Graphics Reference
In-Depth Information
TMVP candidates provide the best tradeoff between coding efficiency and memory
bandwidth reduction. Furthermore, the general impact of sub-sampling the motion
information was measured. While the 8
8 subsampling show no difference in
coding efficiency compared to 4
4, the current 16
16 scheme results in a coding
efficiency loss of 0.1 % BD-rate which is negligible and can be considered as being
in the noise margin.
5.3
Fractional Sample Interpolation
Interpolation tasks arise naturally in the context of video coding because the
true displacements of objects from one picture to another are independent of the
sampling grid of cameras. Therefore, in MCP, fractional-sample accuracy is used to
more accurately capture continuous motion. Samples available at integer positions
are filtered to estimate values at fractional positions. This spatial domain operation
can be seen in the frequency domain as introducing phase delays to individual
frequency components. An ideal interpolation filter for band-limited signals induces
a constant phase delay to all frequencies and does not alter their magnitudes. The
efficiency of MCP is limited by many factors—the spectral content of original and
already reconstructed pictures, camera noise level, motion blur, quantization noise
in reconstructed pictures, etc.
Similar to H.264/AVC, HEVC supports motion vectors with quarter-pixel
accuracy for the luma component and one-eighth pixel accuracy for chroma
components. If the motion vector has a half or quarter-pixel accuracy, samples
at fractional positions need to be interpolated using the samples at integer-sample
positions. The interpolation process in HEVC introduces several improvements over
H.264/AVC that contributes to the significant coding efficiency increase of HEVC.
In this section, these differences are first explained and then the complexity and
coding efficiency characteristics of the HEVC interpolation process are presented.
5.3.1
Overview
In order to improve the filter response in the high frequency range, luma and chroma
interpolation filters have been re-designed and the tap-lengths were increased. The
luma interpolation process in HEVC uses a symmetric 8-tap filter for half-sample
positions and an asymmetric 7-tap filter for quarter-sample positions. For chroma
samples, a 4-tap filter was introduced.
The intermediate values used in interpolation process are kept at a higher
accuracy in HEVC to improve coding efficiency. This is done as follows (please
refer to Fig.
5.9
for notation throughout the text, where integer-sample values are
shown with dark squares and the fractional-sample values are shown with white
squares):
