NV12 yuv pixel format

Written by Paul Bourke
August 2016

The NV12 image format is commonly found as the native format from various machine vision, and other, video cameras. It is yet another variant where colour information is stored at a lower resolution than the intensity data. In the NV12 case the intensity (Y) data is stored as 8 bit samples, and the colour (Cr, Cb) information as 2x2 subsampled image, this is otherwise known as 4:2:0. The Y data is stored in one plane, the Cr and Cb channels are either interleaved in another plane or sometimes (in contrast to the official specification) in two planes.

Supposedly the official MicroSoft statement is:
A format in which all Y samples are found first in memory as an array of unsigned char with an even number of lines (possibly with a larger stride for memory alignment), followed immediately by an array of unsigned char containing interleaved Cb and Cr samples (such that if addressed as a little-endian WORD type, Cb would be in the LSBs and Cr would be in the MSBs) with the same total stride as the Y samples. This is the preferred 4:2:0 pixel format.

For example, where nx and ny are the width and height of the image respectively and the Cr and Cb are stored in separate planes.

   for (j=0;j<ny;j++) {
      for (i=0;i<nx;i++) {
          indexy = j*nx+i;
          y[indexy] = fgetc(fptr);
      }
   }
   for (j=0;j<ny;j+=2) {
      for (i=0;i<nx;i+=2) {
         indexuv = j*nx/4+i/2;
         u[indexuv] = fgetc(fptr);
      }
   }
   for (j=0;j<ny;j+=2) {
      for (i=0;i<nx;i+=2) {
         indexuv = j*nx/4+i/2;
         v[indexuv] = fgetc(fptr);
      }
   }

The YUV to RGB conversion can be based upon a few standards.

BITMAP3 YUV_to_RGB(int y,int u,int v)
{
   int r,g,b;
   BITMAP3 bm = {0,0,0};
   
   // u and v are +-0.5
   u -= 128;
   v -= 128;

   /* Conversion
   r = y + 1.370705 * v;
   g = y - 0.698001 * v - 0.337633 * u;
   b = y + 1.732446 * u;
   */
   r = y + 1.402 * v;  
   g = y - 0.34414 * u - 0.71414 * v;  
   b = y + 1.772 * u; 

   // Clamp to 0..1
   if (r < 0) r = 0;
   if (g < 0) g = 0;
   if (b < 0) b = 0;
   if (r > 255) r = 255;
   if (g > 255) g = 255;
   if (b > 255) b = 255;

   bm.r = r;
   bm.g = g;
   bm.b = b;

   return(bm);
}