ProVideo Coalition.com: Creating Motion Graphics: Keyframes by Chris & Trish Meyer

The one exception we make to the “rule” on the previous page is when working in standard definition D1 or DV formats with a 4:3 image aspect ratio (IAR). In this case, we nearly always work at the native frame size, not one of its square-pixel equivalents. That said, a lot of people prefer to work square in this format - so let’s discuss both cases.

One of the reasons is that we do not feel comfortable scaling images up from 486 lines to 540 lines (or 480 up to 534, in the DV case) if the source was interlaced. If you did not separate the fields to start with, then lines from one point in time (one field in a frame) will be combined with line from another point in time (the other field in a frame). If you did remember to separate fields - and you almost always should with interlaced footage - then you’re stretching up a half-resolution image (243 lines) up to 540 before you work on it.

An even worse problem arises if you need to field render the output. You cannot field render at 540 (or 534) lines, and then use the Stretch section to squeeze it back down to 486 (480) lines - your fields will be “munged” in the process, screwing up your motion and making the resulting render much more difficult to handle down the road. This is only a problem with SD NTSC comps, as all other common non-square-pixel frame sizes require stretching and squashing in only the horizontal direction, which will not affect fields.

Don’t ever, ever, ever scale an interlaced render in the vertical direction; you will screw up its fields.

However, if you find your design calls for a lot of accurate squares and rectangles, you may prefer to work in square pixels anyway. In that case, the best square pixel size for these formats are 720x540 for D1 and 720x534 for DV (remember, DV is D1 with 6 lines missing - it doesn’t have a different PAR than D1!). If any of your source footage was interlaced, be sure to separate the fields in Interpret Footage before stretching those clips horizontally from 486 to 540 (or 480 to 534).

When it comes time to render, be sure to do the vertical stretch in a final D1 or DV-size comp, which will be rendered before the interlacing is introduced in Render Settings. If you use After Effects, fellow user Lloyd Alvarez has created an After Effects script that will nest a 720x540 comp into a D1-sized 720x486 comp for you.

An additional bonus tip for After Effects users: Turn on the Collapse Transformations switch in the final comp so that the transformations are calculated across both comps. This will increase speed and quality (because it skips another anti-aliasing stage).

Again, just to reinforce: if you work in the 720x540 comp, but render the D1 comp it is nested into, then the squashing will occur before interlacing takes place, and life will be good.

If this still makes you nervous, but you still want to work square, the other alternative is to work at 648x486 for D1 (or 648x480 for DV) and then scale horizontally out to 720 in a final comp or when you render - the image will be a little softer, but you won’t have to worry about your fields.

If you would like more information on pixel aspect ratios, we wrote a pair of articles for Artbeats on the subject. Click to download PDFs for Part 1 and Part 2.