CS50图像模糊函数仅中间像素计算正确且出现段错误求助

问题概述 #

编写了CS50作业中的图像模糊函数，手动处理8种边缘情况，但运行BMP文件时出现段错误（Seg fault），仅中间像素计算结果正确。使用check50测试3x3样本图像时，实际输出与预期差异较大。

原代码 #

// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
    RGBTRIPLE blurred[height][width];
    int i = 0;
    int j = 0;
    int sumred;
    int sumgreen;
    int sumblue;
    for (i = 0; i < height; i++)
    {
        for (j = 0; j < width; j++)
        {
            if (i == 0 && j == 0) //edge case 1: top left corner
            {
                // ([i][j] + [i][j+1] + [i+1][j] + [i+1][j+1])/4
                sumred = image[i][j].rgbtRed + image[i][j+1].rgbtRed + image[i+1][j].rgbtRed + image[i+1][j+1].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 4.0);
                sumgreen = image[i][j].rgbtGreen + image[i][j+1].rgbtGreen + image[i+1][j].rgbtGreen + image[i+1][j+1].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 4.0);
                sumblue = image[i][j].rgbtBlue + image[i][j+1].rgbtBlue + image[i+1][j].rgbtBlue + image[i+1][j+1].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 4.0);
            }

            if (i == 0 && j > 0 && j < (width - 1)) //edgecase 2: top roll
            {
                // ([i][j-1] + [i][j] + [i][j+1] + [i+1][j-1] + [i+1][j] + [i+1][j+1])/6
                sumred = image[i][j-1].rgbtRed + image[i][j].rgbtRed + image[i][j+1].rgbtRed + image[i+1][j-1].rgbtRed + image[i+1][j].rgbtRed + image[i+1][j+1].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 6.0);
                sumgreen = image[i][j-1].rgbtGreen + image[i][j].rgbtGreen + image[i][j+1].rgbtGreen + image[i+1][j-1].rgbtGreen + image[i+1][j].rgbtGreen + image[i+1][j+1].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 6.0);
                sumblue = image[i][j-1].rgbtBlue + image[i][j].rgbtBlue + image[i][j+1].rgbtBlue + image[i+1][j-1].rgbtBlue + image[i+1][j].rgbtBlue + image[i+1][j+1].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 6.0);
            }

            if (i == 0 && j == (width - 1)) //edge case 3: top right corner
            {
                sumred = image[i][j-1].rgbtRed + image[i][j].rgbtRed + image[i+1][j-1].rgbtRed + image[i+1][j].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 4.0);
                sumgreen = image[i][j-1].rgbtGreen + image[i][j].rgbtGreen + image[i+1][j-1].rgbtGreen + image[i+1][j].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 4.0);
                sumblue = image[i][j-1].rgbtBlue + image[i][j].rgbtBlue + image[i+1][j-1].rgbtBlue + image[i+1][j].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 4.0);
            }

            if (i > 0 && i < (height - 1) && j == 0) //edge case 4: left most column
            {
                sumred = image[i-1][j].rgbtRed + image[i-1][j+1].rgbtRed + image[i][j].rgbtRed + image[i][j+1].rgbtRed + + image[i+1][j].rgbtRed + image[i+1][j+1].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 6.0);
                sumgreen = image[i-1][j].rgbtGreen + image[i-1][j+1].rgbtGreen + image[i][j].rgbtGreen + image[i][j+1].rgbtGreen + image[i+1][j].rgbtGreen + image[i+1][j+1].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 6.0);
                sumblue = image[i-1][j].rgbtBlue + image[i-1][j+1].rgbtBlue + image[i][j].rgbtBlue + image[i][j+1].rgbtBlue + image[i+1][j].rgbtBlue + image[i+1][j+1].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 6.0);
            }

            if (i == (height - 1) && j == 0) // edge case 5: bottom left corner
            {
                sumred = image[i-1][j].rgbtRed + image[i-1][j+1].rgbtRed + image[i][j].rgbtRed + image[i][j+1].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 4.0);
                sumgreen = image[i-1][j].rgbtGreen + image[i-1][j+1].rgbtGreen + image[i][j].rgbtGreen + image[i][j+1].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 4.0);
                sumblue = image[i-1][j].rgbtBlue + image[i-1][j+1].rgbtBlue + image[i][j].rgbtBlue + image[i][j+1].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 4.0);
            }

            if (i == (height - 1) && j > 0 && j < (width -1)) // edge case 6: bottom roll
            {
                sumred = image[i-1][j-1].rgbtRed + image[i-1][j].rgbtRed + image[i-1][j+1].rgbtRed + image[i][j-1].rgbtRed + image[i][j].rgbtRed + image[i][j+1].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 6.0);
                sumgreen = image[i-1][j-1].rgbtGreen + image[i-1][j].rgbtGreen + image[i-1][j+1].rgbtGreen + image[i][j-1].rgbtGreen + image[i][j].rgbtGreen + image[i][j+1].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 6.0);
                sumblue = image[i-1][j-1].rgbtBlue + image[i-1][j].rgbtBlue + image[i-1][j+1].rgbtBlue + image[i][j-1].rgbtBlue + image[i][j].rgbtBlue + image[i][j+1].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 6.0);
            }

            if (i == (height - 1) && j == (width - 1)) // edge case 7: bottom right corner
            {
                sumred = image[i-1][j-1].rgbtRed + image[i-1][j].rgbtRed + image[i][j-1].rgbtRed + image[i][j].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 4.0);
                sumgreen = image[i-1][j-1].rgbtGreen + image[i-1][j].rgbtGreen + image[i][j-1].rgbtGreen + image[i][j].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 4.0);
                sumblue = image[i-1][j-1].rgbtBlue + image[i-1][j].rgbtBlue + image[i][j-1].rgbtBlue + image[i][j].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 4.0);
            }

            if (i > 0 && i < (height - 1) && j == (width - 1)) // edge case 8: right most column
            {
                sumred = image[i-1][j-1].rgbtRed + image[i-1][j].rgbtRed + image[i][j-1].rgbtRed + image[i][j].rgbtRed + image[i+1][j-1].rgbtRed + image[i+1][j].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 6.0);
                sumgreen = image[i-1][j-1].rgbtGreen + image[i-1][j].rgbtGreen + image[i][j-1].rgbtGreen + image[i][j].rgbtGreen + image[i+1][j-1].rgbtGreen + image[i+1][j].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 6.0);
                sumblue = image[i-1][j-1].rgbtBlue + image[i-1][j].rgbtBlue + image[i][j-1].rgbtBlue + image[i][j].rgbtBlue + image[i+1][j-1].rgbtBlue + image[i+1][j].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 6.0);
            }

            else // everything else srrounded by 8 pixels
            {
                sumred = image[i-1][j-1].rgbtRed + image[i-1][j].rgbtRed + image[i-1][j+1].rgbtRed + image[i][j-1].rgbtRed + image[i][j].rgbtRed + image[i][j+1].rgbtRed + image[i+1][j-1].rgbtRed + image[i+1][j].rgbtRed + image[i+1][j+1].rgbtRed;
                blurred[i][j].rgbtRed = round(sumred / 9.0);
                sumgreen = image[i-1][j-1].rgbtGreen + image[i-1][j].rgbtGreen + image[i-1][j+1].rgbtGreen + image[i][j-1].rgbtGreen + image[i][j].rgbtGreen + image[i][j+1].rgbtGreen + image[i+1][j-1].rgbtGreen + image[i+1][j].rgbtGreen + image[i+1][j+1].rgbtGreen;
                blurred[i][j].rgbtGreen = round(sumgreen / 9.0);
                sumblue = image[i-1][j-1].rgbtBlue + image[i-1][j].rgbtBlue + image[i-1][j+1].rgbtBlue + image[i][j-1].rgbtBlue + image[i][j].rgbtBlue + image[i][j+1].rgbtBlue + image[i+1][j-1].rgbtBlue + image[i+1][j].rgbtBlue + image[i+1][j+1].rgbtBlue;
                blurred[i][j].rgbtBlue = round(sumblue / 9.0);
            }
        }
    }

    for (i = 0; i < height; i++)
    {
        for (j = 0; j < width; j++)
        {
            image[i][j].rgbtRed = blurred[i][j].rgbtRed;
            image[i][j].rgbtGreen = blurred[i][j].rgbtGreen;
            image[i][j].rgbtBlue = blurred[i][j].rgbtBlue;
        }
    }
    return;
}

测试用3x3图像 #

• 第一行：(10, 20, 30), (40, 50, 60), (70, 80, 90)
• 第二行：(110, 130, 140), (120, 140, 150), (130, 150, 160)
• 第三行：(200, 210, 220), (220, 230, 240), (240, 250, 255)

问题原因与修复 #

1. 段错误根源

原代码中所有边缘情况判断都使用独立的if语句，而非else if。这导致边缘像素在执行完对应边缘分支后，会继续判断后续条件，最终进入else分支。以左上角像素（i=0,j=0）为例，进入else分支后会访问image[-1][-1]，触发数组越界，直接导致段错误。

2. 计算结果错误根源

边缘像素会先后执行边缘分支和else分支，else分支的计算结果会覆盖之前的正确值，只有中间像素（不触发任何边缘if条件）仅执行else分支，结果正确。

3. 代码笔误

边缘情况4的sumred计算中多了一个加号：+ + image[i+1][j].rgbtRed，这会导致语法错误或错误的数值累加。

修复措施

• 将所有边缘情况的if改为else if，确保每个像素仅进入一个分支；
• 删除边缘情况4中多余的加号。

内容的提问来源于stack exchange，提问作者Butterworth Filter