error in implementation of dct in opencv - c++

Question

why the following errors occur when i try to implement dct in opencv?
Assertion failed (type == CV_32FC1 || type == CV_64FC1) in dct,
here is the code:
#include <cv.h>
#include "opencv2/imgproc/imgproc.hpp"
#include <highgui.h>
#include <iostream>
using namespace cv;
using namespace std;
int main()
{
Mat image,dimage(image);
image = imread( "lena.jpg", 1 );
if( !image.data )
{
printf( "No image data \n" );
return -1;
}
namedWindow( "Display Image", CV_WINDOW_AUTOSIZE );
imshow( "Display Image", image );
dimage=Mat::zeros(image.rows,image.cols,CV_32F);
dct(image, dimage, DCT_INVERSE == 0 );
namedWindow( "DCT Image", CV_WINDOW_AUTOSIZE );
imshow( "DCT image", dimage );
waitKey(0);
return 0;
}

Answer 1

you have to convert your image from uchar to float first (and maybe back to uchar later):
// also please use the c++ headers, not the old c-ones !
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
Mat image = imread( "lena.jpg", 0 ); // grayscale
Mat fimage;
image.convertTo(fimage, CV_32F, 1.0/255); // also scale to [0..1] range (not mandatory)
// you're lucky with lena (512x512), for odd sizes, you have to
// pad it to pow2 size, or use dft() instead:
Mat dimage;
dct( fimage, dimage );
// process dimage,
// then same way back:
dct( dimage, dimage, DCT_INVERSE );
dimage.convertTo(image, CV_8U); // maybe scale back to [0..255] range (depending on your processing)
imshow("result",image);
waitKey();