CS50 - PSet4 (Filter) - Blur - c

Question

I'm running the following code
void blur(int height, int width, RGBTRIPLE image[height][width])
{
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
int div = 1;
if ( j + 1 < width - 1 && j + 1 > 0)
{
div++;
}
else
{
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
}
if ( i + 1 < height - 1 && i + 1 > 0)
{
div++;
}
else
{
image[i + 1][j].rgbtRed = 0;
image[i + 1][j].rgbtRed = 0;
image[i + 1][j].rgbtRed = 0;
}
if ( j + 1 < width - 1 && j + 1 > 0 && i + 1 < height - 1 && i + 1 > 0)
{
div++;
}
else
{
image[i + 1][j + 1].rgbtRed = 0;
image[i + 1][j + 1].rgbtRed = 0;
image[i + 1][j + 1].rgbtRed = 0;
}
if ( i - 1 < height - 1 && i - 1 > 0)
{
div++;
}
else
{
image[i - 1][j].rgbtRed = 0;
image[i - 1][j].rgbtRed = 0;
image[i - 1][j].rgbtRed = 0;
}
if ( i - 1 < height - 1 && i - 1 > 0 && j + 1 < width - 1 && j + 1 > 0)
{
div++;
}
else
{
image[i - 1][j + 1].rgbtRed = 0;
image[i - 1][j + 1].rgbtRed = 0;
image[i - 1][j + 1].rgbtRed = 0;
}
if ( j - 1 < width - 1 && j - 1 > 0)
{
div++;
}
else
{
image[i][j - 1].rgbtRed = 0;
image[i][j - 1].rgbtRed = 0;
image[i][j - 1].rgbtRed = 0;
}
if ( j - 1 < width - 1 && j - 1 > 0 && i + 1 < height - 1 && i + 1 > 0)
{
div++;
}
else
{
image[i + 1][j - 1].rgbtRed = 0;
image[i + 1][j - 1].rgbtRed = 0;
image[i + 1][j - 1].rgbtRed = 0;
}
if ( j - 1 < width - 1 && j - 1 > 0 && i - 1 < height - 1 && i - 1 > 0)
{
div++;
}
else
{
image[i - 1][j - 1].rgbtRed = 0;
image[i - 1][j - 1].rgbtRed = 0;
image[i - 1][j - 1].rgbtRed = 0;
}
int averageR = (float)(image[i][j].rgbtRed + image[i][j + 1].rgbtRed + image[i][j - 1].rgbtRed + image[i + 1][j].rgbtRed + image[i - 1][j].rgbtRed + image[i + 1][j + 1].rgbtRed + image[i + 1][j - 1].rgbtRed + image[i - 1][j + 1].rgbtRed + image[i + 1][j - 1].rgbtRed) / div;
int averageG = (float)(image[i][j].rgbtGreen + image[i][j + 1].rgbtGreen + image[i][j - 1].rgbtGreen + image[i + 1][j].rgbtGreen + image[i - 1][j].rgbtGreen + image[i + 1][j + 1].rgbtGreen + image[i + 1][j - 1].rgbtGreen + image[i - 1][j + 1].rgbtGreen + image[i + 1][j - 1].rgbtGreen) / div;
int averageB = (float)(image[i][j].rgbtBlue + image[i][j + 1].rgbtBlue + image[i][j - 1].rgbtBlue + image[i + 1][j].rgbtBlue + image[i - 1][j].rgbtBlue + image[i + 1][j + 1].rgbtBlue + image[i + 1][j - 1].rgbtBlue + image[i - 1][j + 1].rgbtBlue + image[i + 1][j - 1].rgbtBlue) / div;
image[i][j].rgbtRed = round(averageR);
image[i][j].rgbtGreen = round(averageG);
image[i][j].rgbtBlue = round(averageB);
}
}
return;
}
Unfortnately, everytime I try to run it in the blur part it appears this message:
UndefinedBehaviorSanitizer:DEADLYSIGNAL
==8771==ERROR: UndefinedBehaviorSanitizer: SEGV on unknown address 0x7f7a4ec2890a (pc 0x000000427faa bp 0x7ffd07cd9170 sp 0x7ffd07cd7560 T8771)
==8771==The signal is caused by a WRITE memory access.
#0 0x427fa9 (/home/ubuntu/pset4/filter/filter+0x427fa9)
#1 0x4232b1 (/home/ubuntu/pset4/filter/filter+0x4232b1)
#2 0x7f7a4db1bb96 (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
#3 0x402dd9 (/home/ubuntu/pset4/filter/filter+0x402dd9)
UndefinedBehaviorSanitizer can not provide additional info.
==8771==ABORTING
Any idea what this might be?

Answer 1

You are accessing out of the bounds of your image 2D array.
This happens in several points.
Consider your loops:
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
}
}
i and j are indexes that can vary respectively from 0 and height and from 0 and width.
But for both these loops you access beyond this limit. For example if j = width-1:
if ( j + 1 < width - 1 && j + 1 > 0)
{
div++;
}
else
{
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
}
The first conditional won't be true as width-1+1 > width -1, so you will end up in the else section, in which you will access
image[i][width].rgbtRed = 0;
that is out of bounds!
And the same happens whenever i = height-1: rows such as image[i + 1][j] will access
image[height][j].rgbtRed = 0;
that is out of bounds as well.
Accessing arrays out of bounds results in undefined behavior, and that's what your sanitizer detects.
Note: although it is unrelated to the error you are asking for, you might also want to correct the mistake in
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
and in similar parts all over the function. Note that only Red level is affected (three times!); I suppose that you intended affect also Green and Blue, instead.
How to proceed
Implementing the whole function for you would be beyond the scope of this question, but I'll try to suggest how to proceed.
For each pixel, you currently
Count the elements that are not in the edge of the image (div++)
Set to 0 the elements that should not contribute to the average (that are actually out of bounds and caused your undefined behavior)
Sum all the values of the elements surrounding the pixel (including those you set to 0) and divide by div
But what forces you to sum all the elements? Just sum those which are actually useful! You already have a correct place in which to perform this sum: it's the place in which you increment div.
Something like that:
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
int div = 1;
int curRedSum = 0, curGReenSum = 0, curBlueSum = 0;
if ( j + 1 < width - 1 && j + 1 > 0)
{
div++;
curRedSum += image[i][j + 1].rgbtRed;
curGreenSum += image[i][j + 1].rgbtGreen;
curBlueSum += image[i][j + 1].rgbtBlue;
}
if ( i + 1 < height - 1 && i + 1 > 0)
{
div++;
curRedSum += image[i+1][j].rgbtRed;
curGreenSum += image[i+1][j].rgbtGreen;
curBlueSum += image[i+1][j].rgbtBlue;
}
/* ... */
float averageR = (float)(curRedSum / ( float )div );
float averageG = (float)(curGreenSum / ( float )div );
float averageB = (float)(curBlueSum / ( float )div );
image[i][j].rgbtRed = round(averageR); // Shouldn't this assignemnt be performed on a copy of the image?
image[i][j].rgbtGreen = round(averageG); // Shouldn't this assignemnt be performed on a copy of the image?
image[i][j].rgbtBlue = round(averageB); // Shouldn't this assignemnt be performed on a copy of the image?
}
}
Disclaimer: this suggestion is supposed to be just an hint. I don't guarantee this will compile and work without any further adjustment.