Texas Instruments TMS320DM648 manual Chrominance Re-sampling, 2x Co-Sited Scaling

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Video Output Filtering

4.4.2 Chrominance Re-sampling Operation

Chrominance re-sampling computes chrominance values at sample points corresponding to output luminance samples based on the input interspersed chrominance samples. This filter performs the conversion between interspersed YCbCr 4:2:2 format and co-sited YCbCr 4:2:2 format. The vertical portion of the conversion from YCbCr 4:2:0 to interspersed YCbCr 4:2:2 must be performed in software.

The chrominance re-sampling filters calculate the implied value of Cb and Cr co-sited with luminance sample points based upon nearby interspersed Cb and Cr samples. The resulting values are clamped to between 01h and FEh before being output. Chrominance re-sampling is shown in Figure 4-15.

Figure 4-15. Chrominance Re-sampling

a b c d e f g h i j k l

YCbCr 4:2:2 interspersed source pixels

YCbCr 4:2:2 co-sited output results

- Luma (Y)	-Chroma (Cb/Cr)
sample	samples

Cb’f = (-3Cbab+ 33Cbcd + 101Cbef - 3Cbgh ) / 128 Cr’f = (-3Crab+ 33Crcd + 101Cref - 3Crgh ) / 128

4.4.3 Scaling Operation

The 2x -scaling mode is used to double the horizontal resolution of output luminance and chrominance data. This allows processed CIF resolution images to be output at full size. Vertical scaling must be performed in software. Scaling for co-sited source is shown in Figure 4-16and scaling for interspersed source is shown in Figure 4-17.

For a co-sited source, the source luminance pixels are output unchanged for every even pixel (a, b, c, etc., in Figure 4-16). Odd luminance pixels (a',b',c',etc.) are generated from neighboring source (even) pixels using a four tap filter. The chrominance source pixels are output unchanged for every other even pixel (a, c, e, etc.). Other even output pixel (b, d, f, etc.) chrominance values are generated from neighboring source chrominance pixels using a four tap filter.

For an interspersed source, the luminance is output identically to the co-sited case. Chrominance output is generated using a four tap filter with one of two different coefficient sets depending on which source chrominance pixel the output pixel is closest.

Note that because input scaling is limited to 2x, full BT.656 width output is not achieved from CIF source images. The horizontal location of the reduced image can be adjusted using HOFFSET.

Figure 4-16.			2x Co-Sited Scaling
a	b		c	d	e	f	g
YCbCr 4:2:2 co-sited
source pixels
a	a’ b	b’	c	c’ d	d’ e e’	f f’	g

2⋅ upscaled output

Y’b = Yb

Cb’c = Cbc

Cr’c = Crc

- Luma (Y)	- Chroma (Cb/Cr)
sample	samples

Y’d’ = (-1Yc+ 17Yd + 17Ye - 1Yf ) / 32

Cb’d = (-1Cba+ 17Cbc + 17Cbe - 1Cbg ) / 32 Cr’d = (-1Cra+ 17Crc + 17Cre - 1Crg ) / 32

104 Video Display Port SPRUEM1 –May 2007

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