CS50 - PSet4（滤镜）- 模糊

Question

我正在运行以下代码

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;
}

不幸的是，每次我尝试在模糊部分运行它时都会出现以下消息：

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

知道这可能是什么吗？

Answer 1

您的访问超出了image 2D 数组的范围。

这发生在几个方面。

考虑你的循环：

for (int i = 0; i < height; i++)
{
    for (int j = 0; j < width; j++)
    {
        
    }
}

i 和 j 是可以分别从 0 和 height 以及从 0 和 width 变化的索引。

但是对于这两个循环，您访问的权限都超出了此限制。例如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;
}

第一个条件不会像 width-1+1 > width -1 那样为真，因此您最终将进入 else 部分，您将在其中访问

image[i][width].rgbtRed = 0;

这超出了范围！
每当i = height-1 时也会发生同样的情况：image[i + 1][j] 等行将访问

image[height][j].rgbtRed = 0;

这也超出了范围。

越界访问数组会导致未定义的行为，这就是您的消毒剂检测到的。

---

注意：虽然它与您要求的错误无关，但您可能还想更正中的错误

image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;
image[i][j + 1].rgbtRed = 0;

并在所有类似的部分功能。请注意，只有 Red 级别会受到影响（三倍！）；我想你打算同时影响 Green 和 Blue。

---

如何进行

为您实现整个功能将超出此问题的范围，但我会尝试建议如何继续。

对于每个像素，您当前

1. 计算不在图像边缘的元素 (div++)
2. 将不应影响平均值的元素设置为 0（实际上是超出范围并导致您未定义的行为）
3. 将像素周围元素的所有值（包括您设置为 0 的值）相加并除以 div

但是是什么迫使您将所有元素相加？只需将那些真正有用的相加即可！你已经有了一个正确的地方来执行这个求和：这是你增加div的地方。

类似的东西：

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?
    }
}

免责声明：此建议只是一个提示。我不保证这将编译和工作没有任何进一步的调整。