How to write on a file from Top to Bottom in C - c

Question

I would like to ask if its possible to write on a file from top to bottom?
The current code below will flip and image Vertically.
This uses left to right writing to a file.
On the other hand, I am having problem on how would I do the flip image Horizontally if I follow the left to right pattern on writing to a file.
Note: I have other option here, which is to swap lines from top to bottom.
Who knows maybe there's a way to follow the Vertical Flip pattern.
I would appreciate any help you guys can give. Thanks.
//Flip Vertically
for(y = WIDTH-1 ; y >= 0 ; y--) {
for(x = 0 ; x < HEIGHT; x++) {
p = (unsigned char *)datap + ((WIDTH) * y * 3) + (x * 3);
// output pixel
if(fwrite(p, sizeof(char), 3, outfp) != 3) {
cleanUpBeforeExit(infp, outfp, datap, E_FILE_WRITE_ERROR);
}
}
}

Answer 1

Beware that the terms that you use are very ambiguous. Flip vertically, flip horizontally can be understood two ways, as can top-to-bottom when speaking of a file.
Hence I'll give a general answer.
Assume you write the pixels in the usual scanning order, top row first, left pixel first, i.e. by addressing Image[Column + Row * Pitch] in a double loop (outer on Row, inner on Column). You can also reverse the loop traversal order.
Then you will obtain a left-right mirroring (pixels move horizontally) by writing Image[(Width - 1 - Column) + Row * Pitch], and a top-bottom mirroring (pixels move vertically) with Image[Column + (Height - 1 - Row) * Pitch].
You can combine the two modifications to achieve a 180° rotation.

Answer 2

When writing to a file would normally write data from start-of-file towards end-of-file. Writing in any other order will in most cases make your program more complicated and give worse performance.
So what you need is to access your data in the correct order depending on how you want to flip the data.
Something like:
#include <stdio.h>
#define WIDTH 2
#define HEIGHT 3
void print_element(char* p)
{
// Here I just print to stdout but you can just save it to a file
printf("%c %c %c ", *p, *(p+1), *(p+2));
}
int main()
{
char data[3 * HEIGHT][3 * WIDTH] = {{'a', 'b', 'c', 'd', 'e', 'f'},
{'g', 'h', 'i', 'j', 'k', 'l'},
{'m', 'n', 'o', 'p', 'q', 'r'}};
char* datap = (char*)data;
char* p;
printf("Normal\n");
for(int y = 0; y < HEIGHT; y++)
{
for(int x = 0 ; x < WIDTH; x++)
{
p = datap + ((WIDTH) * y * 3) + (x * 3);
print_element(p);
}
printf("\n");
}
printf("\nFlip horizontal\n");
for(int y = HEIGHT-1 ; y >= 0 ; y--)
{
for(int x = 0 ; x < WIDTH; x++)
{
p = datap + ((WIDTH) * y * 3) + (x * 3);
print_element(p);
}
printf("\n");
}
printf("\nFlip vertical\n");
for(int y = 0; y < HEIGHT; y++)
{
for(int x =WIDTH-1; x >= 0; x--)
{
p = datap + ((WIDTH) * y * 3) + (x * 3);
print_element(p);
}
printf("\n");
}
printf("\nFlip both\n");
for(int y = HEIGHT-1 ; y >= 0 ; y--)
{
for(int x = WIDTH-1; x >= 0; x--)
{
p = datap + ((WIDTH) * y * 3) + (x * 3);
print_element(p);
}
printf("\n");
}
return 0;
}
This will produce:
Normal
a b c d e f
g h i j k l
m n o p q r
Flip horizontal
m n o p q r
g h i j k l
a b c d e f
Flip vertical
d e f a b c
j k l g h i
p q r m n o
Flip both
p q r m n o
j k l g h i
d e f a b c