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 decompress

Answer 1

Here's a fragment of code what I use to load jpeg's from memory. Maybe it will require a bit of fixing, because I extracted it from different files in my project. It will load both - grayscale and rgb images (bpp will be set either to 1 or to 3).

struct Image
{
    int bpp;
    int width;
    int height;
    unsigned char* data;
};

struct jerror_mgr
{
    jpeg_error_mgr base;
    jmp_buf        jmp;
};

METHODDEF(void) jerror_exit(j_common_ptr jinfo)
{
    jerror_mgr* err = (jerror_mgr*)jinfo->err;
    longjmp(err->jmp, 1);
}

METHODDEF(void) joutput_message(j_common_ptr)
{
}

bool Image_LoadJpeg(Image* image, unsigned char* img_data, unsigned int img_size)
{
    jpeg_decompress_struct jinfo;
    jerror_mgr jerr;

    jinfo.err = jpeg_std_error(&jerr.base);
    jerr.base.error_exit = jerror_exit;
    jerr.base.output_message = joutput_message;
    jpeg_create_decompress(&jinfo);

    image->data = NULL;

    if (setjmp(jerr.jmp)) goto bail;

    jpeg_mem_src(&jinfo, img_data, img_size);

    if (jpeg_read_header(&jinfo, TRUE) != JPEG_HEADER_OK) goto bail;

    jinfo.dct_method = JDCT_FLOAT; // change this to JDCT_ISLOW on Android/iOS

    if (!jpeg_start_decompress(&jinfo)) goto bail;

    if (jinfo.num_components != 1 && jinfo.num_components != 3) goto bail;

    image->data = new (std::nothrow) unsigned char [jinfo.output_width * jinfo.output_height * jinfo.output_components];
    if (!image->data) goto bail;

    {
        JSAMPROW ptr = image->data;
        while (jinfo.output_scanline < jinfo.output_height)
        {
            if (jpeg_read_scanlines(&jinfo, &ptr, 1) != 1) goto bail;

            ptr += jinfo.output_width * jinfo.output_components;
        }
    }

    if (!jpeg_finish_decompress(&jinfo)) goto bail;

    image->bpp = jinfo.output_components;
    image->width = jinfo.output_width;
    image->height = jinfo.output_height;

    jpeg_destroy_decompress(&jinfo);

    return true;

bail:
    jpeg_destroy_decompress(&jinfo);
    if (image->data) delete [] data;

    return false;
}