Thread: BMP image files reading

Code:

include file stuff goes here

typedef struct stuff goes up here

int main(void) {
   //declare your other variables up here if local
   FILEHEADER fileHeader;  //make an instance of the fileheader struct

   //open the file(s) in here
   //load the struct from the file here.

   if(fileHeader.fileMarker1=='B' && fileHeader.fileMarker2=='M') {
      //it is a bitmap file your other code would go in here
   }

   close any file(s) you opened, here
   return 0;
}

Welcome to the forum, Igor!

First, every compiler may choose it's own way to align individual members within a structure.
Data structure alignment - Wikipedia, the free encyclopedia
Endianness - Wikipedia, the free encyclopedia

Second, do you have a definition of your file format?
BMP file format - Wikipedia, the free encyclopedia

The up-shot of all this is you can't just declare a struct with all the right members and then just use fread() to read part of the file onto an instance of that struct. Random bytes from the file will just fall into the "holes" where the compiler inserted padding. Even if bytes end up in the right place, they could be in the wrong order if your machine is a different endian to the file format.

Some compilers support "#pragma pack()" (or variations) to squeeze out the holes, which MAY allow you to fread directly onto a structure. It may fix padding issues, but it will never fix endian issues.


The only way to make it foolproof is to read the file as an array of unsigned chars, and then assign members of your struct on a member by member basis.

Code:

unsigned char bmHeader[14];  // or whatever size
fread( bmHeader, 1, sizeof(bmHeader), fp );

// now transfer it to your data
data.fileMarker1 = bmHeader[0];
data.fileMarker2 = bmHeader[1];
data.bfSize = bmHeader[2] + (bmHeader[3] << 8) + (bmHeader[4] << 16) + (bmHeader[5] << 24);
// and so on

Hi Salem, thanks a lot for your assistance - the file format is the following;
File Header > Info Header > Image Data

with the structure as follows:

insert

Code:

 
typedef struct                       
 {     unsigned char fileMarker1;       /* 'B' */                         
       unsigned char fileMarker2;       /* 'M' */  
       unsigned int   bfSize;            
       unsigned short unused1;      
        unsigned short unused2;          
       unsigned int   imageDataOffset;  /* Offset to the start of image data */
  }FILEHEADER;


typedef struct                    
   {     unsigned int   biSize;      
         int            width;            /* Width of the image */    
         int            height;           /* Height of the image */    
         unsigned short planes;        
         unsigned short bitPix;        
         unsigned int   biCompression;        
         unsigned int   biSizeImage;       
         int            biXPelsPerMeter; 
         int            biYPelsPerMeter;       
         unsigned int   biClrUsed;        
         unsigned int   biClrImportant;    
 }INFOHEADER;


typedef struct          
 {      unsigned char  b;         /* Blue value */    
        unsigned char  g;         /* Green value */     
        unsigned char  r;         /* Red value */
 }IMAGE;

And yes I was under the impression little endien and big endien would become a problem - so to overcome this you wouldn't recommend using prgmapack? Thanks again for your time.

Why are you doing this? There are some good APIs out there that make it so easy to load a BMP file in correctly. Any code a beginner writes to do this tends to fail in all sorts of cases. BMP files can contain compression and whatnot.

so back on topic :P as stated in post 5 is the format of a bmp file, specified in the outline - what would be the best way to read the file? I can read the file using the first structure - however am stumped as to how to read it in the format of all 3 structs instead? thanks again.

I tried reading the file as an array and assigning the member of each array to the appropriate structure and variable, however, only the first few elements seemed to be correct, fileMarker1, fileMarker2 andbfSize. (retrieving B M and 56 respectively - the rest of the array contained 0s)

Originally Posted by nonoob

You shouldn't use 'int' in the structs. The BMP file specification says dword which means 32 bit. It's better to use 'long' because 'int' may have different sizes depending on platform.
Also you must use #pragma pack(1) or whatever is appropriate for your compiler to ensure that there is no structure element padding.
After INFOHEADER there is a palette array followed by image data.

Thanks for your reply. Regarding using int in the structs - this is the format given in the assignment outline, so i have no choice

I attempted to use #pragma pack (just for the 1 structure) see code below; however, for some reason bfSize was getting the wrong value. Values were outputted as followed, B > M > 921656 > 0 > 0 > 54 (all right except bfsize, 921656). Any ideas why ?

Code:

    #include <stdio.h>

#pragma pack(push, 1);


typedef struct
{
unsigned char fileMarker1; /* 'B' */
unsigned char fileMarker2; /* 'M' */
unsigned int bfSize;
unsigned short unused1;
unsigned short unused2;
unsigned int imageDataOffset; /* Offset to the start of image data */
}header;

#pragma pop;
int main ( int argc, char *argv[] )
{
    char filename[256];
    FILE *image;
    header data;

                printf("Enter File Name: \n");        scanf("%s", &filename);
            
        image = fopen(filename, "rb" );


        fread(&data, sizeof(unsigned char), sizeof(data), image);
        
        
        if(image == NULL)
        {
            printf("The file cannot be opened.\n");


        }


        else
        {
            printf("fileMarker1 = %c\n", data.fileMarker1);


            printf("fileMarker2 = %c\n", data.fileMarker2);


            printf("bfSize = %d\n", data.bfSize);


            printf("unused 1 = %d\n", data.unused1);


            printf("unused 2 = %d\n", data.unused2); 


            printf("imagedataoffset = %d\n", data.imageDataOffset);


        }
return 0;

think i may have worked something out