convolution implementation in c++ - c++

Question

I want to implement 2D convolution function in C++ by myself, without using filter2D(). I'm trying to iterate all pixels of input image and kernel, then, assign new value to each pixel of dst.
However, I got this error.
Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)
I found that this error tells I'm accessing nullptr, but I could not solve the problem. Here is my c++ code.
cv::Mat_<float> spatialConvolution(const cv::Mat_<float>& src, const cv::Mat_<float>& kernel)
{
// declare variables
Mat_<float> dst;
Mat_<float> flipped_kernel;
float tmp = 0.0;
// flip kernel
flip(kernel, flipped_kernel, -1);
// multiply and integrate
// input rows
for(int i=0;i<src.rows;i++){
// input columns
for(int j=0;j<src.cols;j++){
// kernel rows
for(int k=0;k<flipped_kernel.rows;k++){
// kernel columns
for(int l=0;l<flipped_kernel.cols;l++){
tmp += src.at<float>(i,j) * flipped_kernel.at<float>(k,l);
}
}
dst.at<float>(i,j) = tmp;
}
}
return dst.clone();
}

Answer 1

To simplify let's suppose you have kernel 3x3
k(0,0) k(0,1) k(0,2)
k(1,0) k(1,1) k(1,2)
k(2,0) k(2,1) k(2,2)
to calculate convolution you are scanning input image (marked as I) from left to fright, from top to bottom
and for every pixel of input image you assign one value calculated from the formula below:
newValue(y,x) = I(y-1,x-1) * k(0,0) + I(y-1,x) * k(0,1) + I(y-1,x+1) * k(0,2)
+ I(y,x-1) * k(1,0) + I(y,x) * k(1,1) + I(y,x+1) * k(1,2) +
+ I(y+1,x-1) * k(2,0) + I(y+1,x) * k(2,1) + I(y+1,x+1) * k(2,2)
------------------x------------>
|
|
| [k(0,0) k(0,1) k(0,2)]
y [k(1,0) k(1,1) k(1,2)]
| [k(2,0) k(2,1) k(2,2)]
|
(y,x) of input Image (I) is anchor point of kernel, to assign new value to I(y,x)
you need to multiply every k coefficient by corresponding point of I - your code doesn't do it.
First you need to create dst matrix with dimenstion as original image, and the same type of pixel.
Then you need to rewrite your loops to reflect formula described above:
cv::Mat_<float> spatialConvolution(const cv::Mat_<float>& src, const cv::Mat_<float>& kernel)
{
Mat dst(src.rows,src.cols,src.type());
Mat_<float> flipped_kernel;
flip(kernel, flipped_kernel, -1);
const int dx = kernel.cols / 2;
const int dy = kernel.rows / 2;
for (int i = 0; i<src.rows; i++)
{
for (int j = 0; j<src.cols; j++)
{
float tmp = 0.0f;
for (int k = 0; k<flipped_kernel.rows; k++)
{
for (int l = 0; l<flipped_kernel.cols; l++)
{
int x = j - dx + l;
int y = i - dy + k;
if (x >= 0 && x < src.cols && y >= 0 && y < src.rows)
tmp += src.at<float>(y, x) * flipped_kernel.at<float>(k, l);
}
}
dst.at<float>(i, j) = saturate_cast<float>(tmp);
}
}
return dst.clone();
}

Answer 2

Your memory access error is presumably happening due to the line:
dst.at<float>(i,j) = tmp;
because dst is not initialized. You can't assign something to that index of the matrix if it has no size/data. Instead, initialize the matrix first, as Mat_<float> is a declaration, not an initialization. Use one of the initializations where you can specify a cv::Size or the rows/columns from the different constructors for Mat (see the docs). For example, you can initialize dst with:
Mat dst{src.size(), src.type()};