A simple C# method that _attempts_ to convert a BMP formatted byte array to NV12

BMPtoNV12.cs

public void BMPtoNV12(byte[] yuv420sp, byte[] argb, int width, int height)
        {
            int frameSize = width * height;

            int yIndex = 0;
           
            int uvIndex = frameSize;

            uint a;
            int R, G, B, Y, U, V;
            int index = 0;
            for (int j = 0; j < height; j++)
            {
                //int index = width * j;

                for (int i = 0; i < width; i++)
                {
                    a = (argb[index] & 0xff000000) >> 24; // a is not used obviously
                    R = (argb[index] & 0xff0000) >> 16;
                    G = (argb[index] & 0xff00) >> 8;
                    B = (argb[index] & 0xff) >> 0;
                    // well known RGB to YUV algorithm
                    Y = (( 66 * R + 129 * G + 25 * B + 128) >> 8) + 16;
                    U = ((-38 * R - 74 * G + 112 * B + 128) >> 8) + 128;
                    V = ((112 * R - 94 * G -  18 * B + 128) >> 8) + 128;

//NV12
//Related to I420, NV12 has one luma "luminance" plane Y and one plane with U and V values interleaved.

//In NV12, chroma planes(blue and red) are subsampled in both the horizontal and vertical dimensions by a factor of 2.

//For a 2×2 group of pixels, you have 4 Y samples and 1 U and 1 V sample.

//It can be helpful to think of NV12 as I420 with the U and V planes interleaved.

//Here is a graphical representation of NV12.Each letter represents one bit:

//For 1 NV12 pixel: YYYYYYYY UVUV
//For a 2 - pixel NV12 frame: YYYYYYYYYYYYYYYY UVUVUVUV
//For a 50 - pixel NV12 frame: Y×8×50(UV)×2×50
//For a n - pixel NV12 frame: Y×8×n(UV)×2×n

                    yuv420sp[yIndex++] = (byte)((Y < 0) ? 0 : ((Y > 255) ? 255 : Y));
                    if (j % 2 == 0 && i % 2 == 0)
                    {
                        yuv420sp[uvIndex++] = (byte)((U < 0) ? 0 : ((U > 255) ? 255 : U));
                        yuv420sp[uvIndex++] = (byte)((V < 0) ? 0 : ((V > 255) ? 255 : V));
                    }
                    
                    index++;


                }
            }
        }