Pixel Aspect Ratios

On aspect ratios.

Pixels

Analogue television scanned the image starting in the upper left hand corner of the screen. Analogue television did not have pixels but rather a beam of electrons. The quantity of electrons fired from a gun slid between more and less so the spot on the monitor was more of a streak than a dot. Color TV introduced a dot screen. Subsequently computer screens created a grid of glowing elements. These elements are pixels.

Pixel is short for picture element. Although we normally think of these as square spots of light on our computer monitors they can also be thought of as a location. We can determine a location through a co-ordinate system. So pixel 128, 320 is the pixel 128 units from the left and 320 units down from the top. Of course nothing is this simple.

Pixel number order can change with picture format. Some early forms of the TIF format start numbering from the lower left. Early graphic display programming also followed a co-ordinate system with 0,0 the world center. The down axis would be a negative number so programmers would number the pixels from up from the lower left.

Today, image formats generally places 0,0 in the upper left hand corner of the screen.

What is aspect ratio

The word “aspect” stems from a Middle English word denoting the action or a way of looking at something. The aspect ratio of a frame is a means of measuring the cinematic frame. While the film gate can be square or rectangular there are restrictions on the frame proportion. The lens always projects a circle. The physics of light generally results in a darkening of the edges of this circle and a loss of focus at the edges. Masking this projected circle with a rectangle gives us the familiar cinematic frame. The film image is bounded by the film edges creating hard vertical edges on the left and right. The “film gauge” limits the horizontal size of the actual negative. 8mm, 16mm, 35mm, 72mm refer to the width of the film stock.

There are very few other limitations on the proportion of the frame. The distribution of film to theatres requires some standardization. In 1932 the Academy of Motion Picture Arts and Sciences made the Academy Ratio a standard ratio. It was common until 1953. A short review of film frame aspect ratios involves numerous frame sizes. These are ArriScope, Cinerama, Cinerama 70mm, CinemaScope, CinemaScope 55, Dimension 150, Frearless SuperFilm, Grandeur 70mm, Imax, J-D-C Scope, Magnascope, Matted 1.66:1, matted 1.85:1, Metroscope, Panavision, Realife 70mm, Shawscope, Super 35mm, SuperScope, Super Panavision 70mm, Super Technirama 70mm, Techniscope, Technovision, Technirama 35mm, Todd-AO, Todd-AO-35, Ultra Panavision 70mm MGM Camera 65, Vitascope, VistiVision, WarnerScope and Widescreen 1.85:1. Most of these formats are not used anymore. Over the years these choices have narrowed though the European and American standards differ. Film gauges 16mm and 8mm are not included in this list.

The 16:9 format was proposed by Dr Kerns H. Powers, a member of SMPTE Working Group On High Definition Electronic Production. When overlapped with their center points aligned, he found that all of those aspect ratio rectangles fit within an outer rectangle with an aspect ratio of 1.78:1 and all of them also covered a smaller common inner rectangle with the same aspect ratio 1.78:1.[4] The geometric mean of the extreme aspect ratios, 4:3 (1.33:1) and 2.35:1, is also 1.77:1 which is coincidentally close to 16:9 (1.78:1).

http://en.wikipedia.org/wiki/Aspect_ratio_%28image%29#Why_16:9.3F

The standard analogue television screen ratio of the 1950’s was 1.33:1. This is the proportion of standard Television. The Aspect Ratio is the relationship between the width and height. A Ratio of 1.33:1 or 4:3 means that for every 4 units wide it is 3 units high (4 / 3 = 1.33). Currently there are five common aspect ratios. These are 4:3, 3:2, 16:9, 1.85:1, and 2.39:1.

A physical restraint of the projection system coupled with the desire for wider rectangles in theatrical release resulted in several anamorphic formats. These systems use an anamorphic lens for the camera as well as projector. The anamorphic lens projects a vertical oval rather than a circle. The negative is squashed horizontally. Anamorphic projection lens creates a wide oval and produces a wide rectangle using the same size negative.

Universal Distribution

The Standard Digitized Video was the outcome of a search for a digital standard for 4:3 analogue video. This format introduced the anamorphic pixel. The ratio of square vertical pixels to horizontal pixels had been set at 640x480 this is a 1.33:1 aspect ratio. The advent of digital video cameras produced another standard: Digital Video or DV. The opportunity to increase the horizontal resolution by adding anamorphic pixels created the pixel aspect ratio. Think of this number as a multiplier. For the following formula let’s call it PAR.

X times PAR divided by Y equals the Frame Aspect Ratio

The professional DV format is 720 horizontal by 486 vertical with a pixel aspect of 0.9 wide to 1 high

Written 720 x 486 (.9)

We can determine the frame aspect ratio with…

720 x 0.9 / 486 = 1.33

note: Since the recording of analogue video produces artifacts in the bottom six lines of the picture most consumer digitizing techniques will cut the bottom six lines of video and split the difference.

A computer display is always square pixel. So anamorphic pixels are displayed through the interpretation of the display. The width is “dithered” to conform “rectangular” pixels to “square”

If we look at 16:9 (1.78) formats we can apply this technique of finding the frame aspect ratio

NTSC Widescreen 720 x 480 (1.21)

NTSC D1 Widescreen 872 x 486 (1) not often used

HDTV 720 1280 x 720 (1)

HDV 1080 1440 x 1080 (1.33)

HDTV 1080 1920 x 1080 (1)

All computer monitors have a pixel aspect ration of 1:1 or square. This means that the software must widen or narrow the image when editing.

Note: Flash is a vector based program and thus has no pixels to measure.

Letterboxing

Distributing a 4:3 on a 16:9 or 16:9 on a 4:3 frame aspect ratio requires pillar-boxing and letter-boxing. Letterboxing adds black pixels to the top and bottom of the frame to adjust for the distribution frame aspect of 4:3. Pillar-boxing adds black to the left and right of the picture to achieve 16:9.