c++ - Examples or tutorials of using libjpeg-turbo's TurboJPEG

Question

The instructions for libjpeg-turbo here describes the TurboJPEG API thus: "This API wraps libjpeg-turbo and provides an easy-to-use interface for compressing and decompressing JPEG images in memory". Great, but are there some solid examples of using this API available? Just looking to decompress a fairly vanilla jpeg in memory.

I've found a few bits such as https://github.com/erlyvideo/jpeg/blob/master/c_src/jpeg.c, which appears to be using the TurboJPEG API, but are there any more solid/varied examples?

The source for libjpeg-turbo is well documented, so that does help.

Answer 1

Ok, I know that you did already solve your problem, but as some people, just like me, could be searching some simple example I will share what I created. It is an example, compressing and decompressing an RGB image. Otherwise I think that the API documentation of TurboJPEG is quite easy to understand!

Compression:

#include <turbojpeg.h>

const int JPEG_QUALITY = 75;
const int COLOR_COMPONENTS = 3;
int _width = 1920;
int _height = 1080;
long unsigned int _jpegSize = 0;
unsigned char* _compressedImage = NULL; //!< Memory is allocated by tjCompress2 if _jpegSize == 0
unsigned char buffer[_width*_height*COLOR_COMPONENTS]; //!< Contains the uncompressed image

tjhandle _jpegCompressor = tjInitCompress();

tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
          &_compressedImage, &_jpegSize, TJSAMP_444, JPEG_QUALITY,
          TJFLAG_FASTDCT);

tjDestroy(_jpegCompressor);

//to free the memory allocated by TurboJPEG (either by tjAlloc(), 
//or by the Compress/Decompress) after you are done working on it:
tjFree(&_compressedImage);

After that you have the compressed image in _compressedImage. To decompress you have to do the following:

Decompression:

#include <turbojpeg.h>

long unsigned int _jpegSize; //!< _jpegSize from above
unsigned char* _compressedImage; //!< _compressedImage from above

int jpegSubsamp, width, height;
unsigned char buffer[width*height*COLOR_COMPONENTS]; //!< will contain the decompressed image

tjhandle _jpegDecompressor = tjInitDecompress();

tjDecompressHeader2(_jpegDecompressor, _compressedImage, _jpegSize, &width, &height, &jpegSubsamp);

tjDecompress2(_jpegDecompressor, _compressedImage, _jpegSize, buffer, width, 0/*pitch*/, height, TJPF_RGB, TJFLAG_FASTDCT);

tjDestroy(_jpegDecompressor);

Some random thoughts:

I just came back over this as I am writing my bachelor thesis, and I noticed that if you run the compression in a loop it is preferable to store the biggest size of the JPEG buffer to not have to allocate a new one every turn. Basically, instead of doing:

long unsigned int _jpegSize = 0;

tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
          &_compressedImage, &_jpegSize, TJSAMP_444, JPEG_QUALITY,
          TJFLAG_FASTDCT);

we would add an object variable, holding the size of the allocated memory long unsigned int _jpegBufferSize = 0; and before every compression round we would set the jpegSize back to that value:

long unsigned int jpegSize = _jpegBufferSize;

tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
          &_compressedImage, &jpegSize, TJSAMP_444, JPEG_QUALITY,
          TJFLAG_FASTDCT);

_jpegBufferSize = _jpegBufferSize >= jpegSize? _jpegBufferSize : jpegSize;

after the compression one would compare the memory size with the actual jpegSize and set it to the jpegSize if it is higher than the previous memory size.