Compressing IplImage to JPEG using libjpeg in OpenCV - opencv

Question

So I have this problem. I have an IplImage that i want to compress to JPEG and do something with it. I use libjpeg8b.The code exit when it goes the function of jpeg_start_compress() with an error of "Bogus input colorspace" .Here are my code.
#include "highgui.h"
#include <stdio.h>
#include "jpeglib.h"
#include "cv.h"
#include <iostream>
#include <fstream>
using namespace std;
using namespace cv;
#pragma comment(lib, "jpeglib.lib")
bool ipl2jpeg(IplImage *frame, unsigned char **outbuffer, unsigned long*outlen)
{
IplImage *img = new IplImage;
memcpy(img,frame,frame->nSize);
unsigned char *outdata = (uchar *) img->imageData;
struct jpeg_compress_struct cinfo = {0};
struct jpeg_error_mgr jerr;
JSAMPROW row_ptr[1];
int row_stride;
*outbuffer = NULL;
*outlen = 0;
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
jpeg_mem_dest(&cinfo, outbuffer, outlen);
cinfo.image_width = frame->width;
cinfo.image_height = frame->height;
cinfo.input_components = frame->nChannels;
cinfo.in_color_space = JCS_RGB;
jpeg_set_defaults(&cinfo);
jpeg_start_compress(&cinfo, TRUE);
system("pause");
row_stride = frame->width * frame->nChannels;
while (cinfo.next_scanline < cinfo.image_height)
{
row_ptr[0] = &outdata[cinfo.next_scanline * row_stride];
jpeg_write_scanlines(&cinfo, row_ptr, 1);
}
jpeg_finish_compress(&cinfo);
jpeg_destroy_compress(&cinfo);
return true;
}
int main()
{
ofstream fout("text.txt");
unsigned char **buf;
buf = new unsigned char* [120];
for(int i=0;i<500;i++)
{
buf[i] = new unsigned char[120];
}
for(int i=0;i< 120;i++)
{
for(int j=0;j<120;j++)
{
buf[i][j] = 0;
}
}
unsigned long *len = new unsigned long;
*len = 120*120;
Ptr<IplImage> img = cvLoadImage("test.jpg",CV_LOAD_IMAGE_GRAYSCALE);
ipl2jpeg(img,buf,len);
for(int i=0;i< 120;i++)
{
for(int j=0;j<120;j++)
{
fout<<buf[i][j]<<endl;
}
}
return 0;
}

Answer 1

I've never used libjpeg before, but it looks like you're mixing color spaces. You load the image as a grayscale (CV_LOAD_IMAGE_GRAYSCALE), but are telling libjpeg that it's an RGB image (JCS_RGB). Have you tried changing the line
cinfo.in_color_space = JCS_RGB;
to
cinfo.in_color_space = JCS_GRAYSCALE;
?

Answer 2

Is there any reason why you're not using opencv's native JPEG support?
cvSaveImage(frame, "frame.jpeg");
The documentation is here.
EDIT
If you insist on using libjpeg, have a look at this post.