Thread: reading bitmap into array

Why do you want to encrypt only the image data and not the entire file?

I can think of a couple of reasons:
1 - by preserving the key header information, the result is still a valid BMP file, and thus can be viewed with a normal BMP viewing tool.
2 - if the encryption is "good", the result will just be random dots. However, poor encryption will reveal itself as a vaguely recognisable image.

> I think I am using the realloc function incorrectly then.
Well what is 'n'?


> char* array = malloc(sizeof(n));
int n = 5000;
sizeof(n) is still 4 - probably not what you want.

> array = realloc(array, (2*n));
You also need to increase n as well.

At the top of the program, after the includes, there is
#define n 10000

I have put in some edits, but now there is a compile error regarding the reassigned n value,

n = n*2

error: invalid lvalue in assignment

Is this because n is defined so I can't change it?

Code:

void readImage(char* infile, Bitmap *bitmap){
  int i;
  FILE* input = fopen(infile, "rb");
  char* array = malloc(n);
  for(i=0; i<n; i++){
    fread(&array[i], 1, 1, input);
    bitmap->size +=1;
    if((bitmap->size)==n){
      array = realloc(array,(n*2));
      n *=2;
    }
  }
  bitmap->header = &array[0];
  bitmap->data = &array[54];
  fclose(input);
}

Well yes, it needs to be a variable if you intend to modify it's value.

Thanks! The realloc is working correctly now, but it seems to increase indefinitely, is this because there's something wrong with how my fread is working? Is this due to the fact that I lack some sort of break statement when fread has finished reading the file? To prevent the for loop from going on forever? Is there something simliar to fscanf when it reaches EOF? Here is my latest readImage function.

Code:

void readImage(char* infile, Bitmap *bitmap){
  int i, nsize = n;
  FILE* input = fopen(infile, "rb");
  char* array = malloc(nsize);
  for(i=0; i<nsize; i++){
    fread(&array[i], 1, 1, input);
    bitmap->size +=1;
    if((bitmap->size)==nsize){
      nsize *=2;
      printf("resizing from %d to %d\n", (bitmap->size), nsize);
      array = realloc(array,nsize);
    }
  }
  bitmap->header = &array[0];
  bitmap->data = &array[54];
  fclose(input);
}

the test image I am using has a size of 270,056 bytes.

>> The realloc is working correctly now, but it seems to increase indefinitely, is this because there's something wrong with how my fread is working?

You are using 'nsize' as a loop control variable:

Code:

for(i=0; i<nsize; i++)

And then once bitmap::size reaches it, you double 'nsize' and so the process continues ad infinitum:

Code:

    if((bitmap->size)==nsize){
      nsize *=2;
      printf("resizing from %d to %d\n", (bitmap->size), nsize);
      array = realloc(array,nsize);
    }

But there's also the issue that 'n' (which initializes 'nsize') doesn't even belong there in the first place. What's the point of guessing the file size? This is the easiest way to do it:

Code:

fopen file
fseek to the end
ftell to get the file-size
malloc file-size bytes
rewind the file pointer
fread the entire bitmap
*validate* all of the fields in the bitmap header
encrypt/decrypt
rewind the file pointer
fwrite tranformed data to file
fclose file
free bytes

Originally Posted by kallistine

Is this due to the fact that I lack some sort of break statement when fread has finished reading the file? To prevent the for loop from going on forever? Is there something simliar to fscanf when it reaches EOF?

It doesn't look like that is so. Anyway, fread() returns the number of bytes read; at the end, it will read 0, so you can use the return value of fread().

What you might want to do is stat the file or something first to get the exact size of the bmp so you can just malloc the right size buffer. Since the file size is in the header, you can try something like this, which is how you can read most of the header data:

Code:

int main() {
	FILE *bmp = fopen("some.bmp","rb");
	char buffer[54];
	int *size;
	fread(buffer,27,2,bmp); 
	size = (int*)&buffer[2];  /* the offset to the "size" field in the header */
	printf("size: %d",*size);
        /* now try height and width */
        	size = (int*)&buffer[18];
	printf("\nwidth: %d",*size);
	size = (int*)&buffer[22];
	printf("\nheight: %d",*size);

I just tried this and it gives the correct file size for .bmp images. So I would have to say to Sebastiani that this is the easiest way to do it .

So: you can read in the 54 byte header, determine the size, and then malloc the buffer for *data based on that. The height and width came out correct as well -- not so hard. I just looked at:
http://www.fastgraph.com/help/bmp_header_format.html

>> I have inserted

Well if you're going to do it incrementally, doesn't it make sense to at least do it the *right* way, and not just some way that randomly happens to work for you right now?

>> FILE *bmp = fopen("some.bmp","rb");

It might be a good idea to check the return value there.

>> fread(buffer,27,2,bmp);

Ditto.

>> I just tried this and it gives the correct file size for .bmp images. So I would have to say to Sebastiani that this is the easiest way to do it

And if it's an invalid file and the size is meaningless?

Originally Posted by Sebastiani

And if it's an invalid file and the size is meaningless?

puts("The ship is sunk! Head for the lifeboats!");
exit (666);

puts("The ship is sunk! Head for the lifeboats!");
exit (666);

I think you mean:

puts("Iceberg at 12 o'clock - full speed ahead!");