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YCbCr 4:2:2 co-sited
input samples
1/2 scaled co-sited
capture results
Luma (Y)
sample
Y’
h
= (-3Y
e
+ 32Y
g
+ 70Y
h
+ 32Y
i
- 3Y
k
) / 128
-
Chroma (Cb/Cr)
samples
-
a b c d e f g h i j k l
Y’
f
= (-3Y
c
+ 32Y
e
+ 70Y
f
+ 32Y
g
- 3Y
i
) / 128
Cb’
f
= (-1Cb
c
+ 17Cb
e
+ 17Cb
g
- 1Cb
i
) / 32
Cr’
f
= (-1Cr
c
+ 17Cr
e
+ 17Cr
g
- 1Cr
i
) / 32
Y’
g
= (-3Y
d
+ 32Y
f
+ 70Y
g
+ 32Y
h
-3Y
j
) / 128
Cb’
f
= (-1Cb
c
+ 17Cb
e
+ 17Cb
g
- 1Cb
i
) / 32
Cr’
f
= (-1Cr
c
+ 17Cr
e
+ 17Cr
g
- 1Cr
i
) / 32
YCbCr 4:2:2 co-sited
input samples
1/2 scaled
chroma-resampled
capture results
Luma (Y)
sample
-
Chroma (Cb/Cr)
samples
-
a b c d e f g h i
j
k l
3.5.4EdgePixelReplication
VideoInputFiltering
Thefilteringfortheluminanceportionofthescalingfilterchangesdependingonifchrominance
re-samplingisalsoenabled.(Bychangingtheluminancefilter,thechrominancefilterscanremainthe
same.)Theresultingvaluesareclampedtobetween01handFEhandsenttotheY,Cb,andCrcapture
buffers.Scalingforco-sitedcaptureisshowninFigure3-9andscalingforchrominancere-samplingis
showninFigure3-10.
Figure3-9.1/2ScaledCo-SitedFiltering
Figure3-10.1/2ScaledChrominanceRe-sampledFiltering
Notethatbecauseinputscalingislimitedto1/2,trueCIFhorizontalresolutionisnotachievedifthefull
BT.656horizontalline(720pixels)iscaptured.ACIFsizelinecanbecapturedbyselectinga704
pixel-sizedwindowwithintheBT.656line.Thiswindowsizeandlocationonthelineareprogrammed
usingtheVCXSTARTnandVCXSTOPnbits.
Notethatwhen1/2scalingisselected,horizontaltimingappliestotheincomingdata(beforescaling).The
VCTHRLDvalueappliestothedatawrittenintotheFIFOafterscaling.
Alsonotewhenusingthescalar,standardBT.601valuesshouldbeusedforthelumaandchroma
(16-240)data.Usingvaluesbeyondthisrangemayresultinoverflowandunderflow.Thescalardoesnot
saturatethedata;therefore,datagoingbelow00horaboveFFhwillnotbeclipped,resultinginimage
degradation.
Becausethefiltersmakeuseofprecedingandtrailingsamples,filteringartifactscanoccuratthe
beginningoftheBT.656orY/CactivelinebecausenosamplesexistbeforetheSAVcode,andattheend
oftheBT.656activelinebecausenosamplesexistaftertheEAVcode.Inordertominimizeartifacts,the
firstmsamplesaftersample0(wheremisthemaximumnumberofprecedingsamplesusedbyanyofthe
filters)aremirroredtotheleftofsample0andthelastmsamplesbeforethelastsamplearemirroredto
therightofthelastsample.
Figure3-11showsedgepixelreplicationassuminganmvalueof3.Sampleaisthefirstsampleafterthe
SAVcode.Therefore,samplesb-daremirroredtotheleftofsampleatoprovidevaluesforthefilter
calculationsonthefirstfewpixelsintheline.Likewise,samplesn-1ton-3aremirroredtotherightof
thelastsamplentoprovidevaluesforthelastfewpixelsontheline.
SPRUEM1–May2007VideoCapturePort59
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