Regarding spatial resolution conversion programming - bmp image - c

Question

I want to ask something about simple spatial resolution manipulating just by using c language. I have done my programming below, it managed to be compiled but for some reasons the program stucked in the middle when I try to run it. Really hope you guys can help. I am extremely beginner on this.
#include<stdio.h>
#define width 640
#define height 581
int main(void)
{
FILE *fp;
unsignedchar header[54];
unsignedchar img_work[width][height][3];
char input_file[128],output_file[128];
int v, h, w, i, c, s, ave_w[width], ave_h[height], average_h, average_w;
/*------------Reading image------------*/
printf("Enter name of the file¥n---");
scanf("%s",input_file);
printf("The file that would be processed is %s.¥n", input_file);
fp=fopen(input_file,"rb");
fread(header,1,54,fp);
fread(img_work,1,width*height*3,fp);
fclose(fp);
/*------------Spatial Resolution Program------------*/
printf ("enter level of spatialization-- ");
scanf ("%d", &v);
for (i=0; i<v; i++) {
s = s + s;
}
for(c=0; c<3; c++){
for(h=0; h<height; h++){
for(w=0; w<width; w=w+s){
average_w = 0;
for (i=0; i<s; i++) {
ave_w = img_work[w+i][h][c] / s;
average_w = average_w + ave_w;
}
for (i=0; i<width; i=i+s) {
img_work[w+i][h][c] = average_w;
}
}
}
}
for(c=0; c<3; c++){
for(w=0; w<width; w++){
for(h=0; h<height; h=h+s){
average_h = 0;
for (i=0; i<s; i++) {
ave_h = img_work[w][h+i][c] / s;
average_h = average_h + ave_h;
}
for (i=0; i<height; i=i+s) {
img_work[w][h+i][c] = average_h;
}
}
}
}
/*------------Writing File------------*/
printf("Enter the name of the file that would be saved.¥n---");
scanf("%s",output_file);
printf("Name of the file that would be saved is %s.¥n",output_file);
fp=fopen(output_file,"wb");
fwrite(header,1,54,fp);
fwrite(img_work,1,width*height*3,fp);
fclose(fp);
printf("End.¥n");
return 0;
}
I am really a beginner, so, sorry if this is lacking too much.

Answer 1

There are several issues with your code:
s is uninitialised. Hence, when you access its value in the assignment s = s + s, the result is undefined. s may even be negative. Initialise it: s = 1;
You've got the representation of your image wrong. You read the pixel data verbatim from the file. The BMP format is row major, so your pixel data should be img_work[height][width][3] and all accesses should have their first and second dimensions swapped.
The BMP format also requires padding at the end of each row. Your fixed-size width of 640 doesn't require it, but it's worth keeping in mind when you want to make your implementation more general.
You don't really need the auxiliary variables ave_w and ave_h. Most importantly, you don't need them to be arrays.
Your height isn't evenly divisible by s. That means that in the last pass through the loop, h + i will go out of bounds. (The same applies to the width, but the value 640 is safe up to at least a level of 7.) You could calculate an "actual s" that would be adjusted for the top and right sides.
When you calculate the average, it is better to sum the values first and then divide by s once. You are dealing with integers and integer division truncates. For example 3/4 is zero. Consequently, (3/4 + 3/4 + 3/4 + 3+4) is also zero, but (3 + 3 + 3 + 3) / 4 is 3. You can notice the effect for larger levels of reduction, where a predominantly white imagfe becomes darker if you divide on summation.
Here's a program based on yours, that puts the points raised above into practice:
#include <stdio.h>
#define width 640
#define height 581
int main(void)
{
FILE *fp;
unsigned char header[54];
unsigned char img_work[height][width][3];
char input_file[128];
char output_file[128];
int v, h, w, i, c, s;
/*------------Reading image------------*/
printf("Enter name of the file\n---");
scanf("%s",input_file);
printf("The file that would be processed is %s.\n", input_file);
fp=fopen(input_file,"rb");
fread(header,1,54,fp);
fread(img_work,1,width*height*3,fp);
fclose(fp);
/*------------Spatial Resolution Program------------*/
printf("enter level of spatialization-- ");
scanf("%d", &v);
s = 1;
for (i = 0; i < v; i++) {
s = s + s;
}
for (c = 0; c < 3; c++) {
for (h = 0; h < height; h++) {
for (w = 0; w < width; w = w + s) {
int average_w = 0;
int ss = s;
if (w + ss > width) ss = width % s;
for (i = 0; i < ss; i++) {
average_w = average_w + img_work[h][w + i][c];
}
for (i = 0; i < ss; i++) {
img_work[h][w + i][c] = average_w / ss;
}
}
}
}
for (c = 0; c < 3; c++) {
for (w = 0; w < width; w++) {
for (h = 0; h < height; h = h + s) {
int average_h = 0;
int ss = s;
if (h + ss > height) ss = height % s;
for (i = 0; i < ss; i++) {
average_h = average_h + img_work[h + i][w][c];
}
for (i = 0; i < ss; i++) {
img_work[h + i][w][c] = average_h / ss;
}
}
}
}
/*------------Writing File------------*/
printf("Enter the name of the file that would be saved.\n---");
scanf("%s",output_file);
printf("Name of the file that would be saved is %s.\n",output_file);
fp=fopen(output_file,"wb");
fwrite(header,1,54,fp);
fwrite(img_work,1,width*height*3,fp);
fclose(fp);
printf("End.\n");
return 0;
}
That's still a quick-and-dirty program with fixed image sizes. It doesn't enforce that the actual size of the image, which can be read from the header, and the fixed sizes match or that the colour depth is the same or that you even get enough pixel data, for which you should check the return value of fread.