shrink image c++ - 'System.AccessViolationException' - c++

Question

I have a project where we have to develop some image processing functions. One of the functions is shrinking an image.
this is the description of the function
void averageRegions(int blockWidth, int blockHeight)
INPUTS: Integers indicating the width and height of the blocks?to be averaged
OUTPUTS: NONE
When this function is called, you should create a new image that will consist of 1 pixel for every block of size
blockWidth by blockHeight pixels in the original image, with each pixel being the average color of the pixels in that
region in the original image.
Please note that it may be easier if you split this into 2 functions and call your helper function from within this one.
The second function could then just calculate the average value of a block of pixels given to it, and return that
to the original function to be used. However, this implementation is up to you! Complete it as you see fit.
I have completed the code of it however after closing the app I get this error
An unhandled exception of type 'System.AccessViolationException' occurred in MCS2514Pgm2.exe
Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
or this one
Heap Corruption Detected: after Normal block (#126) at 0x004cF6c0 CRT detected that the application wrote to memory after end of heap bugger.
This is the function code
void averageRegions(int blockWidth, int blockHeight)
{
//please add the code
int height = inImage.getHeight();
int width = inImage.getWidth();
pixel** myPixels = inImage.getPixels();
pixel* pixelptr;
int Rsum = 0, Gsum = 0, Bsum = 0;
int Ravg, Gavg, Bavg, pcount = 0, m, n;
outImage.createNewImage(width/blockWidth, height/blockHeight);
pixel** outPixels = outImage.getPixels();
//pixelptr = &myPixels[0][4];
for(int x = 0; x < height; x +=blockHeight)
{
for(int y = 0; y < width; y += blockWidth)
{
for(int i = x; i < blockHeight+x; i++)
{
for(int j = y; j < blockWidth+y; j++)
{
Rsum += myPixels[i][j].red;
Gsum += myPixels[i][j].green;
Bsum += myPixels[i][j].blue;
pcount++;
}
}
Ravg = Rsum/pcount;
Gavg = Gsum/pcount;
Bavg = Bsum/pcount;
for(int i = x; i < blockHeight+x; i++)
{
for(int j = y; j < blockWidth+y; j++)
{
myPixels[i][j].red = Ravg;
myPixels[i][j].green = Gavg;
myPixels[i][j].blue = Bavg;
m = x/blockHeight;
n = y/blockWidth;
outPixels[m][n].red = myPixels[i][j].red;
outPixels[m][n].green = myPixels[i][j].green;
outPixels[m][n].blue = myPixels[i][j].blue;
}
}
pcount=0;
Rsum = 0;
Gsum = 0;
Bsum = 0;
}
}
inImage = outImage;
}
this is the image.h
#ifndef IMAGE
#define IMAGE
#include <atlimage.h>
#include <string>
#include <cstdlib>
#include "globals.h"
#include "pixel.h"
using namespace std;
class image {
public:
image(); //the image constructor (initializes everything)
image(string filename); //a image constructor that directly loads an image from disk
~image(); //the image destructor (deletes the dynamically created pixel array)
void createNewImage(int width, int height); //this function deletes any current image data and creates a new blank image
//with the specified width/height and allocates the needed number of pixels
//dynamically.
bool loadImage(string filename); //load an image from the specified file path. Return true if it works, false if it is not a valid image.
//Note that we only accept images of the RGB 8bit colorspace!
void saveImage(string filename); //Save an image to the specified path
pixel** getPixels(); //return the 2-dimensional pixels array
int getWidth(); //return the width of the image
int getHeight(); //return the height of the image
void viewImage(CImage* myImage); //This function is called by the windows GUI. It returns the image in format the GUI understands.
private:
void pixelsToCImage(CImage* myImage); //this function is called internally by the image class.
//it converts our pixel struct array to a standard BGR uchar array with word spacing.
//(Don't worry about what this does)
pixel** pixels; // pixel data array for image
int width, height; // stores the image dimensions
};
#endif
And this is Pixel.h
#ifndef PIXEL_H
#define PIXEL_H
class pixel
{
public:
unsigned char red; //the red component
unsigned char green; //the green component
unsigned char blue; //the blue component
};
#endif
Can any one tell me why I am getting this error
In addition:
the error is taking me to this line in dbgdel.cpp
/* verify block type */
_ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));

Answer 1

This error occurs because you are accessing outside of memory allocated for an array. There are several places in your code where this can happen.
If height is not a multiple of blockHeight, or width is not a multiple of blockWidth, your i/j loops will access elements outside of the memory allocated for myPixels.
Another possibility is when writing to outPixels if blockHeight and blockWidth are not equal. Your computation of m and n may have the blockHeight and blockWidth swapped (you're dividing x by blockHeight).

Answer 2

In
for (int x = 0; x < height; x += blockHeight)
say height is 100 and blockHeight is 33
x == 0. 0 < 100, so the body is entered and iterates 0 -> 32
x == 33. 33 < 100, so the body is entered and iterates 33 -> 65
x == 66. 66 < 100, so the body is entered and iterates 66 -> 98
x == 99. 99 < 100, so the body is entered and iterates 99 -> 131
Sadly there is no 100 -> 131.