There are too few online tutorials on using the libyuv library to scale the NV12 format under Linux. It is simply a torment for the blogger, because my ability to read the source code is really poor! ! ! But no matter what, write down the income of the past few days, hoping to help everyone.

1. Introduction to libyuv library

libyuv is Google's open source library that realizes conversion, rotation, and scaling between various YUV and RGB. It is cross-platform, can be compiled and run on Windows, Linux, Mac, Android and other operating systems, x86, x64, arm architecture, and supports SIMD instruction acceleration such as SSE, AVX, and NEON.

Compile method (note that the gcc version must be above 4.8 for compilation), clone the libyuv library to the local warehouse, enter the libyuv directory, and compile:

git clone https://github.com/lemenkov/libyuv.git
cd libyuv
make -f linux.mk

A static library of libyuv.a will be generated in the current directory. I did not compile and install libyuv locally. Just extract the libyuv.a static library and include header files and run them separately (the required files are downloaded from the attachment).
Run make to generate an executable file. Execute the executable file to reduce the picture in NV12 format with 1920*1080 pixels to 1280*720.

2. Introduction to YUV420

Libyuv cannot directly scale images in NV12 format. You must first convert NV12 to I420 (the memory distribution format is the same as YV12). In fact, NV12 and YV12 belong to YUV420, but the memory distribution of UV components is different. Let's first understand the difference between the two. Without further ado, the picture above (pay attention to the memory distribution of the UV component, YV12 is stored sequentially in each of the three components of YUV, while the UV component of NV12 is stored interleaved):

In fact, for this conversion based on the libyuv library, the memory distribution of the following YV12 is more correct (the red box is the Y component, the blue is the U component, and the yellow box is the V component. Note that this is for the specified YUV components The memory range is very important!!! For example: For a picture with YV12 pixels of WxH, the Y component is W and the UV is W/2):

That's it for basic knowledge.

3. The program realizes the reduction of NV12 format pictures

Implementation algorithm: use libyuv::NV12ToI420() function of libyuv library to convert NV12 to I420, then use libyuv::I420Scale() function to reduce I420 from pixel 1920*1080 to 1280*720, and finally use libyuv::I420ToNV12() Convert function back to NV12 format

#include <stdio.h>
#include <stdlib.h>
#include "libyuv.h"

int NV12Scale(uint8 *psrc_buf, int psrc_w, int psrc_h, uint8 *pdst_buf, int pdst_w, int pdst_h, libyuv::FilterModeEnum pfmode);

int main(int argc, const char *argv[])
{
    const int src_w = 1920;
    const int src_h = 1080;
    const int dst_w = 1280;
    const int dst_h =  720;

    libyuv::FilterModeEnum fmode = libyuv::kFilterNone;

    char inputPathname[30]  = "videotestsrc_1920x1080.nv12";
    char outputPathname[30] = "videotestdst_1280x720.nv12";

    FILE * fin  = fopen(inputPathname,  "rb");
    FILE * fout = fopen(outputPathname, "wb+");

    /* Allocate memory for nv12 */
    uint8 *src_buf = (uint8 *)malloc((src_w * src_h * 3) >> 1);
    uint8 *dst_buf = (uint8 *)malloc((dst_w * dst_h * 3) >> 1);

    /* Read file data to buffer */
    fread(src_buf, sizeof(uint8), (src_w * src_h * 3) >> 1, fin);

    /* Scale NV12 */
    NV12Scale(src_buf, src_w, src_h, dst_buf, dst_w, dst_h, fmode);

    /* Write the data to file */
    fwrite(dst_buf, sizeof(uint8), (dst_w * dst_h * 3) >> 1, fout);

    free(src_buf);
    free(dst_buf);

    fclose(fin);
    fclose(fout);

    return 0;
}

int NV12Scale(uint8 *psrc_buf, int psrc_w, int psrc_h, uint8 *pdst_buf, int pdst_w, int pdst_h, libyuv::FilterModeEnum pfmode)
{
    uint8 *i420_buf1 = (uint8 *)malloc((psrc_w * psrc_h * 3) >> 1);
    uint8 *i420_buf2 = (uint8 *)malloc((pdst_w * pdst_h * 3) >> 1);

    /* NV12_1920x1080 -> I420_1920x1080 */
    libyuv::NV12ToI420(&psrc_buf[0],                           psrc_w,
                       &psrc_buf[psrc_w * psrc_h],             psrc_w,
                       &i420_buf1[0],                          psrc_w,
                       &i420_buf1[psrc_w * psrc_h],            psrc_w >> 1,
                       &i420_buf1[(psrc_w * psrc_h * 5) >> 2], psrc_w >> 1,
                       psrc_w, psrc_h);

    /* I420_1920x1080 -> I420_1280x720 */
    libyuv::I420Scale(&i420_buf1[0],                          psrc_w,
                      &i420_buf1[psrc_w * psrc_h],            psrc_w >> 1,
                      &i420_buf1[(psrc_w * psrc_h * 5) >> 2], psrc_w >> 1,
                      psrc_w, psrc_h,
                      &i420_buf2[0],                          pdst_w,
                      &i420_buf2[pdst_w * pdst_h],            pdst_w >> 1,
                      &i420_buf2[(pdst_w * pdst_h * 5) >> 2], pdst_w >> 1,
                      pdst_w, pdst_h,
                      pfmode);

    /* I420_1280x720 -> NV12_1280x720 */
    libyuv::I420ToNV12(&i420_buf2[0],                          pdst_w,
                       &i420_buf2[pdst_w * pdst_h],            pdst_w >> 1,
                       &i420_buf2[(pdst_w * pdst_h * 5) >> 2], pdst_w >> 1,
                       &pdst_buf[0],                           pdst_w,
                       &pdst_buf[pdst_w * pdst_h],             pdst_w,
                       pdst_w,pdst_h);

    free(i420_buf1);
    free(i420_buf2);

    return 0;
}

4. Accessories

Program, test NV12 file and libyuv static library and header file.

yuv viewer (yvplayer).

5. NV12 plus rectangular box effect

If you want to know about the effect of adding rectangular boxes on NV12 format pictures, you can read my other blog post.
Set the rectangular border of nv12