Keying compressed / low color sampling footagea comparision between uncompressed 4:4:4 RGB and digital consumer video formats
I rendered Jeff Greulich's "Chastity" model over Red (255,0,0) Green (0,255,0) and Blue (0,0,255) backgrounds in Lightwave 3D. I then compressed this image to MiniDV and HDV formats and keyed them using After Effects' Keylight, using default settings. The variations in color were caused by spill removal algorithm at default settings. In order to get HDV footage into AE, i rendered out an uncompressed 1080p HTDV avi clip. This extra conversion shouldn't affect the quality.
The images in this page are at 100% size, the various formats are compared pixel by pixel. The greater resolution of HDV isn't taken into account here.
If the result is to be shown at standard television resolution, HDV has a big advantage due to it's greater native resolution: even though some artifacts are visible at full size, they will be averaged out to the extent of being virtually nonexistant, when the footage is scaled to standard resolution. In this test, the HDV footage hasn't been downscaled, the comparision is pixel for pixel - remember that there's four times more of them in HDV than in MiniDV. The test area used here is much smaller a portion of HDV screen than MiniDV screen. In my real-life tests, HDV outperforms Digibeta, and is my acquisition format of choice for SD greenscreen composites.
This comparision shows the artifacts caused by the compressed formats on an extreme situation, full saturation background and stationary objects. Less saturation, with less accurate BG color would probably slightly IMPROVE the results of MiniDV and HDV, as more than one channel would be used while keying. On the other hand, motion could cause artifacts on HDV footage.
While some of the color sampling / compression artifacts seem really bad here, in real life situations, with some processing, rather good results can be obtained using any of these formats, with any background color.
The results clearly show that green backdrop is the best choice for both MiniDV and HDV. This was as expected, knowing how color sampling / compression algorithms work. MiniDV is better with red and blue backgrounds, while HDV looks slightly better with green. Uncompressed 4:4:4 is perfect every time, as expected.
If you wish to see an example of a video we've done with greenscreen / HDV combo, go to http://www.poetsofthefall.com/ and have a look at "Lift" music video (biggest version is 640 pixels wide, which is quite close to D1 resolution). Basicly ALL shots in the video are greenscreen, with CGI backgrounds. The video was mastered at 1280*720p HDTV resolution, and result looked rather spiffy at that size too.