Write to memory buffer instead of file with libjpeg? - c++

Question

I have found this function which uses libjpeg to write to a file:
int write_jpeg_file( char *filename )
{
struct jpeg_compress_struct cinfo;
struct jpeg_error_mgr jerr;
/* this is a pointer to one row of image data */
JSAMPROW row_pointer[1];
FILE *outfile = fopen( filename, "wb" );
if ( !outfile )
{
printf("Error opening output jpeg file %s\n!", filename );
return -1;
}
cinfo.err = jpeg_std_error( &jerr );
jpeg_create_compress(&cinfo);
jpeg_stdio_dest(&cinfo, outfile);
/* Setting the parameters of the output file here */
cinfo.image_width = width;
cinfo.image_height = height;
cinfo.input_components = bytes_per_pixel;
cinfo.in_color_space = color_space;
/* default compression parameters, we shouldn't be worried about these */
jpeg_set_defaults( &cinfo );
/* Now do the compression .. */
jpeg_start_compress( &cinfo, TRUE );
/* like reading a file, this time write one row at a time */
while( cinfo.next_scanline < cinfo.image_height )
{
row_pointer[0] = &raw_image[ cinfo.next_scanline * cinfo.image_width * cinfo.input_components];
jpeg_write_scanlines( &cinfo, row_pointer, 1 );
}
/* similar to read file, clean up after we're done compressing */
jpeg_finish_compress( &cinfo );
jpeg_destroy_compress( &cinfo );
fclose( outfile );
/* success code is 1! */
return 1;
}
I would actually need to write the jpeg compressed image just to memory buffer, without saving it to a file, to save time. Could somebody give me an example how to do it?
I have been searching the web for a while but the documentation is very rare if any and examples are also difficult to come by.

Answer 1

You can define your own destination manager quite easily. The jpeg_compress_struct contains a pointer to a jpeg_destination_mgr, which contains a pointer to a buffer, a count of space left in the buffer, and 3 pointers to functions:
init_destination (j_compress_ptr cinfo)
empty_output_buffer (j_compress_ptr cinfo)
term_destination (j_compress_ptr cinfo)
You need to fill in the function pointers before you make the first call into the jpeg library, and let those functions handle the buffer. If you create a buffer that is larger than the largest possible output that you expect, this becomes trivial; init_destination just fills in the buffer pointer and count, and empty_output_buffer and term_destination do nothing.
Here's some sample code:
std::vector<JOCTET> my_buffer;
#define BLOCK_SIZE 16384
void my_init_destination(j_compress_ptr cinfo)
{
my_buffer.resize(BLOCK_SIZE);
cinfo->dest->next_output_byte = &my_buffer[0];
cinfo->dest->free_in_buffer = my_buffer.size();
}
boolean my_empty_output_buffer(j_compress_ptr cinfo)
{
size_t oldsize = my_buffer.size();
my_buffer.resize(oldsize + BLOCK_SIZE);
cinfo->dest->next_output_byte = &my_buffer[oldsize];
cinfo->dest->free_in_buffer = my_buffer.size() - oldsize;
return true;
}
void my_term_destination(j_compress_ptr cinfo)
{
my_buffer.resize(my_buffer.size() - cinfo->dest->free_in_buffer);
}
cinfo->dest->init_destination = &my_init_destination;
cinfo->dest->empty_output_buffer = &my_empty_output_buffer;
cinfo->dest->term_destination = &my_term_destination;

Answer 2

There is a predefined function jpeg_mem_src defined in jdatasrc.c. The simplest usage example:
unsigned char *mem = NULL;
unsigned long mem_size = 0;
struct jpeg_compress_struct cinfo;
struct jpeg_error_mgr jerr;
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
jpeg_mem_dest(&cinfo, &mem, &mem_size);
// do compression
// use mem buffer
Do not forget to deallocate your buffer.

Answer 3

I have tried Mark's solution and on my platform it always gives SEGMENTATION FALUT error when it executes
cinfo->dest->term_destination = &my_term_destination;
And I turned to the jpeglib source codes (jdatadst.c) and found this:
jpeg_mem_dest (j_compress_ptr cinfo, unsigned char ** outbuffer, unsigned long * outsize)
just below the method jpeg_stdio_dest(), and I've tried it by simply fill in the address of the buffer(char*) and the address of the buffer size(int). The destination manager automatically allocates memory for the buffer and the program need to free the memory after use.
It successfully runs on my platform, Beaglebone Black with the pre-installed Angstrom Linux. My libjpeg version is 8d.

Answer 4

All you need to do is pass a FILE-like object to jpeg_stdio_dest().

Answer 5

unsigned char ***image_ptr
unsigned char* ptr;
unsigned char** image_buf;
for(int i=0;i<h;i++){
image_buf[i] = new unsigned char[w*o];
}
ptr = image_buf[0];
while (info.output_scanline < info.image_height) {
jpeg_read_scanlines(&info,&ptr,1);
ptr = image_buf[c];
c++;
}
*image_ptr = image_buf;
This is all you need to read.
JSAMPROW row_pointer;
while (info.next_scanline < info.image_height) {
row_pointer = &image_buf[info.next_scanline][0];
(void) jpeg_write_scanlines(&info, &row_pointer, 1);
}
And this is all you need to write.