Adobe Premiere Pro is perhaps one of the more versatile and common professional video editing tools. An issue that has often plagued me in using it, however, is an easy method (or any kind of method at all, really) of centering a cropped image or video that has been layered onto a background which may also be an image or video. This means, for instance, a background image or video with a resolution of say, 1920 x 1080 pixels and an image or video on top of it with the same or similar original resolution (but less after cropping is applied). In the Premiere program and timeline panels, it might look something like the image below.
The moon image (originally 1620 x 1080 pixels) is then cropped using Premiere’s ‘crop’ effect tool which is simply dragged onto it in the V2 track. The background image has a resolution of 1920 x 1080 pixels.
The perhaps weird thing about Premiere’s crop effect tool is that cropping is based on percentages rather than pixels. This means that in order to crop the moon image into something like this…
… I would need to apply percentages to the left, top, right and bottom through trial and error until the image appeared satisfactorily cropped. The values in the Effect Controls panel in the case of the cropping done to the above moon image are as shown below.
If all you were dealing with was images, perhaps cropping them using an image editor before importing into Premiere would be easier. However, with video, where the same cropping method applies, it would make sense or at least be more convenient to do everything within Premiere. So the question now is how to center the moon image with respect to the background. Note that the Position values in the image above, i.e. ‘960.0’ and ‘540.0’ refer to the center of the background image (960 being exactly half of 1920 horizontal pixels and 540 being half of 1080 vertical pixels). The Anchor Point values, i.e. ‘810.0’ and ‘540.0’ refer to the center of the original moon image which had a resolution of 1620 x 1080).
Therefore to center the cropped moon image relative to the background, you could try to do so by hand and eye which will be imprecise in most cases or you could use some logic and math. First, understand that the number of pixels cropped on the left in this case is 50.5% of 1620 (or 818.1 pixels) and on the right is 29% of 1620 (or 469.8 pixels). Together they add up to 1287.9 cropped pixels. Theoretically, we should have an even amount on both sides of the cropped image before proceeding. This means 1287.9 / 2 = 643.95 pixels on both sides of the cropped moon.
We then need to subtract this value from the side that had more pixels cropped, i.e. the left side which had 818.1 pixels cropped. The difference we get is calculated as, 818.1 – 643.95 =174.15. We then subtract this amount from the Position value, i.e. 960. The result is 960 – 174.15 = 785.85 or approximately 785.9 pixels since Premiere allows only up to one decimal place there. This value of ‘785.9’ will replace the ‘960’. Basically, a push to the left. As far as the cropped moon image is concerned, this is its new horizontal center relative to the background image.
We use the same logic for the vertical resolution. The number of pixels cropped at the top is 36% of 1080 (or 388.8 pixels) and at the bottom is 37% of 1080 (or 399.6 pixels). Note that in this case the original moon image and the background image have the same vertical resolution but the cropped percentages are based on the image cropped, not the background. Together the total amount cropped adds up to 788.4 pixels. This means 788.4 / 2 = 394.2 pixels on each side (top and bottom), theoretically. We then need to subtract this value from the side that had more pixels cropped, i.e. the bottom side which had 399.6 pixels cropped.
The difference we get is calculated as, 399.6 – 394.2 = 5.4. We then subtract this amount from the vertical Position value, i.e. 540. The result is 540 – 5.4 = 534.6 which is not a very big change in this particular case since the cropped amounts (i.e. 36% and 37%) were fairly close too. You might have also noticed that 5.4 pixels is 1% of 1080 split in two. This value of ‘534.6’ will replace the ‘540’. A little nudge upward. As far as the cropped moon image is concerned, this is its new vertical center relative to the background. The new Position values and centered cropped moon image are as shown below.
So the moon image that was cropped (manually) is now dead center relative to the background because the (center) Position values relative to the background image have been adjusted precisely based on the cropped amounts of the moon image. Perhaps in a future version Adobe will make this process automatic but until then I hope this article has been helpful.