flip image horizontal in c - c

Question

My function is getting an Image and I am trying to show the mirror of it.
horizonal flip. I tried to do something like swap function, but it prints the original picture.
The size of the image is m*n and the function knows the values of m and n.
Here is my code:
void flipfunc(Img *img)
{
int y;
int x;
const int middleX = m / 2;
char tmp;
char* p;
for (x = 0; x < middleX; ++x)
{
p = image->data + x * m;
for (y = 0; y <3*n; y+=3)
{
// swap pixels
tmp = p[y];
p[y] = p[3*n - 1 - y];
p[3*n - 1 - y] = tmp;
tmp = p[y+1];
p[y+1] = p[3*n - 1 - (y+1)];
p[3*n - 1 - y] = tmp;
tmp = p[y+2];
p[y+2] = p[3*n - 1 - (y+2)];
p[3*n - 1 - (y+2)] = tmp;
}
}
}
/* Image type - contains height, width, and RGB data */
struct Img {
unsigned long X;
unsigned long Y;
char *data;
};

Answer 1

Leaving aside the issues that other mentioned in the comments, I'll try to answer with a few hints:
1) In your function, you want to do an in-place mirroring of the given rgb image. That's reasonable.
2) You were thinking in the right direction with your "middleX" and your pixel-swapping approach. BUT it seems you did it wrong: You ignore the top half of your image completely, and instead swap each row of the bottom half twice! That's why you end up with the same image in the end. So why don't you just apply your "middle" logic to the inner loop instead of the outer loop?

Answer 2

Images are usually stored in a row-major raster order. That is, all pixels of the first row come before (=their addresses are smaller than) pixels of the second row, the third row, etc.
To visualize an example image of width 5 and height 4:
RGB RGB RGB RGB RGB
RGB RGB RGB RGB RGB
RGB RGB RGB RGB RGB
RGB RGB RGB RGB RGB
Each letter represents a byte in memory; spaces are just for clarity.
To find a specific sample in the image, you must do some arithmetic. For example, locate a G sample of pixel with x=1 and y=2. I marked it with a capital G:
,,, ,,, ,,, ,,, ,,,
,,, ,,, ,,, ,,, ,,,
,,, ,G. ... ... ...
... ... ... ... ...
To find its offset in the data array, count the number of samples that come before it (I marked them with ,). If you fiddle with it enough, you can discover this formula:
offset = y * 3 * sizeX + x * 3 + k;
where k is 0, 1, or 2, depending on which colour you are looking for.
Your code has (for example)
x * m + y
which is not correct at all.
BTW you shouldn't use variable names like m or n - they are extremely confusing. Just use
image->sizeX
or, if you want to use temporary variables:
unsigned long sizeX = image->sizeX;

Answer 3

This should be working:
void flipfunc(Img *img)
{
int y;
int x;
int l, r;
char tmp;
for (y = 0; y < img->y; ++ y) {
for (l = 0, r = img->x - 1; l < r; ++ l, -- r) {
for (d = 0; d < 3; ++ d) {
tmp = img->data[(y * img->x + l) * 3 + d];
img->data[(y * img->x + l) * 3 + d] = img->data[(y * img->x + r) * 3 + d];
img->data[(y * img->x + r) * 3 + d] = tmp;
}
}
}
}
If you want something, let's say, more optimized, build it with -O2.