Pixel Aspect Ratio Tool

Three separate ratios describe any raster image, and confusing them is the source of most stretched video. Storage Aspect Ratio is simply the stored pixel grid, 1920 divided by 1080 for example. Pixel Aspect Ratio is the shape of one sample: 1.0 means square, anything else means the sample is wider or taller than it is high. Display Aspect Ratio, the shape the viewer actually sees, is the product of the two, DAR = SAR x PAR. Reducing width and height by their greatest common divisor gives the familiar integer form: 1920 and 1080 share a GCD of 120, yielding 16:9. Standard definition digital video is where non-square samples appear. ITU-R BT.601 fixes 720 active samples per line for both 525-line and 625-line systems, so 720x576 stored PAL displayed at 4:3 needs a PAR of 12/11, or about 1.0909, while 720x480 NTSC at 4:3 needs 10/11, about 0.9091. The anamorphic 16:9 variants use 16/11 and 40/33, the latter being the 1.2121 figure seen on widescreen NTSC DVDs. When resizing, height follows from width divided by the display ratio, so a 16:9 source scaled to 1280 wide becomes 720 tall. Two constraints bite in practice. Most codecs based on 8x8 transforms and 4:2:0 chroma subsampling require even dimensions, and many encoders prefer multiples of 8 or 16, so rounded results should be nudged to the nearest even number rather than left odd. Second, mismatched ratios force a choice: letterboxing and pillarboxing preserve geometry at the cost of unused screen area, cropping preserves scale but discards content, and stretching preserves neither. Modern HD and UHD formats defined in ITU-R BT.709 and BT.2020 use square pixels exclusively, so PAR only matters when handling legacy standard definition material, anamorphic camera output, or files whose container metadata declares a PAR the pixel grid does not support.

Non-square pixels are a direct inheritance from analogue broadcast. When the CCIR, now the ITU Radiocommunication Sector, adopted Recommendation 601 in 1982 to define component digital video, the committee chose a single sampling frequency of 13.5 MHz that would work for both the 525-line 60 Hz systems used in North America and Japan and the 625-line 50 Hz systems used across Europe. That common clock produces 720 active samples per line in both cases, even though the two systems have different line counts and different active line durations. The result was a shared, hardware-friendly digital standard whose samples are not square in either variant, and every DV camcorder, DVD and standard definition broadcast chain that followed carried that geometry forward. The 16:9 display ratio itself was an engineering compromise rather than an artistic choice. Kerns H. Powers of the David Sarnoff Research Center analysed the span of ratios then in circulation, from 4:3 television through 1.85:1 and 2.35:1 cinema, and showed that a ratio of about 1.78:1 is the geometric mean that lets any of them be framed within a common rectangle with the least wasted area. SMPTE and the wider standards community adopted 16:9 on that basis, and it was written into the HDTV parameters of ITU-R BT.709 in 1990 alongside 1920x1080 square-pixel sampling. The later shift to computer-native imaging, web video and mobile capture made square pixels universal, but PAR metadata survives in container formats precisely so that decades of BT.601 footage can still be displayed with correct proportions.