1、下采样(subsample or downsample)

图像下采样原理：原始图像的分辨率为WxH,对其进行n倍下采样得到(W/n)x(H/n)分辨率的图像，其中n应该是M和N的公约数才行。n倍下采样,即每n个像素抽取一个像素。
将YUV视频分为亮度分量(Y)和色度分量（U、V），对视频的每一帧都进行n采样，就可以实现对YUV视频的下采样。

void downsample(int x_sample, int y_sample, int width, int height, unsigned char* pY, unsigned char* pU, unsigned char* pV, unsigned char* pS_Y, unsigned char* pS_U, unsigned char* pS_V) {//YUV分量的起始位置unsigned char* src_img_py = pY;unsigned char* src_img_pu = pU;unsigned char* src_img_pv = pV;//YUV分量的目标位置unsigned char* dst_img_py = pS_Y;unsigned char* dst_img_pu = pS_U;unsigned char* dst_img_pv = pS_V;unsigned char* start_src = src_img_py;unsigned char* start_src_u = src_img_pu;unsigned char* start_src_v = src_img_pv;int cpy_x_end = width / x_sample;int cpy_y_end = height / y_sample;int cpy_x_end_c = (cpy_x_end >> 1);int cpy_y_end_c = (cpy_y_end >> 1);int i, j = 0;for (i = 0; i < cpy_y_end; i++) {src_img_py = start_src;for (j = 0; j < cpy_x_end; j++) //Luma{*dst_img_py = *src_img_py;dst_img_py++;src_img_py += x_sample;//每n个像素取一个}start_src += y_sample * width;}for (i = 0; i < cpy_y_end_c; i++) {src_img_pu = start_src_u;src_img_pv = start_src_v;for (j = 0; j < cpy_x_end_c; j++) //Chroma{*dst_img_pu = *src_img_pu;dst_img_pu++;src_img_pu += x_sample;*dst_img_pv = *src_img_pv;dst_img_pv++;src_img_pv += x_sample;}start_src_u += y_sample * width / 2;start_src_v += y_sample * width / 2;}
}

2、源码

int main(){
FILE* fin = fopen("Gaslamp.yuv", "rb+");if (!fin) {std::cout << "Open file failed!" << std::endl;return -1;}FILE* fout = fopen("Gaslamp_1.yuv", "wb+");if (!fout) {std::cout << "Open file failed!" << std::endl;return -1;}int width = 7680;int height = 3840;int ySize = width * height;int uSize = ySize / 4;int vSize = ySize / 4;uint8_t* PY = new uint8_t[ySize]();uint8_t* PU = new uint8_t[uSize]();uint8_t* PV = new uint8_t[vSize]();uint8_t* PS_Y = new uint8_t[ySize / 4]();uint8_t* PS_U = new uint8_t[uSize / 4]();uint8_t* PS_V = new uint8_t[vSize / 4]();for (int frame = 0; frame < 5; frame++) {fread(PY, 1, ySize, fin);fread(PU, 1, uSize, fin);fread(PV, 1, vSize, fin);//YUV分量的起始位置unsigned char* src_img_py = PY;unsigned char* src_img_pu = PU;unsigned char* src_img_pv = PV;//YUV分量的目标位置unsigned char* dst_img_py = PS_Y;unsigned char* dst_img_pu = PS_U;unsigned char* dst_img_pv = PS_V;int i, j = 0;for (i = 0; i < height / 2; i++) {for (j = 0; j < width / 2; ++j) //Luma{*dst_img_py = *src_img_py;dst_img_py++;src_img_py += 2;//每两个像素取一个}src_img_py += width;}for (i = 0; i < height / 4; i++) {for (j = 0; j < width / 4; ++j) //Chroma{*dst_img_pu = *src_img_pu;dst_img_pu++;src_img_pu += 2;*dst_img_pv = *src_img_pv;dst_img_pv++;src_img_pv += 2;}src_img_pu += width/2;src_img_pv += width/2;}fwrite(PS_Y, 1, ySize / 4, fout);fwrite(PS_U, 1, uSize / 4, fout);fwrite(PS_V, 1, vSize / 4, fout);std::cout << "Succeed downsample" << frame << "frame" << std::endl;}	delete[] PY;delete[] PU;delete[] PV;delete[] PS_Y;delete[] PS_U;delete[] PS_V;fclose(fin);fclose(fout);system("pause");return 0;}