DM8127 about IPIPE、IPIPEIF and ISIF mode input size

Hi,all!
DM8127 run ipnc2.8 in the lowpower M2M mode,the camera outputs H264(5M)+D1+MJPEG(5M).
About RSZE、IPIPEIF and ISIF mode input size,we have three questions.

1、In the issdrv_ispApi.c file Iss_ispControl() function.

Set rszA and rezB input size,the code are as folloes:

rszASetOutConfig(pRszCfg->inWidth - 2,pRszCfg->inHeight - 4,pRszCfg->outWidth0,pRszCfg->outHeight0);
rszBSetOutConfig(pRszCfg->inWidth - 2,pRszCfg->inHeight - 4,pRszCfg->outWidth0,pRszCfg->outHeight0);

Q1: Why the input size are inWidth-2 and inHeight-4 ?
     When we set input size are inWidth and inHeight,the camera can not output streams.
     When we set input size are smaller than inWidth-2 and inHeight-4,the camera can output streams, and the streams become smaller.

2、In the issdrv_ispApi.c file Iss_ispControl() function.

Set ipipeif PPLN and LPFR,the code are as folloes:

Iss_ispWriteReg(&ipipeif_reg->PPLN,((pRszCfg->inWidth + 16) - 1),0,13);
Iss_ispWriteReg(&ipipeif_reg->LPFR,((pRszCfg->inHeight + 16) - 1),0,13);

Q2: Why the PPLN and LPFR are (inWidth + 16)-1 and (inHeight + 16)-1? Is it because the follow said:

But when we set PPLN = inWidth + 10 and LPFR = inHeight + 10,the camera can output streams.

3、In the issdrv_captureApi.c file Iss_captCreate() function.

Set isif regesiter, the code are as follows:

if (pObj->createArgs.scParams[0].inHeight == 1920)
{
    isif_reg->HDW = 2577;
    isif_reg->VDW = 1923;
    isif_reg->PPLN = 2577;
    isif_reg->LPFR = 3847;
    isif_reg->LNH = 2563;
    isif_reg->LNV = 1921;
}

Q3: Why are these value? The value must be these?

But when we modify the value are as follows,the camera can also output streams.
isif_reg->HDW = 2563;
isif_reg->VDW = 1921;
isif_reg->PPLN = 2563;
isif_reg->LPFR = 3843;
isif_reg->LNH = 2559;
isif_reg->LNV = 1919;

Thanks a lot!