Read color bytes (RGB) from a bitmap - c

Question

I'm having a problem with a code that I made that should display the bytes of the RGB color values of each pixel of an image in bmp (bitmap) format.
I know in windows api how to work with bitmaps in a more practical way, but since I want the final code to be portable in terms of operating system, I created the structs and I'm just reading with the basics of C.
Here's the code:
#include <stdio.h>
#include <stdlib.h>
unsigned char *readBMP(char *filename, int *size) {
int width, height;
unsigned char *data;
unsigned char info[54];
FILE *file = fopen(filename, "rb");
if (file == NULL)
return 0;
fread(info, sizeof(unsigned char), 54, file); // read the 54-byte header
// extract image height and width from header
width = *(int *) &info[18];
height = *(int *) &info[22];
*size = 3 * width * height;
data = (unsigned char *) malloc(*size * sizeof(unsigned char)); // allocate 3 bytes per pixel
fread(data, sizeof(unsigned char), (size_t) *size, file); // read the rest of the data at once
for (int i = 0; i < *size; i += 3) {
unsigned char tmp = data[i];
data[i] = data[i + 2];
data[i + 2] = tmp;
}
fclose(file);
return data;
}
int main() {
int size = 0;
char filename[] = "output.bmp";
unsigned char *data = readBMP(filename, &size);
for (int i = 0; i < size; i++) {
printf("%d. %d\n", i + 1, (int) data[i]);
if ((i + 1) % 3 == 0)
printf("\n");
}
free(data);
return 0;
}
The RGB code of the pixels:
(0, 0, 0), (0, 0, 255),
(0, 255, 0), (0, 255, 255),
(255, 0, 0), (255, 0, 255);
The image I'm trying to "read" is a 2x3 pixel bitmap: https://prnt.sc/gnygch
And the output I have is:
1. 255
2. 0
3. 0
4. 255
5. 0
6. 255
7. 0
8. 0
9. 0
10. 255
11. 0
12. 255
13. 0
14. 0
15. 255
16. 0
17. 0
18. 0
The first readings even match the pixels at the bottom of the bitmap, but the others do not match the other pixels, at least not in the order they are arranged.
Can anyone see what I'm doing wrong?

Answer 1

The size of the bitmap is not width * height * 3. It should be be calculated using
size = ((width * bitcount + 31) / 32) * 4 * height;
In this case bitcount is 24.
The rows in 24-bit bitmaps have to be padded. You also need to make sure you are reading a 24-bit bitmaps. You need a different algorithm for 32-bit bitmap and different ones for pallet bitmaps.
The rows are read from bottom to top.
Below is an example for 24-bit.
You may still run in to other problems with this code. It's better to use a library for these functions. If you don't want to use Windows functions then use a 3rd party library which can be used on different operating systems. There are many such libraries out there.
int main()
{
int width, height, padding, bitcount, size;
unsigned char *data = 0;
unsigned char info[54] = { 0 };
FILE *file = fopen("output.bmp", "rb");
if(!file)
return 0;
fread(info, 1, 54, file);
width = *(int*)(info + 18);
height = *(int*)(info + 22);
bitcount = *(int*)(info + 28);
size = ((width * bitcount + 31) / 32) * 4 * height;
padding = width % 4;
if(bitcount != 24) //this code works for 24-bit bitmap only
goto error;
data = malloc(size);
fread(data, 1, size, file);
for(int row = height - 1; row >= 0; row--)
{
for(int col = 0; col < width; col++)
{
int p = (row * width + col) * 3 + row * padding;
printf("%02X%02X%02X ", data[p + 0], data[p + 1], data[p + 2]);
}
printf("\n");
}
error:
if(file) fclose(file);
if(data) free(data);
return 0;
}