How to use libjpeg to read a JPEG from a std::istream? - c++

Question

libjpeg can read JPEG data from a FILE* or a buffer. My data is coming from a std::istream. I could read the entire std::istream into a buffer to use with libjpeg, but I'd rather have libjpeg read directly from the std::istream if possible. How can this be done?

Answer 1

You just need to provide wrappers around your istream. Define a struct, for instance
struct JpegStream {
jpeg_source_mgr pub;
std::istream* stream;
byte buffer [4096];
}
Then you need four methods to operate on the stream:
void init_source (j_decompress_ptr cinfo)
{
auto src = (JpegStream*)(cinfo->src);
src->stream-> // seek to 0 here
}
boolean fill_buffer (j_decompress_ptr cinfo)
{
// Read to buffer
JpegStream* src = // as above
src->pub.next_input_byte = src->buffer;
src->pub.bytes_in_buffer = // How many yo could read
return eof() ? FALSE : TRUE;
}
void skip (j_decompress_ptr cinfo, long count)
{
// Seek by count bytes forward
// Make sure you know how much you have cached and subtract that
// set bytes_in_buffer and next_input_byte
}
void term (j_decompress_ptr cinfo)
{
// Close the stream, can be nop
}
and one method to bind them to the JPEG decompression info structure:
void make_stream (j_decompress_ptr cinfo, std::istream* in)
{
JpegStream * src;
/* The source object and input buffer are made permanent so that a series
* of JPEG images can be read from the same file by calling jpeg_stdio_src
* only before the first one. (If we discarded the buffer at the end of
* one image, we'd likely lose the start of the next one.)
* This makes it unsafe to use this manager and a different source
* manager serially with the same JPEG object. Caveat programmer.
*/
if (cinfo->src == NULL)
{
/* first time for this JPEG object? */
cinfo->src = (struct jpeg_source_mgr *)
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, POOL_PERMANENT, sizeof(JpegStream));
src = reinterpret_cast<JpegStream*> (cinfo->src);
}
src = reinterpret_cast<JpegStream*> (cinfo->src);
src->pub.init_source = init_source;
src->pub.fill_input_buffer = fill_buffer;
src->pub.skip_input_data = skip;
src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
src->pub.term_source = term;
src->stream = in;
src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
src->pub.next_input_byte = NULL; /* until buffer loaded */
}
After calling jpeg_create_decompress, call your make_stream function.