Sampling Chroma in DV
NTSC DV employs a 4:1:1 color space with a
luma resolution of 720x480 and a chroma
resolution of 180x480.
Blue screen chroma key techniques require accurate color information
to obtain satisfactory results.
We compare the chroma upsampling in Sony Vegas 9 to Cinelerra and
to a simple inpainting technique that uses the luma plane to increase
chroma resolution.
Comparison of Vegas and Cinelerra
Screen captures of DV encoded video displayed by Vegas and Cinelerra are
magnified 4x.
The sample aspect ratio was set to 3:2 to avoid additional scaling
in the display routines.
Vertical banding in the red colors near the blue
background is evident in both programs.
Interpolation and Inpainting
The following describes linear interpolation and a simple
inpainting technique to upsample the chroma planes in DV video.
It is presented as a image to allow for the
mathematical typesetting.
In a practical implementation it may happen that
ni,4 is zero.
The solution I adopted in my code was to replace
ni,k by
ni,k+ck where
c is a non-zero bias factor and
switch to linear
interpolation when ni,4+ck is smaller than a
certain threshold.
Given the 8 bit color depth of DV video a threshold of 4 or a
bias factor of 1 is reasonable.
Cinelerra Plugin
I have written a Cinelerra plugin called
yuv411 that performs interpolation
and inpainting for DV source. This plugin yields
improved chroma upsampling results.
Note that the red vertical banding is gone from the upsampled images.
The plugin has an additional setting called offset. This
setting shifts the chroma planes to the right by one or two pixels.
For DV source downloaded from a camcorder the offset should be 0.
For a DV file encoded by ffmpeg, the offset should be 1.
This plugin is designed to work with the YUV-8 Bit or
YUVA-8 Bit color models in Cinelerra. It will not work
with any of the RGB color models.
Conclusions
The inpainting algorithm is simple and obtains better
chroma key results than any of the other algorithms.
Vegas may use different upsampling algorithms
for chroma keying and encoding than it does for displaying
video. This has not been tested.
Improved upsampling of the chroma planes helps
with the 4:1:1 to 4:2:0 color space conversion needed to
create DVD compliant mpeg2 video streams.
However, if the project is being rendered back to DV
format, better quality will be obtained without
chroma upsampling.
Last Updated: Thu Mar 31 23:22:16 PDT 2011
