Image Processing Question - Converting Standard-Def to Hi-Def, do I have to lose image data?

Question

I'm writing a small program to convert a standard definition 4:3 video to a hi-def video 16:9 and I'm experiencing a serious stretching effect, as expected I suppose (though I didn't think about it until my code started working). Anyhow, the only way I can think of getting around this stretching effect and still fill the whole 16:9 screen is to cut off the top and bottom of the image.

1) So my question is, when converting from SD to HD, do I have to lose image parts of the image in order to fill the whole screen without any stretching effects?

2) Same question for converting from HD to SD.

I'm new image processing, are there any popular approaches to reducing the stretching in these kinds of operations? Is there a smarter approach to this problem than just cutting off parts of the image or introducing black bars to the image?

Thanks in advance for all your help!

Answer 1

Other than the obvious methods of cropping, letterboxing, and pillarboxing, which either lose image data or necessitate potentially-undesirable black bars, there is also adaptive image resizing. Basically, the intent of these techniques is to be able to create a version of an image with an arbitrary aspect ratio, without losing the essential characteristics of the image or distorting it. One technique is called seam carving, and can be seen here.

If you'd like to test the technique out on some images of your own, the functionality is included in recent versions of ImageMagick, as explained here.

Answer 2

Reduction of quality or loss of content is always a problem in resizing images or video. Generally you scale the image in one direction, and either trim or pad the other direction.

On TV it is common to cut off the left and ride side of a 16:9 frame to put is on 4:3 screen, and to add black side bars to go from 4:3 to 16:9. TV editors don't cut off the top and bottom of a 4:3 frame to fit it on 16:9 because there's almost always important parts of the scene there. The far left and far right of a 16:9 frame don't usually have important elements, although in some cinematic scenes losing the sides makes a huge difference.