Shooting for Streaming, Part 4
On the left is the original video, where we see two observers, two lines of text, and a blurry skateboarder coming down a ramp. The effect is most obvious with the observer, whose position during each of the two fields that comprise the frame is quite distinct. In the image on the right, I applied a "deinterlacing filter" that intelligently combined the two fields into a single frame, minimizing the artifacts significantly. If you produce lots of streaming video from interlaced source, as I do, the ability to enable and master deinterlacing filters is a critical skill.
For me at least, deinterlacing in post has proved such a valuable technique that I tended to forget that it’s like a bandage you put on a cut. In some instances it works well, but you’d rather avoid the cut in the first place. That’s where progressive video comes in. Rather than shooting two fields for each frame, progressive camcorders shoot one complete frame for each frame and, if necessary, divide those frames into fields for displaying on analog TVs. Since progressive camcorders capture a complete frame, there’s no interlacing and no need for deinterlacing. That’s why, if buying new, you should definitely buy a camcorder with progressive capabilities, like the Canon XH A1 used in our testing.
But that’s where we started. The question on the table is whether, if you’re currently shooting in interlaced mode, a progressive camcorder will deliver better-quality results. To set up this discussion, let’s discuss scene-related characteristics that tend to highlight deinterlacing artifacts and therefore favor progressive camcorders. Then we’ll analyze some delivery-related factors that control whether your viewers will actually see the difference between a progressive or interlaced camcorder.
When Interlacing Artifacts Matter
Deinterlacing artifacts are most pronounced during high-motion sequences with lots of sharp detail, as we saw in Figure 1, but in many instances, only when the shutter speed is fast enough to preserve the detail. These three criteria make perfect sense. If there is minimal or no motion, there is little or no difference between the two fields, so no interlacing artifacts to start with.
If there is minimal sharp detail—say, when shooting fast-moving clouds or waves breaking on the shore—it’s very challenging to perceive interlacing issues. For example, if you look at the blue back wall in Figure 1, you really can’t tell that there’s an interlacing problem. Rather, it’s the text on the wall, the skateboarder, and the observer, along with the edges of the platform and back wall, that highlight the problem.
Finally, the shutter speed has to be high enough to preserve the detail during the motion. For example, in one scene I filmed, shot at a shutter speed of 1/100 in both interlaced and progressive modes, a young girl walking is a bit sharper in the deinterlaced frame (center) than the progressive frame (right). That’s because a shutter speed of 1/100 wasn’t fast enough to preserve the moving detail, most notably in her legs.
For completeness, on the left is the original frame of the interlaced video, pre-deinterlacing. The center frame shows the results after deinterlacing, while the frame on the right is the progressive frame, which obviously needed no processing. I shot this sequence of shots in HDV mode, which accounts for the funky interlaced pattern on the left, and all others in DV mode for reasons explained below.
As you probably know, shutter speed often isn’t a choice so much as a compromise. In this gymnasium, the lighting was perfect for playing and watching but inadequate for shooting, and I had to pump the gain in both camcorders up to 12dB to support the 1/100 shutter speed and produce even decent exposure. Increased gain would have enabled a faster shutter speed but may have degraded the quality of the compressed footage more noticeably than the additional preserved detail would improve it.
Contrast this image with another shot outside at a shutter speed of 1/2000. Even with my 120mph (har, har) golf swing, the progressive camcorder captured the image perfectly, and you see clear jaggies on the interlaced club, even after deinterlacing.
I should state for the record that all tools do not deinterlace equally well, and that not all encoders even offer deinterlacing filters. For example, Adobe’s Flash 8 Video Encoder does not deinterlace, though the CS3 version does. Microsoft’s Windows Media Encoder has a tragically poor deinterlacing filter, as does QuickTime Player. When encoding with these tools, you should scale and deinterlace in your video editor, not in the encoding tool.