New to progamming; trying to implement a sobel edge detection algorithm. I am doing this in a function called edges, as per the code snippet below.
void edges(int height, int width, RGBTRIPLE image[height][width])
{
// define kernals
int Gx[3][3];
Gx[0][0] = -1;
Gx[0][1] = 0;
Gx[0][2] = 1;
Gx[1][0] = -2;
Gx[1][1] = 0;
Gx[1][2] = 2;
Gx[2][0] = -1;
Gx[2][1] = 0;
Gx[2][2] = 1;
int Gy[3][3];
Gy[0][0] = -1;
Gy[0][1] = -2;
Gy[0][2] = -1;
Gy[1][0] = 0;
Gy[1][1] = 0;
Gy[1][2] = 0;
Gy[2][0] = 1;
Gy[2][1] = 2;
Gy[2][2] = 1;
// define variables
int Gx_red, Gx_green, Gx_blue;
int Gy_red, Gy_green, Gy_blue;
// define temporary array
RGBTRIPLE edge_image[height][width];
// loop through columns
for (int i = 0; i < height; i++)
{
// loop through rows
for (int j = 0; j < width; j++)
{
// set/reset sobel values for each colour channel
Gx_red = Gx_green = Gx_blue = 0;
Gy_red = Gy_green = Gy_blue = 0;
// 3x3 kernal around [i][j]
for (int x = -1; x < 2; x++)
{
for (int y = -1; y < 2; y++)
{
// 'pixels' outside of image array treated as black pixels
if (i + x > height || i + x < 0 || j + y > width || j + y < 0)
{
// Gx kernal
Gx_red += 0;
Gx_green += 0;
Gx_blue += 0;
// Gy kernal
Gy_red += 0;
Gy_green += 0;
Gy_blue += 0;
}
// Multiply each channel by corresponding value in convolutional array
else if (i + x < height && i + x > 0 && j + y < width && j + y > 0)
{
// Gx kernal
Gx_red += (image[i + x][j + y].rgbtRed * Gx[x][y]);
Gx_green += (image[i + x][j + y].rgbtGreen * Gx[x][y]);
Gx_blue += (image[i + x][j + y].rgbtBlue * Gx[x][y]);
// Gy kernal
Gy_red += (image[i + x][j + y].rgbtRed * Gy[x][y]);
Gy_green += (image[i + x][j + y].rgbtGreen * Gy[x][y]);
Gy_blue += (image[i + x][j + y].rgbtBlue * Gy[x][y]);
}
}
}
// Perform sobel operatation and assign each colour channel value to new array
edge_image[i][j].rgbtRed = sobel(Gx_red, Gy_red);
edge_image[i][j].rgbtGreen = sobel(Gx_green, Gy_green);
edge_image[i][j].rgbtBlue = sobel(Gx_blue, Gy_blue);
}
}
// assign temp array to origional array for output
for (int i = 0; i < height; i++)
for (int j = 0; j < width; j++)
image[i][j] = edge_image[i][j];
}
The edges function calls the sobel function, written as below.
// Calculate sobel value for each channel (capping at 255)
int sobel (int Gx, int Gy)
{
int n = sqrt(Gx^2 + Gy^2);
if (n > 255)
{
n = 255;
}
return n;
}
This is the Input image
This is the Output image
As you can see the output is extremely grainy and the edges are undefined. Clearly, I haven't implemented the sobel alorithm properly, but I am unsure what I have done wrong. Please could you give me some advice on how I could fix this?
*Please note I am aware I could convert the image to greyscale before implementing the sobel algorithm but i'm trying to preserve the colour channels as much as possible. Let me know if I can provide any more information.
First of all, this all does nothing:
// 'pixels' outside of image array treated as black pixels
if (i + x > height || i + x < 0 || j + y > width || j + y < 0)
{
// Gx kernal
Gx_red += 0;
Gx_green += 0;
Gx_blue += 0;
// Gy kernal
Gy_red += 0;
Gy_green += 0;
Gy_blue += 0;
}
Adding zero is a no-op. You can safely delete that.
Your first main issue is here:
// Gx kernal
Gx_red += (image[i + x][j + y].rgbtRed * Gx[x][y]);
Gx_green += (image[i + x][j + y].rgbtGreen * Gx[x][y]);
Gx_blue += (image[i + x][j + y].rgbtBlue * Gx[x][y]);
// Gy kernal
Gy_red += (image[i + x][j + y].rgbtRed * Gy[x][y]);
Gy_green += (image[i + x][j + y].rgbtGreen * Gy[x][y]);
Gy_blue += (image[i + x][j + y].rgbtBlue * Gy[x][y]);
You defined x, y to loop through -1, 0, 1, but defined Gx, Gy as int[3][3] arrays. So when x or y is -1 you are indexing out of bounds.
I would suggest defining
int kx = x + 1; // Kernel x.
int ky = y + 1; // Kernel y.
And then
// Gx kernel
Gx_red += (image[i + x][j + y].rgbtRed * Gx[kx][sy]);
Gx_green += (image[i + x][j + y].rgbtGreen * Gx[kx][sy]);
Gx_blue += (image[i + x][j + y].rgbtBlue * Gx[kx][sy]);
// Gy kernel
Gy_red += (image[i + x][j + y].rgbtRed * Gy[kx][sy]);
Gy_green += (image[i + x][j + y].rgbtGreen * Gy[kx][sy]);
Gy_blue += (image[i + x][j + y].rgbtBlue * Gy[kx][sy]);
Your second main issue is here:
int n = sqrt(Gx^2 + Gy^2);
In C and C++ (and many, but not all other programming languages) ^ means the bit-wise XOR operator, not exponentiation. You want:
int n = sqrt(Gx*Gx + Gy*Gy);