Down-rez methods for 4k Red

I haven’t seen much in the way of comparisons of the resizing filters available for Red footage, so yesterday I ran some quick tests. The “Redline” processor provides a choice of seven different filters for resizing clips: Bell, Mitchell, Lanczos3, Quadratic, Cubic-bspline, CatmulRom (sic), and Gauss. I took something fairly neutral we’d shot at 4k (4096×2048 to be exact) and down-rezzed it to something more feasible (2048×1024), to see how much sharpness was retained. I then decided that an exact 75% reduction in area is a bit too computer-friendly, so I also did a set down-rezzed to HD (letterboxed to 1920×1080).

There are a lot of available options for processing the clips, for the purposes of this test, I turned off sharpening and noise-reduction, set the detail to high, the ISO to 320, and applied a rec.709 gamma curve.

It’s possibly a little difficult to tell from this image, but the Lanczos and Catmull-Rom filters produced the sharpest results compared to the others. Although a difference matte between those two methods suggested there was a difference, I just couldn’t see it. I suspect that the difference in processing time between the two would also be neglible, even over a lot of frames, so I’ll probably just go with Lanczos when the time comes to actually prep the images for the online. What’s interesting is that the default filter used by the Red software is
Mitchell, which is probably the most middle-of-the-road in terms of sharpness.

As tests go, this one’s not bullet-proof, for example I didn’t test to see how the gamma curve affects the process, so I have no idea whether using a different gamma curve affect the interpolation (this would probably depend upon the order the Red software performs its processing, if it interpolates first and then adjusts the gamma, then there would be no difference between curves). The other problem is that this really only provides half the story– until we get this into a grading suite and see how it looks at speed it will be difficult to say for sure that one filter is perceptually better than another. I also can’t compare to how it would look to something similar shot in-camera at 2k, because we didn’t shoot anything 2k (although the general buzz around the Red forums is that shooting 2k produces far inferior results, at least with the current firmware).

The original 4k frame, along with the converted DPX versions, as well as layered Photoshop versions can be downloaded from here… if anyone wants a closer look (please note all images are Copyright 2008 Entitled Productions).

Posted: March 14th, 2008
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