在用libx264做h264压缩的时候，我们可以通过命令ffmpeg -h encoder=libx264来查看它所支持的输入格式

Encoder libx264 [libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10]:

General capabilities: delay threads

Threading capabilities: auto

Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p nv12 nv16 nv21

yuv422p不是YUY2, 可以看见我从USB摄像头直接取出的数据YUY2是不能直接当作输入格式的。(采集YUY2数据查看上一篇文章https://my.oschina.net/noevilme/blog/4462358)

常有3种方案来做转换:

自己按像素点做转换

使用libyuv

使用ffmpeg swscale

我推荐第2种和第3种， 大佬们的算法都有优化的，除非你觉得自己更牛X。据江湖传言libyuv是swscale的几倍速度，我没测过哈。

转换前先了解一下YUY2和I420数据格式，话说YUV各种名称实在有点乱，参考http://fourcc.org/yuv.php

YUY2, YUYV, YUV422  这三个都是YUY2的别称，ffmpeg定义AV_PIX_FMT_YUYV422。

Y U Y V Y U Y V

Y U Y V Y U Y V

Y U Y V Y U Y V

Y U Y V Y U Y V

Y U Y V Y U Y V

Y U Y V Y U Y V

Y U Y V Y U Y V

I420, IYUV, YUV420P, YU12， 前面这几个都是I420的名字，其中YUV420P又是几个格式的统称，特定环境下就是I420，ffmpeg定义AV_PIX_FMT_YUV420P。

Y Y Y Y Y Y Y Y

Y Y Y Y Y Y Y Y

Y Y Y Y Y Y Y Y

Y Y Y Y Y Y Y Y

U U U U U U U U

V V V V V V V V

一、使用libyuv::YUY2ToI420

// Convert YUY2 to I420.

LIBYUV_API

int YUY2ToI420(const uint8_t* src_yuy2,

int src_stride_yuy2,

uint8_t* dst_y,

int dst_stride_y,

uint8_t* dst_u,

int dst_stride_u,

uint8_t* dst_v,

int dst_stride_v,

int width,

int height);

上面是接口定义，stride参数是指图片格式行距。

int libyuv_convert(const char *input_file, const char *output_file, int width,

int height) {

FILE *in_fp = fopen(input_file, "rb");

if (!in_fp) {

std::cout << "open input failure" << std::endl;

return 1;

}

FILE *out_fp = fopen(output_file, "wb");

if (!out_fp) {

std::cout << "open output failure" << std::endl;

return 1;

}

uint8_t *yuy2_image = new uint8_t[width * height * 2];

uint8_t *i420_image = new uint8_t[width * height * 3 / 2];

while (fread(yuy2_image, 1, width * height * 2, in_fp) ==

(size_t)width * height * 2) {

uint8_t *i420_y = i420_image;

uint8_t *i420_u = i420_y + width * height;

uint8_t *i420_v = i420_u + width * height / 4;

libyuv::YUY2ToI420(yuy2_image, width * 2, i420_y, width, i420_u,

width / 2, i420_v, width / 2, width, height);

fwrite(i420_image, 1, width * height * 3 / 2, out_fp);

}

delete[] i420_image;

delete[] yuy2_image;

fclose(in_fp);

fclose(out_fp);

return 0;

}

二、使用sws_scale

swscale库是ffmpeg的一部分，所以在操作的时候用AVFrame结构更加方便，不然得自己定义一个二维数组。

int ffmpeg_convert2(const char *input_file, const char *output_file, int width,

int height) {

SwsContext *context;

FILE *in_fp, *out_fp;

in_fp = fopen(input_file, "rb");

if (!in_fp) {

std::cout << "open input failure" << std::endl;

return 1;

}

out_fp = fopen(output_file, "wb");

if (!out_fp) {

std::cout << "open out file failure" << std::endl;

fclose(in_fp);

return 1;

}

context = sws_getContext(width, height, AV_PIX_FMT_YUYV422, width, height,

AV_PIX_FMT_YUV420P, SWS_FAST_BILINEAR, nullptr,

nullptr, nullptr);

AVFrame *av_frame_in = av_frame_alloc();

auto yuy2_image_size =

av_image_alloc(av_frame_in->data, av_frame_in->linesize, width, height,

AV_PIX_FMT_YUYV422, 1);

AVFrame *av_frame_out = av_frame_alloc();

auto i420_image_size =

av_image_alloc(av_frame_out->data, av_frame_out->linesize, width,

height, AV_PIX_FMT_YUV420P, 1);

while (fread(av_frame_in->data[0], 1, yuy2_image_size, in_fp) ==

(size_t)yuy2_image_size) {

sws_scale(context, av_frame_in->data, av_frame_in->linesize, 0, height,

av_frame_out->data, av_frame_out->linesize);

fwrite(av_frame_out->data[0], 1, i420_image_size, out_fp);

}

sws_freeContext(context);

av_freep(&av_frame_in->data[0]);

av_freep(&av_frame_out->data[0]);

av_frame_free(&av_frame_in);

av_frame_free(&av_frame_out);

fclose(in_fp);

fclose(out_fp);

return 0;

}

调用函数

#include

#include

#include

#include

#include

#include

extern "C" {

#include

#include

#include

#include

#include

};

int main(int argc, char **argv) {

const char *input_file =

"1280x720_yuy2.yuv";

const char *output_file = "yuv_1280x720_i420.yuv";

libyuv_convert(input_file, output_file, 1280, 720);

ffmpeg_convert2(input_file, "ff_1280x720_i420.yuv", 1280, 720);

return 0;

}