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If true circles (the effect is more noticeable with large ones) have an oval appearance instead of a perfectly circular one on the monitor you are viewing a monitor with a non 1:1 pixel aspect ratio. Check the image below:

Pixels or 'pels' (from "picture element", "pics" is a common slang term for pictures) are usually as wide as they are tall, at least in the computer graphics and printing industries. However, on some monitors and SDTV sets they are rectangular or oval instead of square or circular. As a result of this, since monitors and TV screens, etc, are grids of physical phosphor dots (see pixel for more information), if the physical dots are wider than tall, then objects will have a distorted appearance if the source of the image assumes that the pixels will be square.

Square pixels

Rectangular pixels

This is not dissimilar to printing a photograph on rubber and physically stretching it, or to some applications of Slit-scan photography.

Why on Earth Would Pixels not be Square?

It's a long story...

Classical analogue video does not have pixels at all! Analogue video systems including NTSC/M which is used the the U.S. address the image raster in terms of lines only. NTSC/M specifies 480 lines, interlaced but it does not explicitly specify how "high" or how "wide" those lines are. There was never any technical need to address this topic since the aspect-ratio was fixed at 4:3 and the center was fixed so, since production practice had always been to leave some space at the edge of the frame in case a badly configured set cropped it off, it simply didn't matter. A line of video was continuous in both chroma and luma.

Technically, CRT gun type displays and Tube video scanners used on old-style cameras did not have physical pixels even though CRT displays have discrete phosphor dots.

In the 90s when computer editing of video became possible this caused enormous problems. Digital electronics must quantize everything, and this requires pixels.

Unfortunately while, obviously, it made sense to view a scanline as being one pixel high, it was not necessarily intuitively obvious how many pixels wide it was. Actual video waves as recorded to tape, etc, did not necessarily strictly maintain the aspect ratio or even the number of scanlines which contain visual information! So it was not a simple arithmetical matter based on the aspect ratio. Besides: the edges of the lines that definitely were part of the visual raster sometimes contained non-visual information that was 'off the screen' so to speak, but not exactly defined, in other words, it was not always concretely clear where the edges of the video were.

For some esoteric reason lost to history, the ITU decided to define the pixel-aspect-ratio of NTSC video as 12 x 3/11 ÷ 13.5 which is not square! Extreme chaos reigned as software vendors argued endlessly about this and about how many pixels there should be, etc, etc, etc.

Fortunately, modern HDTV formats strictly define how many pixels wide the HD scanline is, so that this is no longer a problem... sort of. In actuality, the matter still isn't quite settled. Some tape formats compromise line-width to increase bitrate and therefore assume that there are 480 fewer pixels per line compared to the normal HDTV raster, but that they are wider so the 1440x1080 HDV raster anamorphically 'fills out' the whole HDTV 1920x1080 widescreen (16:9) image aperture aspect-ratio.

My Computer Monitor has Non-square pixels, can I do Something about this?

Possibly, but it's not going to be very precise. Your monitor has controls, probably via a cumbrous OSD control menu to alter the "width" of the beam projected by the electron guns on the phosphor dots. (That is a great simplification, but bear with us) If you adjust this you may be able to simulate square pixels, or at least stop things being "squashed" or "stretched"

All LCD screens inherently have square pixels.

Categories AV, Hardware