我决定使用 libjpeg 作为处理 jpeg 文件的主库。我已经阅读了 libjpg.txt 文件。我很高兴该库允许以方便的方式读取/写入 DCT 系数。由于编写自己的解码器需要很长时间。
我的工作与无损嵌入有关。目前我需要从文件中读取 DCT 系数，然后修改其中一些并将更改后的系数写入同一个文件中。
好吧，我找到了jpeg_write_coefficients()功能。我天真地认为我可以将其应用于减压对象（struct jpeg_decompress_struct）。但它不起作用，需要一个压缩对象。
我无法相信如此强大的图书馆无法做到这一点。我认为我很可能错过了一些东西。尽管我试图保持专注。也许写系数可以做更复杂的方式。但我不知道该怎么做。
如果您提出您的想法，我将非常高兴。
您可以使用 jpeg_write_coefficients 来编写更改后的 DCT。
以下信息可在libjpeg.txt
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().
所以要改变单个dct，可以使用下面的简单代码： 要访问任何dct coeff，需要改变四个index，cx，bx，by，bi。在我的代码中，我曾经blockptr_one[bi]++;增加一个 dct Coeff
#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);
//change one dct:
int ci = 0; // between 0 and number of image component
int by = 0; // between 0 and compptr_one->height_in_blocks
int bx = 0; // between 0 and compptr_one->width_in_blocks
int bi = 0; // between 0 and 64 (8x8)
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";
/* Try opening a jpeg*/
if( read_jpeg_file( infilename, outfilename ) > 0 )
{
std::cout << "It's Okay..." << std::endl;
}
else return -1;
return 0;
}
你真的应该看看库附带的transupp.h和 jpegtran 的源代码。无论如何，这是我的带有注释的脏代码，部分来自jpegtran。它使您可以一一操作 DCT 系数。
#include "jpeglib.h" /* Common decls for cjpeg/djpeg applications */
#include "transupp.h" /* Support routines for jpegtran */
struct jpeg_decompress_struct srcinfo;
struct jpeg_compress_struct dstinfo;
struct jpeg_error_mgr jsrcerr, jdsterr;
static jpeg_transform_info transformoption; /* image transformation options */
transformoption.transform = JXFORM_NONE;
transformoption.trim = FALSE;
transformoption.force_grayscale = FALSE;
jvirt_barray_ptr * src_coef_arrays;
jvirt_barray_ptr * dst_coef_arrays;
/* Initialize the JPEG decompression object with default error handling. */
srcinfo.err = jpeg_std_error(&jsrcerr);
jpeg_create_decompress(&srcinfo);
/* Initialize the JPEG compression object with default error handling. */
dstinfo.err = jpeg_std_error(&jdsterr);
jpeg_create_compress(&dstinfo);
FILE *fp;
if((fp = fopen(filePath], "rb")) == NULL) {
//Throw an error
} else {
//Continue
}
/* Specify data source for decompression */
jpeg_stdio_src(&srcinfo, fp);
/* Enable saving of extra markers that we want to copy */
jcopy_markers_setup(&srcinfo, JCOPYOPT_ALL);
/* Read file header */
(void) jpeg_read_header(&srcinfo, TRUE);
jtransform_request_workspace(&srcinfo, &transformoption);
src_coef_arrays = jpeg_read_coefficients(&srcinfo);
jpeg_copy_critical_parameters(&srcinfo, &dstinfo);
/* Do your DCT shenanigans here on src_coef_arrays like this (I've moved it into a separate function): */
moveDCTAround(&srcinfo, &dstinfo, 0, src_coef_arrays);
/* ..when done with DCT, do this: */
dst_coef_arrays = jtransform_adjust_parameters(&srcinfo, &dstinfo, src_coef_arrays, &transformoption);
fclose(fp);
//And write everything back
fp = fopen(filePath, "wb");
/* Specify data destination for compression */
jpeg_stdio_dest(&dstinfo, fp);
/* Start compressor (note no image data is actually written here) */
jpeg_write_coefficients(&dstinfo, dst_coef_arrays);
/* Copy to the output file any extra markers that we want to preserve */
jcopy_markers_execute(&srcinfo, &dstinfo, JCOPYOPT_ALL);
jpeg_finish_compress(&dstinfo);
jpeg_destroy_compress(&dstinfo);
(void) jpeg_finish_decompress(&srcinfo);
jpeg_destroy_decompress(&srcinfo);
fclose(fp);
以及函数本身：
void moveDCTAround (j_decompress_ptr srcinfo, j_compress_ptr dstinfo, JDIMENSION x_crop_offset, jvirt_barray_ptr *src_coef_arrays)
{
size_t block_row_size;
JBLOCKARRAY coef_buffers[MAX_COMPONENTS];
JBLOCKARRAY row_ptrs[MAX_COMPONENTS];
//Allocate DCT array buffers
for (JDIMENSION compnum=0; compnum<srcinfo->num_components; compnum++)
{
coef_buffers[compnum] = (dstinfo->mem->alloc_barray)((j_common_ptr) dstinfo, JPOOL_IMAGE, srcinfo->comp_info[compnum].width_in_blocks,
srcinfo->comp_info[compnum].height_in_blocks);
}
//For each component,
for (JDIMENSION compnum=0; compnum<srcinfo->num_components; compnum++)
{
block_row_size = (size_t) sizeof(JCOEF)*DCTSIZE2*srcinfo->comp_info[compnum].width_in_blocks;
//...iterate over rows,
for (JDIMENSION rownum=0; rownum<srcinfo->comp_info[compnum].height_in_blocks; rownum++)
{
row_ptrs[compnum] = ((dstinfo)->mem->access_virt_barray)((j_common_ptr) &dstinfo, src_coef_arrays[compnum], rownum, (JDIMENSION) 1, FALSE);
//...and for each block in a row,
for (JDIMENSION blocknum=0; blocknum<srcinfo->comp_info[compnum].width_in_blocks; blocknum++)
//...iterate over DCT coefficients
for (JDIMENSION i=0; i<DCTSIZE2; i++)
{
//Manipulate your DCT coefficients here. For instance, the code here inverts the image.
coef_buffers[compnum][rownum][blocknum][i] = -row_ptrs[compnum][0][blocknum][i];
}
}
}
//Save the changes
//For each component,
for (JDIMENSION compnum=0; compnum<srcinfo->num_components; compnum++)
{
block_row_size = (size_t) sizeof(JCOEF)*DCTSIZE2 * srcinfo->comp_info[compnum].width_in_blocks;
//...iterate over rows
for (JDIMENSION rownum=0; rownum < srcinfo->comp_info[compnum].height_in_blocks; rownum++)
{
//Copy the whole rows
row_ptrs[compnum] = (dstinfo->mem->access_virt_barray)((j_common_ptr) dstinfo, src_coef_arrays[compnum], rownum, (JDIMENSION) 1, TRUE);
memcpy(row_ptrs[compnum][0][0], coef_buffers[compnum][rownum][0], block_row_size);
}
}