yuv420p转nv21

您可以使用以下代码将YUV420P格式的图像转换为NV21格式的图像：

void convertYUV420PToNV21(unsigned char* yuv420p, int width, int height, unsigned char* nv21) {
    int imageSize = width * height;
    unsigned char* yPlane = yuv420p;
    unsigned char* uPlane = yuv420p + imageSize;
    unsigned char* vPlane = yuv420p + imageSize * 5 / 4;
    int yIndex = 0, uvIndex = 0;
    for (int i = 0; i < height; i++) {
        for (int j = 0; j < width; j++) {
            nv21[yIndex++] = yPlane[i * width + j];
            if (i % 2 == 0 && j % 2 == 0) {
                nv21[imageSize + uvIndex] = vPlane[uvIndex];
                nv21[imageSize + uvIndex + 1] = uPlane[uvIndex];
                uvIndex++;
            }
        }
    }
}

其中，yuv420p是YUV420P格式的图像数据，width和height分别是图像的宽度和高度，nv21是转换后的NV21格式的图像数据。