Altera Video and Image Processing Suite - Manual (Page 123)

Figure 7-1: Resampling 4.4.4 to a 4.2.2 Image
The figure below shows the location of samples in a co-sited 4:2:2 image.
1 2 3 4
1
2
Sample No 5 6 7 8
++ ++ ++ ++
3
4
= Y’
+
= Cb
+ = Cr
++
= CbCr
++
= Y’CbCr
++ ++ ++ ++
The Chroma Resampler IP core supports only the cosited form of horizontal resampling—the form for
4:2:2 data in ITU Recommendation BT.601, MPEG-2, and other standards.
Note: For more information about the ITU standard, refer to Recommendation ITU-R BT.601, Encoding
Parameters of Digital Television for Studios, 1992, International Telecommunications Union,
Geneva.
You can configure the Chroma Resampler IP core to change image size at run time using control packets.
4:4:4 to 4:2:2
The nearest-neighbor algorithm is the simplest way to down-scale the chroma channels. The nearest-
neighbor algorithm discards the Cb and Cr samples that occur on even columns (assuming the first
column is numbered 1). This algorithm is very fast and cheap but, due to aliasing effects, it does not
produce the best image quality.
To get the best results when down-scaling, apply a filter to remove high-frequency data and thus avoid
possible aliasing. The filtered algorithm for horizontal subsampling uses a 9-tap filter with a fixed set of
coefficients. The coefficients are based on a Lanczos-2 function that the Scaler II IP core uses. Their
quantized form is known as the Turkowski Decimator.
4:2:2 to 4:4:4
The nearest-neighbor algorithm is the simplest way to up-scale the chroma channels. The nearest-
neighbor algorithm duplicates each incoming Cb and Cr sample to fill in the missing data. This algorithm
is very fast and cheap, but it tends to produce sharp jagged edges in the chroma channels.
The filtered algorithm uses the same upscaling method as the Scaler II IP core—that is a four-tap filter
with Lanczos-2 coefficients. Use this filter with a phase offset of 0 for the odd output columns (those with
existing data) and an offset of one-half for the even columns (those without direct input data). A filter
with phase offset 0 has no effect, so the function implements it as a pass-through filter. A filter with phase
offset of one-half interpolates the missing values and has fixed coefficients that bit-shifts and additions
implement. This algorithm performs suitable upsampling and does not use memory or multipliers. It uses
more logic elements than the nearest-neighbor algorithm and is not the highest quality available.
The best image quality for upsampling is obtained by using the filtered algorithm with luma-adaptive
mode enabled. This mode looks at the luma channel during interpolation and uses this to detect edges.
Edges in the luma channel make appropriate phase-shifts in the interpolation coefficients for the chroma
channels.
7-2 4:4:4 to 4:2:2
UG-VIPSUITE
2015.05.04
Altera Corporation Chroma Resampler IP Core
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