libjpeg ver. 6b jpeg_stdio_src vs jpeg_mem_src

问题

I am using Libjpeg version 6b. In version 8 they have a nice function to read data out of the memory called jpeg_mem_src(...), unfortunately ver. 6b does not have this function.

What can I use to read compressed data directly from memory? All I see is jpeg_stdio_src which reads from the hard drive.

回答1:

Write your own...

/* Read JPEG image from a memory segment */
static void init_source (j_decompress_ptr cinfo) {}
static boolean fill_input_buffer (j_decompress_ptr cinfo)
{
    ERREXIT(cinfo, JERR_INPUT_EMPTY);
return TRUE;
}
static void skip_input_data (j_decompress_ptr cinfo, long num_bytes)
{
    struct jpeg_source_mgr* src = (struct jpeg_source_mgr*) cinfo->src;

    if (num_bytes > 0) {
        src->next_input_byte += (size_t) num_bytes;
        src->bytes_in_buffer -= (size_t) num_bytes;
    }
}
static void term_source (j_decompress_ptr cinfo) {}
static void jpeg_mem_src (j_decompress_ptr cinfo, void* buffer, long nbytes)
{
    struct jpeg_source_mgr* src;

    if (cinfo->src == NULL) {   /* first time for this JPEG object? */
        cinfo->src = (struct jpeg_source_mgr *)
            (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
            SIZEOF(struct jpeg_source_mgr));
    }

    src = (struct jpeg_source_mgr*) cinfo->src;
    src->init_source = init_source;
    src->fill_input_buffer = fill_input_buffer;
    src->skip_input_data = skip_input_data;
    src->resync_to_restart = jpeg_resync_to_restart; /* use default method */
    src->term_source = term_source;
    src->bytes_in_buffer = nbytes;
    src->next_input_byte = (JOCTET*)buffer;
}

and then to use it:

...
    /* Step 2: specify data source (eg, a file) */
    jpeg_mem_src(&dinfo, buffer, nbytes);
...

where buffer is a pointer to the memory chunk containing the compressed jpeg image, and nbytes is the length of that buffer.

回答2:

Answering to poor s093294 who has been waiting for an answer for more than a year. I can't comment, so creating a new answer is the only way.

ERREXIT is a macro in libjpeg. Include jerror.h and you're all set.

回答3:

Or you can also try to use GNU's fmemopen() function which should be declared in stdio.h header file.

FILE * source = fmemopen(inbuffer, inlength, "rb");
if (source == NULL)
{
    fprintf(stderr, "Calling fmemopen() has failed.\n");
    exit(1);
}

// ...

jpeg_stdio_src(&cinfo, source);

// ...

fclose(source);

来源：https://stackoverflow.com/questions/5280756/libjpeg-ver-6b-jpeg-stdio-src-vs-jpeg-mem-src

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c++

libjpeg