How to convert RGB to NV12 color space using IPP

Question

Video encoders like Intel® Media SDK require NV12 video input format.

NV12 format is YUV 4:2:0 format ordered in memory with a Y plane first, followed by packed chroma samples in interleaved UV plane.

Example:
YYYYYY
YYYYYY
UVUVUV

RGB color format, refers to Pixel-Order RGB (byte per pixel, lower byte is Red):
RGBRGBRGBRGBRGB
RGBRGBRGBRGBRGB

I did some Web research, and found out that regarding NV12, YUV is defined as YCbCr color space. There are currently at least 2 possible YCbCr formats apply NV12:

• BT.601 - Applies SDTV
• BT.709 - Applies HDTV

My question is: Is there as IPP function that converts RGB color space to NV12?

Answer 1

I found out that IPP function exists:

ippiRGBToYCbCr420_8u_C3P2R

It was hard to find, because the function name or description does not mention NV12.
The function uses BT.601 standard.

Here is a code sample for converting RGB to NV12 in BT.601 standard:

void Rgb2NV12(const unsigned char I[], int image_width, int image_height, unsigned char J[])
{
    IppStatus ipp_status;
    int srcStep = image_width*3;
    int dstYStep = image_width;
    int dstCbCrStep = image_width;
    IppiSize roiSize = {image_width, image_height};

    const Ipp8u* pSrc = (Ipp8u*)I;

    Ipp8u *pDstY    = (Ipp8u*)J;                            //Y color plane is the first image_width*image_height pixels of J.
    Ipp8u *pDstCbCr = (Ipp8u*)&J[image_width*image_height]; //In NV12 format, UV plane starts below Y.

    ipp_status = ippiRGBToYCbCr420_8u_C3P2R(pSrc, srcStep, pDstY, dstYStep, pDstCbCr, dstCbCrStep, roiSize);

    //if (ipp_status != ippStsNoErr), Handle errors...          
}

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Converting RGB to NV12 with BT.709 standard:

As for 2019, BT.709 (HDTV) standard is probably more relevant than BT.601 (SDTV).

IPP lacks a function for direct conversion from RGB to NV12 in BT.709 standard.
There is a function that converts BGR to NV12.
The solution includes two stages:

1. Convert from RGB to BGR (swap channels).
   Code sample uses ippiSwapChannels_8u_C3R for RGB to BGR conversion.
2. Convert from BGR to NV12.
   Code sample uses ippiBGRToYCbCr420_709CSC_8u_C3P2R for BGR to NV12 conversion.

The sample function requires some extra memory space for storing the intermediate BGR image.
A pointer to the sketch memory is passed to the function (the memory should be allocated outside the function).

Here is a code sample for converting RGB to NV12 in BT.709 standard:

//sketch_buff - Temporary buffer for storing image in BGR format.
//              Size of sketch_buff must be at least image_width*image_height*3 bytes.
void Rgb2NV12_709(const unsigned char I[],
                  const int image_width, 
                  const int image_height,
                  unsigned char sketch_buff[],
                  unsigned char J[])
{
    IppStatus ipp_status;    
    int srcStep = image_width*3;
    int dstBgrStep = image_width*3;
    int dstYStep = image_width;
    int dstCbCrStep = image_width;
    IppiSize roiSize = {image_width, image_height};

    const Ipp8u* pRGB = (Ipp8u*)I;
    Ipp8u* pBGR = (Ipp8u*)sketch_buff; //BGR image is stored in sketch_buff
    Ipp8u *pDstY    = (Ipp8u*)J;                            //Y color plane is the first image_width*image_height pixels of J.
    Ipp8u *pDstCbCr = (Ipp8u*)&J[image_width*image_height]; //In NV12 format, UV plane starts below Y.

    const int bgrOrder[3] = {2, 1, 0};

    //Swap Red and Blue color channels - convert from RGB to BGR
    //Store the result into sketch_buff (sketch buffer is allocated outside the function)
    ipp_status = ippiSwapChannels_8u_C3R(pRGB, srcStep, pBGR, dstBgrStep, roiSize, bgrOrder);

    //if (ipp_status != ippStsNoErr), Handle errors...

    //Convert BGR to NV12 in BT.709 standard
    ipp_status = ippiBGRToYCbCr420_709CSC_8u_C3P2R(pBGR, srcStep, pDstY, dstYStep, pDstCbCr, dstCbCrStep, roiSize);

    //if (ipp_status != ippStsNoErr), Handle errors...
}