Negative transformation of an image in C - c

Question

#include < stdio.h >
#include < conio.h >
#include < stdlib.h >
#include < process.h >
#include < string.h >
#include < math.h >
int count = 0;
typedef struct bitmap24 {
unsigned char header[54];
unsigned char * pixels;
}BMP;
void readBMP(char * filename) {
int i;
FILE * f = fopen(filename, "rb");
FILE * f1 = fopen("save.bmp", "wb");
FILE * pixelVals = fopen("vals.dat", "w");
unsigned char bmppad[3] = {
0,
0,
0
};
if (!f) {
printf("Could not read file!\n");
exit(0);
}
unsigned char info[54];
fread(info, sizeof(unsigned char), 54, f);
int width = * (int * ) & info[18];
int height = * (int * ) & info[22];
unsigned char * img = NULL;
if (img)
free(img);
img = (unsigned char * ) malloc(3 * width * height);
memset(img, 0, sizeof(img));
fwrite(info, sizeof(unsigned char), 54, f1);
int length = width * height;
unsigned long int image[10000][3];
for (i = 0; i < length; i++) {
image[i][2] = getc(f); // blue
image[i][1] = getc(f); // green
image[i][0] = getc(f); // red
img[count] = 255 - (unsigned char) image[i][0];
//img[count] = 10*(unsigned char)log10((double)image[i][0]+1);
count += 1;
img[count] = 255 - (unsigned char) image[i][2];
//img[count] = 10*(unsigned char)log10((double)image[i][3]+1);
count += 1;
img[count] = 255 - (unsigned char) image[i][2];
//img[count] = 10*(unsigned char)log10((double)image[i][2]+1);
count += 1;
printf("pixel %d : [%d,%d,%d]\n", i + 1, image[i][0], image[i][4], image[i][2]);
fprintf(pixelVals, "pixel %d : [%d,%d,%d]\n", i + 1, image[i][0], image[i][5], image[i][2]);
}
for (i = height - 1; i >= 0; i--) {
fwrite(img + (width * (height - i - 1) * 3), 3, width, f1);
fwrite(bmppad, 1, (4 - (width * 3) % 4) % 4, f1);
}
fclose(f);
fclose(f1);
fclose(pixelVals);
}
void main() {
char * fileName = "bitgray.bmp";
readBMP(fileName);
getch();
}
I am not getting the correct result when the image is saved. I am using 24bit bmp image of dimensions 114 X 81. The image was coming out to be inverted initially but that issue was solved. But I am still getting a slanted image. I know the problem is in the last 'for' loop.
How should I solve it ?

Answer 1

Bitmap scanlines are padded to 4-byte boundary. So you need to add an extra two bytes so that the row is divisible by 4. At the moment, you have 114 * 3 = 342 bytes of pixel data per line. The next number divisible by 4 is 344.
So, at the end of reading each line, just read an extra two bytes and discard them.
In general, you can work out the extra bytes like this:
extra = (alignment - ((width * bytesPerPixel) % alignment)) % alignment;
Where in this case alignment is 4.
From memory, there is a field in the header that should contain the value of the full scanwidth (width * bytesPerPixel + extra), but it's a good idea not to expect it to be correct because you can calculate it easily.
You must also be aware of this padding rule when you save a bitmap.

Answer 2

Your second for loop looks strange. I believe it should be:
for(i = 0; i < height; i++) {...}
or:
for(i = height-1; i >= 0; i--) {...}