Richard Knop Richard Knop - 2 months ago 17
C Question

Write to memory buffer instead of file with libjpeg?

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

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;