This page discusses picture aspect ratios and pixel aspect ratios. It is easy to become confused, especially when trying to produce graphics for broadcast use and hopefully this will be of help. Much of what follows will eventually change if and when high-resolution video is adopted. The figures given here deal specifically with European video standards (PAL/SECAM 625 line). The same issues exist in the NTSC standard (used in USA and Japan and elsewhere) but the numbers are different. If you are in any way hazy about the terms bitmap and or pixels, please read here first. Bitmaps
and Pixels
The
above example is of a very tiny bitmap image just 4 pixels wide
and 3 pixels high. This is the shape of an ordinary (non widescreen)
television picture. Because it is 4 pixels by 3 in shape, this is called a 4 x 3 pixel image (pronounced "four by three”). Now if you divide 4 by 3, you get a value of 1.333 and that number is its picture aspect ratio. 4 x 3 or 1.33 ( really 1.33:1) - both can be used as terms to describe the aspect ratio of this image. Occasionally, it may also be referred to as 12 x 9. Same thing. Below, is an 8 x 6 pixel image that is exactly twice as many pixels across and up as the first example, so of course they both have the same 4 x 3 aspect ratio. As you will see, it conveys an image of a red square on a grey background, but obviously there is not much you can draw on a canvas this small. (Bear in mind that each pixel can only be on or off; you cannot light up part of a pixel).
Multiply the rows and columns by 100 and
you will have a 4 x 3 image made up of 400 x 300 pixels and this
is quite adequate to convey a readable image:
What You Need to Know about "Non-square Pixels" Analogue television pictures, the type that most people still watch, are made up of horizontal lines, not pixels. The number of lines is about 600. When you create a digital image (a bitmap) that is going to end up on television, you need therefore to decide how many pixels to use. Use too few, and the result will be a nasty blocky picture. Use too many and you will be squandering disk space and bogging your computer down as it has to juggle millions of pixels that will never be seen. The numbers that were chosen for a European TV picture were 768 x 576 pixels. (If you divide 768 by 576 you will see it is still our 4 x 3 ratio). This is where confusion can set in for the unwary; the 768 x 576 pixel size is not in fact what is used for broadcasting. Even an application such as Photoshop may mislead you by offering 768 x 576 as a standard PAL picture size. Photoshop
note: this problem was
finally resolved in Photoshop CS, released in late 2003. You
will still encounter the problem if you are using an earlier
version, that is up to Photoshop 7. In
reality, broadcasters and every digital VTR will expect you to produce
your finished work at 720 x 576 pixels resolution.
Producing Bitmap Graphics for TV in Correct Aspect Ratios The aim of what follows is to guarantee that when you draw an image, it will be correctly seen on TV. For instance, you want your circles to remain circular. First, you must ascertain whether your software application
is going to correctly output your artwork or not. Generally speaking,
high end broadcast applications like Aura and Animo correctly deal
with the issue of non-square pixels. If they do, you do not have
a problem. When you view shapes inside these programs, you will
know that they will still be correct when they are sent to your
video source. The clearest indication that all is well is normally
when you come to the output stage. If the software specifically
offers PAL D1 or 720x576 pixels with a pixel aspect ratio of either
1.066 or 1.422, you can be sure that your images will be the shape
they are meant to be. The reason for the two different pixel aspect
ratios will become apparent below. Working in 4 x 3 In an application that is not specifically designed for video use such as Paint Shop Pro, Flash or Photoshop, choose a working screen shape of 768 x 576, or if you prefer, you can work at a higher resolution as long as the picture aspect ratio is 4 x 3. When the project is to be output, you will need to squeeze the image to be 720 x 576 pixels. Effectively, this means you have to make the pixel aspect ratio 1.0667. This means that circles will all be slightly oval looking. There is no way to give a general guide for how to do this, as every program is different; Photoshop for example has the means of performing batch operations and these are not too difficult to set up; then the software will happily transform thousands of images automatically. With most batch programs, you turn on a "recorder", re-size the image then turn off the "recorder" and store this action as a macro. When you come to run the batch file, you will load in the files to be processed, run your re-size macro and be given a choice where to output the processed files. The size conversion may not be necessary if you are fortunate and have a video board that will automatically re-size the incoming image correctly. If this is the case and it is working correctly, when you look at its output on a video monitor the circles will look circular and the image will not be displaced off-centre slightly or cropped. (You will need to have a video monitor with an underscan function to see the whole image).
Widescreen is a slightly different problem. Here again, if you are using a high-end program specifically tailored for video usage, the software should take care of the nasties for you. Whatever software you use for widescreen, the important thing is that as you are working you work in a frame that is the right shape: 16 x 9. In
programs that do not alter the output pixel aspect ratio, (programs
such as Photoshop), proceed as follows: while you are producing
the animation or graphics, work at a resolution of 1024 x 576 pixels. Do
discuss the needs of your client if you are working with a post-production
facility. They may, for example, be happier to be given your output
files on a CD or DVD rather than tape and may also prefer to have
the files at 1024 x 576 pixels as this will give them a little more
flexibility regarding scene framing. The higher horizontal resolution
also means that they will get better looking stills. Summary 4 x 3 format
work
at 768 x 576 pixels output
at 720 x 576 pixels
16 x 9 format
Further reading: A very thorough discussion of the problem complete with pretty pictures can be seen in a discussion of how After Effects deals with the issue at this (USA) website: http://www.creativecow.net/articles/gerard_rick/pixel_madness/ and in case you are now confident that aspect ratios are easy peasy, this site will convince you otherwise: http://www.lurkertech.com/lg/pixelaspect.html
This page written Nov. 02, last amended March 04
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