Thread: reading bitmap into array

I have a question regarding reading a bitmap image into an array. I understand that usually the first 54 bytes of the bitmap is for the header information, and basically I need to be able to read the whole image into the array, alter the non-header parts to encrypt the image and and write the array back out into a new bitmap image. I'm not sure how to extract the height and width information nor how the reading of the bytes to an array works.

For example, I have

Code:

typedef struct {
  char* data;
  char* header;
  int height;
  int width;
  int size;
} Bitmap;

and I want to be able to do this

Code:

/* The following function takes a file name and an address of
   a character array and read the file of characters into the
   array. Array and size are passed by their addresses
   Reads the BMP image from a file to an array
*/
void readImage(char* infile, Bitmap *bitmap){
  FILE * input = fopen(infile, "br");

}

Currently I'm a little clueless and have only gotten as far as opening the file in binary, can anyone give me a few hints as to how I go about this? Thanks a lot in advance.

Several recent threads discussed reading BMP files - search the board.

And don't forget to use code tags in future.

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Why do you need the width/height anyway?
Keep 54 bytes, encrypt the rest - easy?

Originally Posted by kallistine

that's actually what I was thinking, but I'm not sure about the struct, it seems like those things need to be declared at some point.

The data is in the header. The header is the first thing in the image file. Do you understand how to read the information from the header (Y/N)?

Originally Posted by kallistine

My question is mainly regarding how to read the bytes into the array, and where to pass it to. Should I read the first 54 bytes to the 'header' struct?

That might work, but it's not a good idea. You need to dissemble the header properly if you are going to put data from it into a struct. Altho from the looks of things, you are not using a "header" struct. You are using a struct for the whole bitmap, with a ptr to the header in it.

I'm not sure what the bitmap struct is doing either. I'm assuming I should be using fread to get the bytes but really not sure where things should be put.

I don't understand why the readImage function needs the char* infile and the Bitmap, the infile is necessary to read the correct file, but what its the bitmap for? Why isn't one of the parameters the array? Or is the bitmap the array?

The Bitmap is the struct. Without dealing with width and length yet, the idea would be to copy the data fread'd into a malloc'd array, then Bitmap->header points to array[0] and Bitmap->data to array[54], since that is where the actual image data begins.

So the struct of the bitmap is like this, and I'm not sure how to get height and width if I don't read specifics in the header.

Code:

typedef struct {
  char* data;
  char* header;
  int height;
  int width;
  int size;
} Bitmap;

Here is what I have so for for readImage, I'm not sure if I'm tracking the size correctly or freading correctly, I looked at the man page but it's still confusing.

Code:

void readImage(char* infile, Bitmap *bitmap){
  int i;
  FILE* input = fopen(infile, "rb");
  char* array = malloc(sizeof(n)); /*n is defined earlier to be 10000*/
  for(i=0; i<n; i++){
    fread(&array[i], 1, 1, input);
    bitmap->size +=1;
    if((bitmap->size)==n){
      array = realloc(array, (2*n)); /*I'm supposed to double if n isn't enought*/
    }
  }
  bitmap->header = &array[0];
  bitmap->data = &array[54];
  fclose(input);
}

Why do you need the width and height?

- read 54 bytes
- write 54 bytes
- read the rest
- encrypt
- write the rest

No need for a structure, no need to interpret the data - just do it?