  This paper describes a technique for performing intra prediction of the
chroma planes based on the reconstructed luma plane in the frequency domain.
This prediction exploits the fact that while RGB to YUV color conversion has
the property that it decorrelates the color planes globally across an image,
there is still some correlation locally at the block level. Previous proposals
compute a linear model of the spatial relationship between the luma plane (Y)
and the two chroma planes (U and V). In codecs that use lapped transforms this
is not possible since transform support extends across the block boundaries and
thus neighboring blocks are unavailable during intra-prediction. We design a
frequency domain intra predictor for chroma that exploits the same local
correlation with lower complexity than the spatial predictor and which works
with lapped transforms. We then describe a low-complexity algorithm that
directly uses luma coefficients as a chroma predictor based on gain-shape
quantization and band partitioning. An experiment is performed that compares
these two techniques inside the experimental Daala video codec and shows the
lower complexity algorithm to be a better chroma predictor.
