Beginner having trouble with invalid memory

[Iain Emerson]

The following code is sending me into an Access violation reading location 0x025e2000 - I'm a novice programmer and I'm hoping somebody can point me in the right direction

CvCapture * Capture=NULL;
Capture = cvCaptureFromCAM(-1);
 
IplImage* FrameImage = 0;    
IplImage* MirrorImage = 0;    
 
for(;;)
    {
        FrameImage = cvQueryFrame(Capture);
 
        MirrorImage = cvCloneImage(FrameImage);
        cvFlip(MirrorImage, NULL, 1);
 
int b, r, g;    //colour channels
        int x, y;        //For indexing through image
 
        cvNamedWindow( "OutPut", CV_WINDOW_AUTOSIZE );
 
        for ( x = 0; x < MirrorImage->width; x++ )
        {
            for ( y = 0; y < MirrorImage->height; y++ )
            {
                b = ((uchar *)(MirrorImage->imageData + x*MirrorImage->widthStep))[y*MirrorImage->nChannels+0];
                g = ((uchar *)(MirrorImage->imageData + x*MirrorImage->widthStep))[y*MirrorImage->nChannels+1];
                r = ((uchar *)(MirrorImage->imageData + x*MirrorImage->widthStep))[y*MirrorImage->nChannels+2];
 
                if ( !( g > 170 && r < 150 && b < 150 ) )
                    {
                    (FrameImage->imageData + x*FrameImage->widthStep)[y*FrameImage->nChannels+0]=(MirrorImage->imageData + x*MirrorImage->widthStep)[y*MirrorImage->nChannels+0];
                    (FrameImage->imageData + x*FrameImage->widthStep)[y*FrameImage->nChannels+1]=(MirrorImage->imageData + x*MirrorImage->widthStep)[y*MirrorImage->nChannels+1];
                    (FrameImage->imageData + x*FrameImage->widthStep)[y*FrameImage->nChannels+2]=(MirrorImage->imageData + x*MirrorImage->widthStep)[y*MirrorImage->nChannels+2];
                    }
            }
        }
        cvShowImage(WinName, FinImage);
        cvShowImage("OutPut", FrameImage);
}
 

I'm trying to achieve a green screen application, but the code falls over while indexing through the image pixels, at x=482, y=64.

I look forward to your wisdom.

[weaknessforcats]

FrameImage and MirrorImpage are pointers. These pointers contain 0. The address 0 is not in your process address space, hence the Access Violation.

Either create FrameImage and MirrorImage as stack arrays ot allocate the memory yourself and use them as heap arrays.

In short, you will get errors whenever you try to access memory that your process does not own.

[Liam Kurmos]

expanding the above answer in case it's not clear to you:

"...allocate the memory yourself and use them as heap arrays."

this means use the new keyword to allocate the memory

[newb16]

They are not null, they are assigned something.
FrameImage = cvQueryFrame(Capture);
MirrorImage = cvCloneImage(FrameImage);
The other thing is that you never free/release them after you process them.

[weaknessforcats]

I didn't see that. oops.

However, you can't return an array from a function. You can return the address of the array but not the number of elements. So in this case, you have no idea how large the array is. In ths case, the function needs two output arguments. One for the array address and one for the number of elements. The return value should be a success/fail code.