alt Hacker News> Videos with non-square pixels are pretty rare...
Before HD, almost all video was non-square pixels. DVD is 720x480. SD channels on cable TV systems are 528x480.
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Even with modern digital codecs and streaming, there's usually chroma subsampling[1], so the color channels may have non-square "pixels" even if overall pixels are nominally square. I most often see 4:2:0 subsampling, which still has square pixels, but at half resolution in each dimension. However 4:2:2 is also fairly common, and it has half resolution in only one dimension, so the pixels are 2:1. You'd have trouble getting a video decoding library to mess this up though.
Displaying content from a DVD on a panel with square pixels (LCD, plasma, etc.) required stretching or omitting some pixels. For widescreen content you'd need to stretch that 720x480 to 848x480, and for 4:3 content you'd need to stretch it to 720x540, or shrink it to 640x480, depending on the resolution of the panel.
CRTs of course had no fixed horizontal resolution.
Edit: I just realized I forgot about PAL DVDs which were 720x576. But the same principle applies.
Just look at Japanese television… most channels get broadcast at 1440x1080i for 16:9 content instead the full 1920x1080i (to save bandwidth for other things, I assume), so it's still very common with HD too.
I'm confused... what does DVD, SD or any arbitrary frame size have to do with the shape of pixels themselves? Is that not only relevant to the display itself and not the file format/container/codec?
My understanding is that televisions would mostly have square/rectangular pixels, while computer monitors often had circular pixels.
Or are you perhaps referring to pixel aspect ratios instead?
>Before HD, almost all video was non-square pixels
Correct. This came from the ITU-R BT.601 standard, one of the first digital video standards authors of which chose to define digital video as a sampled analog signal. Analog video never had a concept of pixels and operated on lines instead. The rate at which you could sample it could be arbitrary, and affected only the horizontal resolution. The rate chosen by BT.601 was 13.5 MHz, which resulted in a 10/11 pixel aspect ratio for 4:3 NTSC video and 59/54 for 4:3 PAL.
>SD channels on cable TV systems are 528x480
I'm not actually sure about America, but here in Europe most digital cable and satellite SDTV is delivered as 720x576i 4:2:0 MPEG-2 Part 2. There are some outliers that use 544x576i, however.