats to maintain compatibility with existing display technologies.
How to Correct for Pixel Aspect Ratio
Most video editing software and media players can correct for pixel aspect ratio. They do this by scaling the image appropriately during playback or export so that it appears with the intended display aspect ratio. This correction ensures that the image isn't distorted.
Example:
Suppose you have a standard-definition PAL DV video with a resolution of 720x576 and a pixel aspect ratio of 1.0667:1. To display it correctly with a 4:3 display aspect ratio, the video player will stretch the image horizontally. Without this correction, the image would appear slightly narrower than intended.
