Monthly Archives: April 2012

Extracting jpeg DCT coefficients

[c]
#include <stdio.h>
#include <assert.h>

#include <cv.h>
#include <highgui.h>

extern "C"
{
#include "jpeglib.h"
#include <setjmp.h>
}

#define DEBUG 0
#define OUTPUT_IMAGES 1

/*
* Extract the DC terms from the specified component.
*/
IplImage *
extract_dc(j_decompress_ptr cinfo, jvirt_barray_ptr *coeffs, int ci)
{
jpeg_component_info *ci_ptr = &cinfo->comp_info[ci];
CvSize size = cvSize(ci_ptr->width_in_blocks, ci_ptr->height_in_blocks);
IplImage *dc = cvCreateImage(size, IPL_DEPTH_8U, 1);
assert(dc != NULL);

JQUANT_TBL *tbl = ci_ptr->quant_table;
UINT16 dc_quant = tbl->quantval[0];

#if DEBUG
printf("DCT method: %x\n", cinfo->dct_method);
printf
(
"component: %d (%d x %d blocks) sampling: (%d x %d)\n",
ci,
ci_ptr->width_in_blocks,
ci_ptr->height_in_blocks,
ci_ptr->h_samp_factor,
ci_ptr->v_samp_factor
);

printf("quantization table: %d\n", ci);
for (int i = 0; i < DCTSIZE2; ++i)
{
printf("% 4d ", (int)(tbl->quantval[i]));
if ((i + 1) % 8 == 0)
printf("\n");
}

printf("raw DC coefficients:\n");
#endif

JBLOCKARRAY buf =
(cinfo->mem->access_virt_barray)
(
(j_common_ptr)cinfo,
coeffs[ci],
0,
ci_ptr->v_samp_factor,
FALSE
);
for (int sf = 0; (JDIMENSION)sf < ci_ptr->height_in_blocks; ++sf)
{
for (JDIMENSION b = 0; b < ci_ptr->width_in_blocks; ++b)
{
int intensity = 0;

intensity = buf[sf][b][0]*dc_quant/DCTSIZE + 128;
intensity = MAX(0, intensity);
intensity = MIN(255, intensity);

cvSet2D(dc, sf, (int)b, cvScalar(intensity));

#if DEBUG
printf("% 2d ", buf[sf][b][0]);
#endif
}
#if DEBUG
printf("\n");
#endif
}

return dc;

}

IplImage *upscale_chroma(IplImage *quarter, CvSize full_size)
{
IplImage *full = cvCreateImage(full_size, IPL_DEPTH_8U, 1);
cvResize(quarter, full, CV_INTER_NN);
return full;
}

GLOBAL(int)
read_JPEG_file (char * filename, IplImage **dc)
{
/* This struct contains the JPEG decompression parameters and pointers to
* working space (which is allocated as needed by the JPEG library).
*/
struct jpeg_decompress_struct cinfo;

struct jpeg_error_mgr jerr;
/* More stuff */
FILE * infile; /* source file */

/* In this example we want to open the input file before doing anything else,
* so that the setjmp() error recovery below can assume the file is open.
* VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
* requires it in order to read binary files.
*/

if ((infile = fopen(filename, "rb")) == NULL) {
fprintf(stderr, "can’t open %s\n", filename);
return 0;
}

/* Step 1: allocate and initialize JPEG decompression object */

cinfo.err = jpeg_std_error(&jerr);

/* Now we can initialize the JPEG decompression object. */
jpeg_create_decompress(&cinfo);

/* Step 2: specify data source (eg, a file) */

jpeg_stdio_src(&cinfo, infile);

/* Step 3: read file parameters with jpeg_read_header() */

(void) jpeg_read_header(&cinfo, TRUE);
/* We can ignore the return value from jpeg_read_header since
* (a) suspension is not possible with the stdio data source, and
* (b) we passed TRUE to reject a tables-only JPEG file as an error.
* See libjpeg.txt for more info.
*/

/* Step 4: set parameters for decompression */

/* In this example, we don’t need to change any of the defaults set by
* jpeg_read_header(), so we do nothing here.
*/

jvirt_barray_ptr *coeffs = jpeg_read_coefficients(&cinfo);

IplImage *y = extract_dc(&cinfo, coeffs, 0);
IplImage *cb_q = extract_dc(&cinfo, coeffs, 1);
IplImage *cr_q = extract_dc(&cinfo, coeffs, 2);

IplImage *cb = upscale_chroma(cb_q, cvGetSize(y));
IplImage *cr = upscale_chroma(cr_q, cvGetSize(y));

cvReleaseImage(&cb_q);
cvReleaseImage(&cr_q);

#if OUTPUT_IMAGES
cvSaveImage("y.png", y);
cvSaveImage("cb.png", cb);
cvSaveImage("cr.png", cr);
#endif

*dc = cvCreateImage(cvGetSize(y), IPL_DEPTH_8U, 3);
assert(dc != NULL);

cvMerge(y, cr, cb, NULL, *dc);

cvReleaseImage(&y);
cvReleaseImage(&cb);
cvReleaseImage(&cr);

/* Step 7: Finish decompression */

(void) jpeg_finish_decompress(&cinfo);
/* We can ignore the return value since suspension is not possible
* with the stdio data source.
*/

/* Step 8: Release JPEG decompression object */

/* This is an important step since it will release a good deal of memory. */
jpeg_destroy_decompress(&cinfo);

fclose(infile);

return 1;
}

int
main(int argc, char **argv)
{
int ret = 0;
if (argc != 2)
{
fprintf(stderr, "usage: %s filename.jpg\n", argv[0]);
return 1;
}
IplImage *dc = NULL;
ret = read_JPEG_file(argv[1], &dc);
assert(dc != NULL);

IplImage *rgb = cvCreateImage(cvGetSize(dc), IPL_DEPTH_8U, 3);
cvCvtColor(dc, rgb, CV_YCrCb2RGB);

#if OUTPUT_IMAGES
cvSaveImage("rgb.png", rgb);
#else
cvNamedWindow("DC", CV_WINDOW_AUTOSIZE);
cvShowImage("DC", rgb);
cvWaitKey(0);
#endif

cvReleaseImage(&dc);
cvReleaseImage(&rgb);

return 0;
}[/c]
收集自:Extracting DCT coefficients from encoded images and video