你可以使用jpeg_write_coefficients来写入你修改后的DCT。
以下信息可在libjpeg.txt中找到。
To write the contents of a JPEG file as DCT coefficients, you must provide
the DCT coefficients stored in virtual block arrays. You can either pass
block arrays read from an input JPEG file by jpeg_read_coefficients(), or
allocate virtual arrays from the JPEG compression object and fill them
yourself. In either case, jpeg_write_coefficients() is substituted for
jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is
* Create compression object
* Set all compression parameters as necessary
* Request virtual arrays if needed
* jpeg_write_coefficients()
* jpeg_finish_compress()
* Destroy or re-use compression object
jpeg_write_coefficients() is passed a pointer to an array of virtual block
array descriptors; the number of arrays is equal to cinfo.num_components.
The virtual arrays need only have been requested, not realized, before
jpeg_write_coefficients() is called. A side-effect of
jpeg_write_coefficients() is to realize any virtual arrays that have been
requested from the compression object's memory manager. Thus, when obtaining
the virtual arrays from the compression object, you should fill the arrays
after calling jpeg_write_coefficients(). The data is actually written out
when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
the file header.
When writing raw DCT coefficients, it is crucial that the JPEG quantization
tables and sampling factors match the way the data was encoded, or the
resulting file will be invalid. For transcoding from an existing JPEG file,
we recommend using jpeg_copy_critical_parameters(). This routine initializes
all the compression parameters to default values (like jpeg_set_defaults()),
then copies the critical information from a source decompression object.
The decompression object should have just been used to read the entire
JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().
jpeg_write_coefficients() marks all tables stored in the compression object
as needing to be written to the output file (thus, it acts like
jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid
emitting abbreviated JPEG files by accident. If you really want to emit an
abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
individual sent_table flags, between calling jpeg_write_coefficients() and
jpeg_finish_compress().
如果要更改一个单独的DCT系数，您可以使用以下简单的代码：
要访问任何DCT系数，您需要更改四个索引：cx、bx、by和bi。
在我的代码中，我使用了blockptr_one[bi]++;来增加一个DCT系数。
#include <stdio.h>
#include <jpeglib.h>
#include <stdlib.h>
#include <iostream>
#include <string>
int write_jpeg_file(std::string outname,jpeg_decompress_struct in_cinfo, jvirt_barray_ptr *coeffs_array ){
struct jpeg_compress_struct cinfo;
struct jpeg_error_mgr jerr;
FILE * infile;
if ((infile = fopen(outname.c_str(), "wb")) == NULL) {
fprintf(stderr, "can't open %s\n", outname.c_str());
return 0;
}
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
jpeg_stdio_dest(&cinfo, infile);
j_compress_ptr cinfo_ptr = &cinfo;
jpeg_copy_critical_parameters((j_decompress_ptr)&in_cinfo,cinfo_ptr);
jpeg_write_coefficients(cinfo_ptr, coeffs_array);
jpeg_finish_compress( &cinfo );
jpeg_destroy_compress( &cinfo );
fclose( infile );
return 1;
}
int read_jpeg_file( std::string filename, std::string outname )
{
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
FILE * infile;
if ((infile = fopen(filename.c_str(), "rb")) == NULL) {
fprintf(stderr, "can't open %s\n", filename.c_str());
return 0;
}
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo, infile);
(void) jpeg_read_header(&cinfo, TRUE);
jvirt_barray_ptr *coeffs_array = jpeg_read_coefficients(&cinfo);
int ci = 0;
int by = 0;
int bx = 0;
int bi = 0;
JBLOCKARRAY buffer_one;
JCOEFPTR blockptr_one;
jpeg_component_info* compptr_one;
compptr_one = cinfo.comp_info + ci;
buffer_one = (cinfo.mem->access_virt_barray)((j_common_ptr)&cinfo, coeffs_array[ci], by, (JDIMENSION)1, FALSE);
blockptr_one = buffer_one[0][bx];
blockptr_one[bi]++;
write_jpeg_file(outname, cinfo, coeffs_array);
jpeg_finish_decompress( &cinfo );
jpeg_destroy_decompress( &cinfo );
fclose( infile );
return 1;
}
int main()
{
std::string infilename = "you_image.jpg", outfilename = "out_image.jpg";
if( read_jpeg_file( infilename, outfilename ) > 0 )
{
std::cout << "It's Okay..." << std::endl;
}
else return -1;
return 0;
}
