What exactly does "High Definition" mean, and what kind of videos should be "allowed" to use this term?
I ask for two reasons.
1. Some people feel that only 1080p video is HD, or that 1080p is synonymous with HD. I have had some people insist that, say, a 720p video is not "really" HD. Others use the term HD to mean basically anything with more than the 480 scan lines of SDTV. And of course, that 480 comes from SDTV in HTSC format; is PAL it's a few more lines (can't recall exactly how many off the top of my head) and the frame rate is 50Hz with progressive scan, so that further muddies the waters.
Personally, I am happy applying HD to 720-line video or higher, but I am not sure if there's an official definition for this.
But the bigger problem is Item 2.
2. Some people also seem to feel that *anything* presented in 720 or 1080 resolution, or any HD format, is HD, regardless of the quality of the underlying video, and regardless of the encoding bit rate. For example, I've seen TV broadcasts at 720p (I don't have a 1080 TV yet) that clearly were encoded with an abysmally low bit rate. Is this HDTV? Maybe the FCC lets them call it that -- do they even regulate the use of the term "HD" for cable/satellite/airwave broadcast, or can any broadcaster stick HD onto any program content?
What does this have to do with fractals? Well, some animations are presented in 720 or 1080 or whatever format greater than 480 lines, and are labeled HD by their authors, but were most likely rendered at significantly less than that actual resolution at the time the actual fractal images were created.
There are many ways this can happen. It's easy to set a video compression program to output 1080 video from low-definition source files. Someone could calculate the fractal frames at, say, 960x540 and then set the encoding software to output 1080 video, a 2X step-up in resolution.
Another way this can happen is with the interpolation techniques that many of us use to make ultra-deep zooming possible. This is where a small number of pixels is used, with digital zooming and interpolation techniques, to generate video data for a large number of video frames. Since there's such a huge amount of redundant information from one frame to the next, only a small fraction of "new" data needs to be calculated to give a very good zooming effect. I have no problem with this, but it can be misused. If, say, a 1280x720 HD video is going to be made this way, it seems only fair to me to insist that every frame be interpolated from a master image with at least that many pixels -- there should be no "undersampling". It's not really HD to start with a 320x180 image and interpolate it up to 1280x720 video. Interpolating to HD resolution from a lower-resolution raw data set is no different than just setting the video encoder to output 1080 from standard-definition video.
I've elaborated on the topic of interpolation and undersampling on my web site at
http://www.hpdz.net/TechInfo/Animation.htm#Frame_Interpolation.
Another trick to reduce computing time, file size, and bit rate is lowering the frame rate. Seems to me that 10 fps isn't HD no matter what the format or bit rate is. Movies, I believe, are usually 24 fps, but I think some might be faster, like the IMAX format.
Finally, there is the question of encoding bitrate. This is a hard problem, because there's so much detail in most fractal animations that anything less than an enormous bit rate tends to look blurry and blocky. But I think most would agree that encoding to at least DVD quality (4Mbps or so) should be part of presenting a final product designated as High-Definition, and in fact, 8-12 Mbps might be an even more satisfactory minimum. I know that YouTube etc probably don't deliver sustained bit rates that high, and not everyone has an internet connection that can sustain that speed. But the loss of detail resulting from the lower bit rates seem to me to cancel out the benefits of presentation in a 720 or 1080 format.
So I just wanted to open up this topic for discussion and see what people think.
I would suggest that those of us who create animations think carefully about the bit rate and resolution issue, as well as the use of undersampling when interpolating. To my eye, I would rather have a smooth, crisp 30 fps video at 320x240 than a blurry, blocky, jittery one at 15 fps 1920x1080.