# A Film Restoration Diary

## The Relevance of Pixel Aspect Ratio

Congratulations if you got through the previous rather technical post about Pixel Aspect Ratio.

The next question is: how does this Pixel Aspect Ratio business affect the process of preparing these kine scans for DVD release?

### Pixels Are Inherently Shapeless

When stored in a digital file on disk or DVD, a video frame is just a series of numbers, where each number (or set of numbers) describes the brightness and/or colour values of a pixel — a video frame being a 2-dimensional array of pixels.

This set of numbers describing each pixel is inherently shapeless — neither square nor rectangular! A pixel gains a shape only when displayed on a screen. And this is where Pixel Aspect Ratio comes into the picture. It’s a hint to the display device to say “when you display this video frame, make the pixels this shape — as described by this Pixel Aspect Ratio”.

You may have noticed that most digital televisions or projectors allow you to choose the desired aspect ratio of the picture from a menu of options. You are simply changing how the shapeless numerical pixels are assigned a shape when physically displayed.

### Revising the PAR Calculation

So let’s now revise our calculation of Pixel Aspect Ratio for our PAL DVDs. As noted in the previous post, the nominal horizontal resolution of a PAL DVD image is “704 or 720” pixels. With modern TV and computer displays, there is no need for the 8-pixel “fudge factor” on either side of the frame. Hence will use the full 720 pixel width, with a height of 576 pixels for our DVDs. We can calculate the aspect ratio of such a frame:

720 / 576 = 1.25

In the last post we also established that our target aspect ratio is 4:3 (approx 1.33), as used in the original black and white television system. Hence when these DVDs are displayed the pixel width will need to “stretch” by a factor of:

1.33 / 1.25 ≈ 1.0666

Conversely this means that when we asymmetrically downsample the scans from 2048 x 1556 pixels to our “PAL SD+” dimensions, the video frames will appear slightly squashed sideways by a factor of:

1 / 1.0666 ≈ 0.938

…or approximately 94%.

We will edit and adjust the video with PAR set to 1.0 — i.e. with the pixels displayed as square pixels, and the image appearing squashed — people will look a bit slimmer than usual!

Pixel Aspect Ratio will become relevant when preparing the final DVD, at which stage a PAR “hint” will be inserted into the video file to tell the display device and its software to horizontally stretch the pixels to restore the correct image aspect ratio.

### Summarising the Workflow

The following table summarises the workflow:

Workflow Stage Resolution PAR Comment
Original scan of 16mm film kine 2048 x 1556 pixels
(“2K”)
1 Square pixels
Downsampled scan for editing and restoration 720 x 576 pixels plus overscan
(“PAL SD+”)
1 Square pixels — image slightly squashed sideways
DVD video file 720 x 576 pixels
(“PAL SD”)
1.0666 Will display on screen with non-square pixels — image will not be squashed sideways

In the next post we will summarise the journey on this project so far and outline the next steps.