maybe one of you can help me out. The following code compiles and runs without any complaint, but the results are not the expected ones. Somehow the image-edge will be detected, but the new values seem to be incorrect - according to the automated testing (check50).

// Detect edges
void edges(int height, int width, RGBTRIPLE image[height][width])
{
    int gxSumRed = 0;
    int gxSumGreen = 0;
    int gxSumBlue = 0;

    int gySumRed = 0;
    int gySumGreen = 0;
    int gySumBlue = 0;

    int sumRedX[height][width];
    int sumGreenX[height][width];
    int sumBlueX[height][width];

    int sumRedY[height][width];
    int sumGreenY[height][width];
    int sumBlueY[height][width];

    int i = 0;
    int j = 0;
    int m = 0;
    int n = 0;
    
    RGBTRIPLE canvas[height+2][width+2];
    
    
    for (i = 0; i <= height + 1; i++)
    {
        for (j = 0; j <= width + 1; j++)
        {
            if (i == 0 || j == 0 || i == height +1 || j == width + 1)
            {
                canvas[i][j].rgbtRed = 0;
                canvas[i][j].rgbtGreen = 0;
                canvas[i][j].rgbtBlue = 0;
            }
        }
    }
    
    for (i = 1; i <= height; i++)
    {
        for (j = 1; j <= width; j++)
        {
            canvas[i][j].rgbtRed = image[i-1][j-1].rgbtRed;
            canvas[i][j].rgbtGreen = image[i-1][j-1].rgbtGreen;
            canvas[i][j].rgbtBlue = image[i-1][j-1].rgbtBlue;
        }
    }
    

    // For each row of the image...
    for (i = 1; i <= height; i++)
    {
        //...take each pixel.
        for (j = 1; j <= width; j++)
        {
            //If (current height greater 0 AND current height less than height - 1) AND (current width greater 0 AND current width less than width - 1)...

    //        if ((i > 0 && i < height - 1) && (j > 0 && j < width - 1))
    //       {
                //..  take 3 rows of the picture..
                for (m = i - 1; m <= i + 1; m++)
                {
                    //...and in each row take 3 pixels.
                    for (n = j - 1; n <= j + 1; n++)
                    {

                        //Calculate new values for each color channel in x-direction..
                        if ((m == i - 1 || m == i + 1) && n == j - 1)
                        {
                            gxSumRed += canvas[m][n].rgbtRed * (-1);
                            gxSumGreen += canvas[m][n].rgbtGreen * (-1);
                            gxSumBlue += canvas[m][n].rgbtBlue * (-1);
                        }


                        if ((m == i - 1 || m == i + 1) && n == j + 1)
                        {
                            gxSumRed += canvas[m][n].rgbtRed * 1;
                            gxSumGreen += canvas[m][n].rgbtGreen * 1;
                            gxSumBlue += canvas[m][n].rgbtBlue * 1;
                        }

                        if (m == i && n == j - 1)
                        {
                            gxSumRed += canvas[m][n].rgbtRed * (-2);
                            gxSumGreen += canvas[m][n].rgbtGreen * (-2);
                            gxSumBlue += canvas[m][n].rgbtBlue * (-2);
                        }


                        if (m == i && n == j + 1)
                        {
                            gxSumRed += canvas[m][n].rgbtRed * 2;
                            gxSumGreen += canvas[m][n].rgbtGreen * 2;
                            gxSumBlue += canvas[m][n].rgbtBlue * 2;
                        }
                        
                        if (n == j)
                        {
                            gxSumRed += 0;
                            gxSumGreen += 0;
                            gxSumBlue += 0;
                        }
                    }
                }

                sumRedX[i-1][j-1] = gxSumRed;
                sumGreenX[i-1][j-1] = gxSumGreen;
                sumBlueX[i-1][j-1] = gxSumBlue;

                gxSumRed = 0;
                gxSumGreen = 0;
                gxSumBlue = 0;

         //   }
        }
    }



    // For each column of the image...
    for (i = 1; i <= width; i++)
    {
        //...take each pixel.
        for (j = 1; j <= height; j++)
        {
            //If (current width greater 0 AND current width less than width - 1) AND (current height greater 0 AND current height less than height - 1)...
       //     if ((i > 0 && i < width - 1) && (j > 0 && j < height - 1))
       //     {
                //..  take 3 columns of the picture..
                for (m = - 1; m <= 1; m++)
                {
                    //...and in each column take 3 pixels.
                    for (n = - 1; n <= 1; n++)
                    {

                        //Calculate new values for each color channel in y-direction..
                        if ( n == j + 1 && (m == i + 1 || m == i - 1))
                        {
                            gySumRed += canvas[m][n].rgbtRed * (-1);
                            gySumGreen += canvas[m][n].rgbtGreen * (.1);
                            gySumBlue += canvas[m][n].rgbtBlue * (-1);
                        }


                        if ( n == j - 1 && (m == i + 1 || m == i - 1))
                        {
                            gySumRed += canvas[m][n].rgbtRed * 1;
                            gySumGreen += canvas[m][n].rgbtGreen * 1;
                            gySumBlue += canvas[m][n].rgbtBlue * 1;
                        }


                        if (n == j)
                        {
                            gySumRed += 0;
                            gySumGreen += 0;
                            gySumBlue += 0;
                        }

                        if ( n == j + 1 && m == 0)
                        {
                            gySumRed += canvas[m][n].rgbtRed * (-2);
                            gySumGreen += canvas[m][n].rgbtGreen * (-2);
                            gySumBlue += canvas[m][n].rgbtBlue * (-2);
                        }


                        if ( n == j - 1 && m == 0)
                        {
                            gySumRed += canvas[m][n].rgbtRed * 2;
                            gySumGreen += canvas[m][n].rgbtGreen * 2;
                            gySumBlue += canvas[m][n].rgbtBlue * 2;
                        }
                    }
                }

                sumRedY[i-1][j-1] = gySumRed;
                sumGreenY[i-1][j-1] = gySumGreen;
                sumBlueY[i-1][j-1] = gySumBlue;

                gySumRed = 0;
                gySumGreen = 0;
                gySumBlue = 0;
          //  }
        }
    }


    for (i = 0; i < height; i++)
    {

        for (j = 0; j < width; j++)
        {
            if (round((float)sqrt((sumRedX[i][j] * sumRedX[i][j]) + (sumRedY[i][j] * sumRedY[i][j]))) < 256)
            {
                image[i][j].rgbtRed = round((float)sqrt((sumRedX[i][j] * sumRedX[i][j]) + (sumRedY[i][j] * sumRedY[i][j])));
            }

            else
            {
                image[i][j].rgbtRed = 255;
            }


            if (round((float)sqrt((sumGreenX[i][j] * sumGreenX[i][j]) + (sumGreenY[i][j] * sumGreenY[i][j]))) < 256)
            {
                image[i][j].rgbtGreen = round((float)sqrt((sumGreenX[i][j] * sumGreenX[i][j]) + (sumGreenY[i][j] * sumGreenY[i][j])));
            }

            else
            {
                image[i][j].rgbtGreen = 255;
            }


            if (round((float)sqrt((sumBlueX[i][j] * sumBlueX[i][j]) + (sumBlueY[i][j] * sumBlueY[i][j]))) < 256)
            {
                image[i][j].rgbtBlue = round((float)sqrt((sumBlueX[i][j] * sumBlueX[i][j]) + (sumBlueY[i][j] * sumBlueY[i][j])));
            }

            else
            {
                image[i][j].rgbtRed = 255;
            }
        }
    }           
    return;
}