Thread: Issues With Edge Detection Program

Hi, I am new to C programming and have only started learning the language in early September.

I am trying to apply a horizontal filter to a small piece of an image and then apply the filter to the whole image. However, I am getting a weird output image and think it is due to my implementation of the Sobel edge detection.

This is what I have so far:

Code:

#include <stdio.h>#include <stdlib.h>
#include "iplib2New.c"


# define n 3
#define FILTER_SIZE 3


//Function prototypes
image_ptr read_pnm(char *filename, int *rows, int *cols, int *type);
int getnum(FILE *fp);
void write_pnm(image_ptr ptr, char *filename, int rows, int cols, int type);
int apply_horizontal_filter(unsigned char small_block[FILTER_SIZE][FILTER_SIZE], int sobel_h[FILTER_SIZE][FILTER_SIZE], int sobel_v[FILTER_SIZE][FILTER_SIZE]);


int ROWS, COLS, TYPE;


int main(int argc, char **argv){
    int rows, cols, type;
    int i=0, j=0, x=0, y=0, value_h = 0; //Loop vars and conditions
    int tp; //File type
    double ave=0;
    char aveStr[10], *aveStrEnd;
    unsigned char small_block[n][n]; //sub block for image


    image_ptr imagePtr, imagePtr2; //image pointer types
    image_ptr imageNegPtr;


    //Creating sobel filters
    int sobel_h[3][3] = {-1, -2, -1, 0, 0, 0, 1, 2, 1}; //Horizontal
    int sobel_v[3][3] = {-1, 0, 1, -2, 0, 2, -1, 0, 1}; //Vertical


    /* check inputs */
    if (argc != 4){
        printf("wrong inputs: use %s infile out1 out2 \n", argv[0]);  
        return 0;
    }


    /* first read-in the image */
    printf("\nReading input image... \n");


    //Pointer to access the whole image
    imagePtr = read_pnm(argv[1], &rows, &cols, &type); //Returns ptr from read_pnm


    printf("\nYour image was read successfully!\n");
    printf("Info about your image: rows = %d, cols = %d, type = %d\n", rows, cols, type);
    unsigned char image2[rows][cols];  /* space for output image */


    //Going over the whole image
    for(i=0; i<rows; i++) //The height of the image
    {
        for(j=0; j<cols; j++) //The width of the image
        {


            //Picking up our sub block of the image
            for(y=0; y<n; y++) //Width of the sub block - cols
            {
                for(x=0; x<n; x++) //Height of the sub block - rows
                {
                    small_block[x][y]=imagePtr[(i+x)*cols+(j+y)];
                }
            }
            //Analyze sub block here
            //printf("Analyzing a sub block...\n");
            value_h = apply_horizontal_filter(small_block, sobel_h, sobel_v);
            
            //Update output image
            image2[i][j] = value_h;
        }
    }


    printf("\nPicked up a piece of image.\n");


    tp = 5;


    imagePtr2 = (image_ptr)image2;


    printf("\nNow writing to image file ... \n");
    printf("rows=%d, cols=%d, type=%d \n", rows, cols, tp);


    write_pnm(imagePtr2, argv[2], rows, cols, tp); //Creating the out1.pgm
 
    /* image negatives  */
    imageNegPtr=(image_ptr)malloc(rows*cols*(sizeof(unsigned char)) );


    printf("\nNow writing negative image file ... \n");
    printf("rows=%d, cols=%d, type=%d \n", rows, cols, type);
 
    write_pnm(imageNegPtr, argv[3], rows, cols, type);  //Creating the out2.pgm


    return 0;
}


int apply_horizontal_filter(unsigned char small_block[n][n], int sobel_h[3][3], int sobel_v[3][3]){
    int x = 0;
    int y = 0;
    int pixel_value_h = 0;
    unsigned char filtered_block[3][3];


    for(x = 0; x<3; x++){
        for(y=0; y<3; y++){
            pixel_value_h += sobel_h[x][y]*small_block[x][y];
        }
    }
    return pixel_value_h;
}

This is the image I am testing as well as the output my code produces for

Hi,

The Sobel operator will return both + and - gradient values, but if you use the raw values you will have an overflow, causing bright values.

Try this for line 123:

Code:

return abs(pixel_value_h);

Please let me know how you get on...