Negative index runtime error - c++

Question

I am using this as my reference to implement my version of Bicubic interpolation for resizing the images. Here is the function that I have so far with some changes.
IplImage * bicubic(IplImage *img, int newWidth, int newHeight)
{
IplImage *img2 ;
img2 = createImage(newWidth,newHeight);
uchar * data = (uchar*)img->imageData;
uchar * Data = (uchar*)img2->imageData;
//int a,b,c,index;
uchar Cc;
uchar C[5];
uchar d0,d2,d3,a0,a1,a2,a3;
int i,j,k,jj;
int x,y;
float dx,dy;
float tx,ty;
tx = (float)img->width /newWidth ;
ty = (float)img->height / newHeight;
printf("New Width = %d, New Height = %d WidthStep = %d", newWidth, newHeight,img->widthStep);
for(i = 0; i< newHeight; i++)
{
for(j = 0; j< newWidth; j++)
{
x = (int)(tx * j);
y = (int)(ty * i);
dx = tx * j - x;
dy = ty * i - y;
for(k = 0;k < 3;k++)
{
for(jj = 0;jj <= 3 ;jj++)
{
int z = (y - 1 + jj);
//if(z > -1){
a0 = data[z * img->widthStep + (x)*img->nChannels +k];//===>Throws of runtime error
d0 = data[z * img->widthStep + (x-1)*img->nChannels +k] - a0 ;
d2 = data[z * img->widthStep + (x+1)*img->nChannels +k] - a0 ;
d3 = data[z * img->widthStep + (x+2)*img->nChannels +k] - a0 ;
a1 = -1.0/3 * d0 + d2 -1.0/6*d3;
a2 = 1.0/2 * d0 + 1.0/2*d2;
a3 = -1.0/6 * d0 - 1.0/2*d2 + 1.0/6*d3;
C[jj] = a0 + a1*dx + a2*dx*dx + a3*dx*dx*dx;
d0 = C[0]-C[1];
d2 = C[2]-C[1];
d3 = C[3]-C[1];
a0 = C[1];
a1 = -1.0/3*d0 + d2 -1.0/6*d3;
a2 = 1.0/2*d0 + 1.0/2*d2;
a3 = -1.0/6*d0 - 1.0/2*d2 + 1.0/6*d3;
Cc = a0 + a1*dy + a2*dy*dy + a3*dy*dy*dy;
Data[i*img2->widthStep +j*img2->nChannels +k ] = Cc;
//}
}
}
}
}
return img2;
}
The problem that I am facing is that when I call this bicubic function, it throws off an invalid access runtime error at the line where I find out the value of a0. I am using VS 2012 debugger and it tells me that the value of z is calculated as -1. This causes the index to access the invalid part of memory of data array.
My question is, why is this happening? Am I missing something in OpenCV's image library that can help in getting right indices so that I dont run into this error? Or am I making some mistake in accessing the correct indices?

Answer 1

for(i = 0; i< newHeight; i++)
{
for(j = 0; j< newWidth; j++)
{
x = (int)(tx * j);
y = (int)(ty * i);
dx = tx * j - x;
dy = ty * i - y;
for(k = 0;k < 3;k++)
{
for(jj = 0;jj <= 3 ;jj++)
{
int z = (y - 1 + jj);
//if(z > -1){
a0 = data[z * img->widthStep + (x)*img->nChannels +k];//===>Throws of runtime error
d0 = data[z * img->widthStep + (x-1)*img->nChannels +k] - a0 ;
On the first iteration, i and j are 0. as are k and jj
This means that:
y = (int)(ty * i); //y = ty * 0 (== 0)
int z = (y - 1 + jj); //z = 0 - 1 + 0 (==-1)
And so in the line:
a0 = data[z * img->widthStep + (x)*img->nChannels +k];//===>Throws of runtime error
the index is:
(-1) * img->widthStep + (x)*img->nChannels +k
simplifies to:
(-1) * img->widthStep + 0 + 0
which is:
-img->widthStep
This is of course out of bounds, leading to the crash.