Resizing HD to SD

Let’s look at a very common problem these days: Down rezing from high definition (HD) to standard definition (SD) in NTSC.

HD comes in two flavors, 720 or 1080, which refers to the vertical dimension of the video frame. Considering that the image has square pixels (as in the previous post) the dimensions are:
Horizontal = 1280 and Vertical = 720 where the aspect ratio is 1280/720 = 1.78 or more commonly known as 16:9 (16/9 = 1.78 .)

And the other flavor where Horizontal = 1920 and Vertical = 1080 that also gives an aspect ratio of 16:9. The full NTSC frame dimensions are Horizontal = 640 and Vertical = 480 with an aspect ratio of 4:3. (Figure 3)

If we want to shrink the image without distortion we have to make a “Letterbox” 4:3 meaning that it will have black horizontal bands at the top and at the bottom of the frame. Otherwise people would look really skinny. (Figure 4)

Let’s say that we are working with 1080. We know what the horizontal dimension should be in SD NTSC; 640 and that the HD horizontal dimension is 1920. To find the resizing factor we compute the ratio of the horizontal dimensions 640/1920 = 1/3. We are shrinking the image by a third. And to keep the proportions in the HD image unaltered we need to also shrink the vertical dimensions by one third. That means that the new vertical dimension is 1080/3 = 360 pixels high. But since the full NTSC frame is 480 we have 480 – 360 = 120 pixels that are unused. That means that each black band above and below the letterbox image is 60 pixels high.

The most common method to fill an entire 4:3 target image with a 16:9 source is to do a Pan and Scan. That means that we use a 4:3 area of interest inside the 16:9 picture and move it around to where the action is in the image. Of course the draw back is that we miss a good portion of the original image. It is a costly and time consuming process that is performed manually to a great extent (figure 5.)

Next post I will cover the "Up Rezing" algorithms and tips to up convert SD to HD.