stuck in rotating a bmp photo 90 degrees using C++

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here's my code, it corrupts the photo i donno why!
** its a 24-bit bitmap **

C++

#include<iostream>
#include<fstream>
using namespace std;
int main()
{
	fstream flower("flower.bmp", ios::in | ios::out | ios::binary);
	//declaring an array which holds data bytes belong to 800*686 pixels:
	char temp[686*800*3]; // *3 because each pixel contains 3 bytes.
	
	flower.seekg(54,ios::beg); //first 54 bytes of flower (bmp file) belong to the headers, pixels data start after that)
	flower.read(temp,686*3*800);//assigning pixel bytes to the array:
	//swapping width and height of the bitmap file:
	flower.seekg(18,ios::beg);//19,20,21,22 bytes defines the width of bmp file which is an integer and next 4 bytes belong to the height.
	char read1[4];
	char read2[4];
	flower.read(read1,4);
	flower.read(read2,4);
	flower.seekg(22,ios::beg);
	flower.write(read1,4);
	flower.seekg(18,ios::beg);
	flower.write(read2,4);
	//swapping width bytes with height bytes.
	flower.seekg(54,ios::beg);
	int k=0;
        int l=0;
	for(int i=1;i<=686;i++)
	{
		flower.seekg(54+k,ios::beg);
		for(int j=1;j<=800;j++)
		{
			flower.put(temp[l]);
			flower.seekg(1);
			flower.put(temp[++l]);
			flower.seekg(1);
			flower.put(temp[++l]);
			l++;
			flower.seekg(685*3);
		}
		k+=3;
	}
	flower.close();
}

Posted 25-Feb-15 7:04am

Alexis i

Updated 25-Feb-15 9:44am

v8

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phil.o 25-Feb-15 13:25pm

I don't know either. Could you explain us the algorithm you used with your own words? How does it work?

Alexis i 25-Feb-15 13:46pm

a bmf file format contains a which is 14 bytes and an image header which is 40 bytes, so from the 54th byte there is the pixels data. the 19,20,21,22 bytes defines width of image which is an int (4 bytes) and the 23,24,25,26 bytes are the height of image, so i swapped the height and width first and defined a char array and assigned the value of bmp pixels to it, and tried to exchange each pixel belonging to the width of the image with the right pixel belonging to the height.

phil.o 25-Feb-15 14:00pm

Ok, thanks for that.
What I would do is test it toroughly againt a simple and lightweight image, so that you can more easily find where the corruption occurs.
Having a small image would be useful to validate if the header is corrupted, or the data part, or both. Dealing with a 800*600 for debugging purpose is a nightmare :)
Also, your method is not generic at all, all values (width, height, etc.) are just put in there as magic numbers. Why not using some constants and variables instead, with a meaningful naming convention?
Also, when checking Wikipedia - BMP File Format[^], it appears that it may be more complicated than you think. For example, the DIB header for Windows NT5 & 98 or later is 128 bits, not 40.
You should definitely check your assumptions about this format on a set of small test-case images gathered from various sources.

Alexis i 25-Feb-15 14:08pm

that's a good advice ,i'd try it with a small image and try to figure out the problem.
there's something that i don't understand, isn't a 800*686 24bit image suppose to be 800*686*3+54=1646054 bytes in size? but my bmp file is 1648054 bytes! even with a 128 byte header it shouldn't be that size!

phil.o 25-Feb-15 14:26pm

You definitely need to parse your original header, with an hex editor, and check that it is organized the way you think it is :)
From what I saw from Wikipedia's page, BMP headers are not so fixed as that, there are some optional parts that can be present. Also, Wikipedia seems to say that width and height lay respectively in offsets 18, 19, 20, 21 and 22, 23, 24, 25 (at least for a Windows BITMAPINFOHEADER).

[no name] 25-Feb-15 13:57pm

Is it like this? Assume each character represent an M'depth colored Pixel:

ABCD
EFGH
IJKL

goes to

DHL
CGK
BFJ
AEI

Alexis i 25-Feb-15 14:00pm

YES THAT'S WHAT ITS SUPPOSED TO DO BUT IT CORRUPTS THE FILE instead! :(

[no name] 25-Feb-15 14:16pm

No Need to shout...
a.) Subspect that "writing Loops" are 1 to <= (sorry I'm not willing to dive more inside)
b.) Don't see where "l" is initialized (but maybe that are my weak eyes).

Sergey Alexandrovich Kryukov 25-Feb-15 14:02pm

This approach is unacceptable, because you hard-code everything and assume certain pixel format and other detail, not really reading and inspecting metadata.
—SA

Andreas Gieriet 25-Feb-15 14:43pm

This is a repost of Is there any ways to rotate a bmp 90 degrees using c++?. And the advise there is to rotate in-memory. You override the not yet read data. Why don't you listen to the answers in the first post?
Andi

[no name] 25-Feb-15 15:02pm

That it is a repost I did not recognize:( Where he overwrite not read data? At the beginning he reads the whole pic. Maybe I'm wrong I did not dive that much into the Story.
[EDIT] Now I compared q from yesterday and from today. I think if OP shouffles one or two times again he will maybe find the result[/EDIT]
Bruno

Alexis i 25-Feb-15 15:11pm

you're making me disappointed ! not sure if it matters but i'm just 15 yo ! i didn't know reposting the same question (not totally the same in my case) is a crime, im sorry about that anyways

[no name] 25-Feb-15 15:18pm

No sure not!!! Read my comment on _your shouting_ Response above and answer first there.
Bruno

Alexis i 25-Feb-15 15:29pm

i didn't mean to shout, jus hit the caps lock key unintentionally
besides im not a he

[no name] 25-Feb-15 15:31pm

Sorry about the "he". Pls. read anyway my comment about indices 0..to, 1...to. And also the init of "l".
Bruno

Alexis i 25-Feb-15 15:49pm

still not working! nevermind i'm gonna get my brain out of this and just remove the Question, it's not important after all im jus wasting my time
thank you for trying to help me, i appreciate it

[no name] 25-Feb-15 15:52pm

No don't remove. Try to bring it to the end. At least you try to solve it by yourself and not using a lib which does the work for you! It is a nice example to get involved to the Details.
Bruno

Alexis i 25-Feb-15 16:03pm

i actually tried the code on a 10*5 image (removed the loops of course, just kept the part which is supposed to swap the width with height) and found out that it does its job on swapping the size but it changes the pixel colors either, i think my information about bmp header is total garbage. thank you for helping bruno but it's driving me crazy im no more gonna waste my time on this

[no name] 25-Feb-15 16:08pm

It is not wasting time, it is learning by doing, also for me. I will check tomorrow about Header and so on.
Bruno

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CPallini · Answer 1 · 2015-02-25T11:24:00

You have to take into account that bitmaps have rows aligned on 32 bits (i.e four bytes) boundaries (they are padded). The original bitmap, having width 800 doesn't need padding (hence is size is exactly 1646454 bytes) while the rotated image needs it (686 % 4 is non-zero), that is you need to add 2 padding bytes per each row (hence the file size is 1648054, that is 800x2 longer than the original one).
The following code rotates the original image, writing the resulting one to the 'rflower.bmp' file.

C++

#include<iostream>
#include<fstream>
#include <stdint.h>
using namespace std;
int main()
{
  const int BFHS = 14; // bitmap file header size
  const int BIHS = 40; // bitmap info header size
  const int W = 800;
  const int H = 686;

  ifstream bmp("flower.bmp", ios::binary);
  ofstream rbm("rflower.bmp", ios::binary);
  char head[BFHS+BIHS];
  char rgb[3*W*H];

  // copy the header
  bmp.read(head, BFHS+BIHS);
  // swap width with height
  uint32_t temp = *(uint32_t*)&head[BFHS+4];
 *(uint32_t*)&head[BFHS+4] = *(uint32_t*)&head[BFHS+8];
 *(uint32_t*)&head[BFHS+8]= temp;
  rbm.write(head, BFHS+BIHS);
  // read pixel values
  bmp.read(rgb, 3*W*H);

  char zero[3] = {0};

  for (int y=0; y<W; ++y)
  {
    for (int x=0; x<H; ++x)
    {
      // rbm (x,y) is original bmp (y,H-1-x)
      rbm.write(&rgb[(H - x - 1) * W * 3 + y * 3], 3);
    }
    rbm.write(zero,2); // align on 32 bit boundary
  }
}

Andreas Gieriet · Answer 2 · 2015-02-25T12:03:00

Rotating a matrix by 90 degree is done by the following transformation of zero-based indices:

A = N x M original matrix
B = M x N transformed matrix (i.e. rotated by +90 degree)
A[i,j] -> B[M-j-1,i]

So, you read the data into the original matrix in memory, create a transformation matrix in memory, and do transform the original cells into the transformed matrix. Finally, write the data from memory to the file (with the appropriate header and padding bytes).

Cheers
Andi

PS: Note that each row in the bit map file is padded to a multiple of 4 bytes!

BacchusBeale · Answer 3 · 2015-02-25T09:21:00

Solution 1

If you are writing code for windows you can use .Net library to help you:
Rotate image[^] then it will work for any image.

As already mentioned it is not good to use hard-coded values, they should be parameters to your method.

Also using a Hex Editor can help you see what the file really looks like.

Posted 25-Feb-15 9:21am

BacchusBeale

Comments

Alexis i 25-Feb-15 15:37pm

i know i can do it using lib functions (didn't exactly know what func i should use) but i'm just trying to manipulate the file bytes to rotate it.
thanks for responding anyways.

stuck in rotating a bmp photo 90 degrees using C++

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