What is the Overlap matching?
When two images of different sizes are used to improve the final result.
Could you be more specific?
Usually this is used when two different versions of the same movie are available at different aspect ratios.
Let’s say you want to restore a movie that is not available in high definition, and you want to upscale a DVD or a laserdisc capture. If you are lucky, an anamorphic DVD is available, so a theoretical 720×576 (PAL) or 720×480 (NTSC) max resolution could be achievable. But, if the original aspect ratio of the movie is 2.35:1, then you have only 720×432 (PAL) or 720×360 (NTSC). Well, enough resolution left to do a decent upscale. However, if the DVD is not anamorphically enhanced, actual resolution drops to 720×324 (PAL) or 720×252 (NTSC).
But what if the only (or the best) available low definition sources are analog? In this case, a 2.35:1 original resolution in laserdisc could be at maximum 576×324 (PAL) or 576×262 (NTSC), while VHS could be 328×324 (PAL) or 328×262 (NTSC)… pretty poor…
So, how it’s possible to improve the quality of such low resolution sources? It’s easy! Overlap a pan&scan version of the same movie over the letterbox version!
The P&S version of a laserdisc could be 576×576 (PAL) or 576×480 (NTSC), while VHS could be 328×576 (PAL) or 328×480 (NTSC). Can you see the differences?
here you can see how a P&S version overlapped to a letterbox version, upscaled, have a better resolution than the latter; this will lead to better details in the center of the image, leaving less detailed image at the borders. It is also possible to use two letterboxed versions (like 1.66:1 and 2.35:1) – even if difference in details will be less than using a P&S version, it nevertheless improve definition a lot.
A lower difference in aspect ratios between sources makes the final resolution only slightly better, but often this avoids all the image cropping and variations that lie in the pan&scan version.
Can I use an open matte version instead of pan& scan?
It is often futile to use an open matte version with a letterboxed version both from the same format, as active image have the same effective vertical resolution; however, it is possible to use an open matte version of an higher resolution source overlapped to a lower resolution source.
So, for example, an open matte DVD (with higher horizontal resolution) overlapped to a letterbox laserdisc; or an open matte laserdisc or DVD (both with higher horizontal resolution) overlapped to a letterboxed VHS, will increase the details clarity of the final upscaled version.
What about high definition formats, like Blu-ray and HDTV sources?
in some rare cases, the open matte version of the same (or comparable) format of the letterbox version has a better resolution – due to several factors like different masters, encoding, bitrates etc. – in such cases, it’s possible to use the Overlap matching method, although usually with lower quality improvement in comparison to previous cases.
NOTE: consider that pan&scan cropping vary usually everytime between shots, and often during a single shot; so this method is really really hard to follow, much more than the “Slice technique”…
Top, upscaled DVD, bottom, upscaled DVD with HDTV overlapped (click for larger image):
(if you can’t see the image: http://s27.postimg.org/e4zlsc6w1/matrix_083217.jpg)
same image as the previous second one, not cropped:
WARNING: this is a very difficult technique, and could be really hard and time consuming to find out the perfect settings; I did this simple match in an hour, and both previous and next shots have completely different setting… be aware!
Comments, improvements, corrections are welcome!