libjpeg-turbo支持直接从JPEG解压成YUV格式，或者反之。这也是我当初想研究它的一个动力。

看了头文件注释，它是支持YUV444(即宏TJSAMP_444)，YUV422(即宏TJSAMP_422)，YUV420(即宏TJSAMP_420)，YUV400(即宏TJSAMP_440)，YUV411(即宏TJSAMP_411)。可惜的是，只支持平面格式(plane)，对于交织的如UYVY或特别的如NV12(即YUV420SP)或NV16(即YUV422SP)，都没看到有支持。在sourceforge上看邮件列表，发现有些描述，但还没研究过，看发布的源码，也未见有说明，估计是不支持的。
本文简单介绍如何从JPEG解压成YUV格式，以及如何将YUV压缩成JPEG。
libjpeg-turbo使用tjBufSizeYUV2函数计算YUV大小，开始时没注意第二个参数pad，默认传递0，发现没效果，而传1或4，却是可以的。解压后的YUV的格式，是由JPEG图片的采样格式决定的，如果JPEG本身是YUV420，则解压得到的YUV，就是YUV420格式。
示例代码如下：

int tjpeg2yuv(unsigned char* jpeg_buffer, int jpeg_size, unsigned char** yuv_buffer, int* yuv_size, int* yuv_type)
{
    tjhandle handle = NULL;
    int width, height, subsample, colorspace;
    int flags = 0;
    int padding = 1; // 1或4均可，但不能是0
    int ret = 0;
 
    handle = tjInitDecompress();
    tjDecompressHeader3(handle, jpeg_buffer, jpeg_size, &width, &height, &subsample, &colorspace);
 
    printf("w: %d h: %d subsample: %d color: %d\n", width, height, subsample, colorspace);
    
    flags |= 0;
    
    *yuv_type = subsample;
    // 注：经测试，指定的yuv采样格式只对YUV缓冲区大小有影响，实际上还是按JPEG本身的YUV格式来转换的
    *yuv_size = tjBufSizeYUV2(width, padding, height, subsample);
    *yuv_buffer =(unsigned char *)malloc(*yuv_size);
    if (*yuv_buffer == NULL)
    {
        printf("malloc buffer for rgb failed.\n");
        return -1;
    }
 
    ret = tjDecompressToYUV2(handle, jpeg_buffer, jpeg_size, *yuv_buffer, width,
            padding, height, flags);
    if (ret < 0)
    {
        printf("compress to jpeg failed: %s\n", tjGetErrorStr());
    }
    tjDestroy(handle);
 
    return ret;
}
 
int tyuv2jpeg(unsigned char* yuv_buffer, int yuv_size, int width, int height, int subsample, unsigned char** jpeg_buffer, unsigned long* jpeg_size, int quality)
{
    tjhandle handle = NULL;
    int flags = 0;
    int padding = 1; // 1或4均可，但不能是0
    int need_size = 0;
    int ret = 0;
 
    handle = tjInitCompress();
   
    flags |= 0;
 
    need_size = tjBufSizeYUV2(width, padding, height, subsample);
    if (need_size != yuv_size)
    {
        printf("we detect yuv size: %d, but you give: %d, check again.\n", need_size, yuv_size);
        return 0;
    }
 
    ret = tjCompressFromYUV(handle, yuv_buffer, width, padding, height, subsample, jpeg_buffer, jpeg_size, quality, flags);
    if (ret < 0)
    {
        printf("compress to jpeg failed: %s\n", tjGetErrorStr());
    }
 
    tjDestroy(handle);
 
    return ret;
}

另外，该库也支持从RGB转换成YUV，或反之。调用相应的函数即可，不再详述。示例代码如下：

int trgb2yuv(unsigned char* rgb_buffer, int width, int height, unsigned char** yuv_buffer, int* yuv_size, int subsample)
{
    tjhandle handle = NULL;
    int flags = 0;
    int padding = 1; // 1或4均可，但不能是0
    int pixelfmt = TJPF_RGB;
    int ret = 0;
 
    handle = tjInitCompress();
   
    flags |= 0;
 
    *yuv_size = tjBufSizeYUV2(width, padding, height, subsample);
 
    *yuv_buffer =(unsigned char *)malloc(*yuv_size);
    if (*yuv_buffer == NULL)
    {
        printf("malloc buffer for rgb failed.\n");
        return -1;
    }
    ret = tjEncodeYUV3(handle, rgb_buffer, width, 0, height, pixelfmt, *yuv_buffer, padding, subsample, flags);
    if (ret < 0)
    {
        printf("encode to yuv failed: %s\n", tjGetErrorStr());
    }
 
    tjDestroy(handle);
 
    return ret;
}
 
int tyuv2rgb(unsigned char* yuv_buffer, int yuv_size, int width, int height, int subsample, unsigned char** rgb_buffer, int* rgb_size)
{
    tjhandle handle = NULL;
    int flags = 0;
    int padding = 1; // 1或4均可，但不能是0
    int pixelfmt = TJPF_RGB;
    int need_size = 0;
    int ret = 0;
 
    handle = tjInitDecompress();
   
    flags |= 0;
 
    need_size = tjBufSizeYUV2(width, padding, height, subsample);
    if (need_size != yuv_size)
    {
        printf("we detect yuv size: %d, but you give: %d, check again.\n", need_size, yuv_size);
        return -1;
    }
 
    *rgb_size = width*height*tjPixelSize[pixelfmt];
 
    *rgb_buffer =(unsigned char *)malloc(*rgb_size);
    if (*rgb_buffer == NULL)
    {
        printf("malloc buffer for rgb failed.\n");
        return -1;
    }
    ret = tjDecodeYUV(handle, yuv_buffer, padding, subsample, *rgb_buffer, width, 0, height, pixelfmt, flags);
    if (ret < 0)
    {
        printf("decode to rgb failed: %s\n", tjGetErrorStr());
    }
 
    tjDestroy(handle);
 
    return ret;
}

以上代码示例，二级指针均在函数内分配内存，需要调用者自行释放，否则会有内存泄漏。

李迟 7.8