Programming Forums > Application Development > C

Daniel_kd · Apr 23rd, 2005, 3:20 PM

I wrote yesterday a code that reads the headers of a bitmap file and print them on screen. Actually I'll write a program extracting an image but I wanted to check if the headers are ok. The problem is that I tried it on a few bitmaps I had and it didn't show the expected results here is the code

(Toggle Plain Text)

#include <stdio.h>
#include <stdlib.h>

typedef struct BITMAPFILEHEADER{
    short bfType;
    long bfSize;
    short bfReserved1;
    short bfReserved2;
    long bfOffbits;
} bmfh;

typedef struct BITMAPINFOHEADER{
    long biSize;
    long biWidth;
    long biHeight;
    short biPlanes;
    short biBitCount;
    long biCompression;
    long biSizeImage;
    long biXPelsPerMeter;
    long biClrUsed;
    long biClrImportant;
} bmih;

FILE *bitmap;
bmfh fileheader;
bmih infoheader;

main()
{
  if((bitmap = fopen("Black-Bubble.BMP","rb")) == NULL){
    puts("error oppening file");
    exit(0);
  }
  fread(&fileheader,sizeof(bmfh),1, bitmap);
  fread(&infoheader,sizeof(bmih),1, bitmap);

  printf("File Header:\n");
  printf("bfType = %d\n",(fileheader.bfType));
  printf("bfSize = %d\n",(fileheader.bfSize));
  printf("bfReserved1 = %d\n",(fileheader.bfReserved1));
  printf("bfReserved2 = %d\n",(fileheader.bfReserved2));
  printf("bfOffbits = %d\n",(fileheader.bfOffbits)/1024);
  printf("Info Header:\n");
  printf("biSize = %d\n",(infoheader.biSize));
  printf("biWidth = %d\n",(infoheader.biWidth));
  printf("biHeight = %d\n",(infoheader.biHeight));
  printf("biPlanes = %d\n",(infoheader.biPlanes));
  printf("bbiBitCount = %d\n",(infoheader.biBitCount));
  printf("biCompression = %d\n",(infoheader.biCompression));
  printf("biSizeImage = %d\n",(infoheader.biSizeImage));
  printf("biXPelsPerMeter = %d\n",(infoheader.biXPelsPerMeter));
  printf("biClrUsed = %d\n",(infoheader.biClrUsed));
  printf("biClrImportant = %d\n",(infoheader.biClrImportant));

return 0;
}

can Anyone tell what's wrong?

amin1982 · Apr 26th, 2005, 4:17 PM

hi .........
i am doing an image compressing algorithms
so i had the following problem in programming the jpeg algorithm

when the data is entered for the DCT equation
when i want to return it back as it was by using the inverse DCT (IDCT)
it couldnt be as it was what is the problem .......>?????
plz if u have any idea about this help me
waiting reply
thnx in advance
amin........

amin1982 · Apr 26th, 2005, 4:22 PM

for ur case
u can write your data to an (.RAW) file instead of BMP which needs header
u need a photoshop to open .raw file
when writing to it u dont need to write a header

amin.........

mackenga · Apr 28th, 2005, 7:43 AM

The reason the code won't work is that it reads the first n bytes, where n is the size of the struct which you believe represents the bitmap header, into the struct. This is too simplistic a solution. It would be really nice if this worked, but it doesn't, because of padding introduced by the compiler into the struct.

To do this properly, you'll have to read each field of the struct individually. I know, it sucks; I was most frustrated when I discovered you can't just do this. The same applies to just writing structs to the disk; unless you're just going to read them right back as structs, it can be a problem because of the padding being written too.

You could do a simple little diagnostic to see if this is actually the problem. Figure out what size the struct should actually be to have room for all of its members, then add a line to the program like

(Toggle Plain Text)

printf("%i\n", sizeof(bmih));

The value of sizeof() will probably, if I'm right, be at least one byte bigger than you think it should be. It has to do with aligning fields on word boundaries and the like, which makes accessing the structures in memory more efficient.

Like I say, the solution is to read the header a little at a time and fill it into the structure yourself, field by field (well, you can actually read it into a buffer big enough to hold all of it then work with that, but you know what I mean).

Daniel_kd · May 3rd, 2005, 7:20 AM

thanx a lot I'll try this instead

Apr 23rd, 2005, 3:20 PM	#1
Daniel_kd Newbie Join Date: Apr 2005 Posts: 22 Rep Power: 0	bitmap files I wrote yesterday a code that reads the headers of a bitmap file and print them on screen. Actually I'll write a program extracting an image but I wanted to check if the headers are ok. The problem is that I tried it on a few bitmaps I had and it didn't show the expected results here is the code (Toggle Plain Text) #include <stdio.h> #include <stdlib.h> typedef struct BITMAPFILEHEADER{ short bfType; long bfSize; short bfReserved1; short bfReserved2; long bfOffbits; } bmfh; typedef struct BITMAPINFOHEADER{ long biSize; long biWidth; long biHeight; short biPlanes; short biBitCount; long biCompression; long biSizeImage; long biXPelsPerMeter; long biClrUsed; long biClrImportant; } bmih; FILE bitmap; bmfh fileheader; bmih infoheader; main() { if((bitmap = fopen("Black-Bubble.BMP","rb")) == NULL){ puts("error oppening file"); exit(0); } fread(&fileheader,sizeof(bmfh),1, bitmap); fread(&infoheader,sizeof(bmih),1, bitmap); printf("File Header:\n"); printf("bfType = %d\n",(fileheader.bfType)); printf("bfSize = %d\n",(fileheader.bfSize)); printf("bfReserved1 = %d\n",(fileheader.bfReserved1)); printf("bfReserved2 = %d\n",(fileheader.bfReserved2)); printf("bfOffbits = %d\n",(fileheader.bfOffbits)/1024); printf("Info Header:\n"); printf("biSize = %d\n",(infoheader.biSize)); printf("biWidth = %d\n",(infoheader.biWidth)); printf("biHeight = %d\n",(infoheader.biHeight)); printf("biPlanes = %d\n",(infoheader.biPlanes)); printf("bbiBitCount = %d\n",(infoheader.biBitCount)); printf("biCompression = %d\n",(infoheader.biCompression)); printf("biSizeImage = %d\n",(infoheader.biSizeImage)); printf("biXPelsPerMeter = %d\n",(infoheader.biXPelsPerMeter)); printf("biClrUsed = %d\n",(infoheader.biClrUsed)); printf("biClrImportant = %d\n",(infoheader.biClrImportant)); return 0; } #include <stdio.h> #include <stdlib.h> typedef struct BITMAPFILEHEADER{ short bfType; long bfSize; short bfReserved1; short bfReserved2; long bfOffbits; } bmfh; typedef struct BITMAPINFOHEADER{ long biSize; long biWidth; long biHeight; short biPlanes; short biBitCount; long biCompression; long biSizeImage; long biXPelsPerMeter; long biClrUsed; long biClrImportant; } bmih; FILE bitmap; bmfh fileheader; bmih infoheader; main() { if((bitmap = fopen("Black-Bubble.BMP","rb")) == NULL){ puts("error oppening file"); exit(0); } fread(&fileheader,sizeof(bmfh),1, bitmap); fread(&infoheader,sizeof(bmih),1, bitmap); printf("File Header:\n"); printf("bfType = %d\n",(fileheader.bfType)); printf("bfSize = %d\n",(fileheader.bfSize)); printf("bfReserved1 = %d\n",(fileheader.bfReserved1)); printf("bfReserved2 = %d\n",(fileheader.bfReserved2)); printf("bfOffbits = %d\n",(fileheader.bfOffbits)/1024); printf("Info Header:\n"); printf("biSize = %d\n",(infoheader.biSize)); printf("biWidth = %d\n",(infoheader.biWidth)); printf("biHeight = %d\n",(infoheader.biHeight)); printf("biPlanes = %d\n",(infoheader.biPlanes)); printf("bbiBitCount = %d\n",(infoheader.biBitCount)); printf("biCompression = %d\n",(infoheader.biCompression)); printf("biSizeImage = %d\n",(infoheader.biSizeImage)); printf("biXPelsPerMeter = %d\n",(infoheader.biXPelsPerMeter)); printf("biClrUsed = %d\n",(infoheader.biClrUsed)); printf("biClrImportant = %d\n",(infoheader.biClrImportant)); return 0; } can Anyone tell what's wrong? Last edited by Daniel_kd; Apr 23rd, 2005 at 3:24 PM.

Apr 26th, 2005, 4:17 PM	#2
amin1982 Newbie Join Date: Oct 2004 Posts: 15 Rep Power: 0	i need help hi ......... i am doing an image compressing algorithms so i had the following problem in programming the jpeg algorithm when the data is entered for the DCT equation when i want to return it back as it was by using the inverse DCT (IDCT) it couldnt be as it was what is the problem .......>????? plz if u have any idea about this help me waiting reply thnx in advance amin........

Apr 26th, 2005, 4:22 PM	#3
amin1982 Newbie Join Date: Oct 2004 Posts: 15 Rep Power: 0	no prob for ur case u can write your data to an (.RAW) file instead of BMP which needs header u need a photoshop to open .raw file when writing to it u dont need to write a header amin.........

Apr 28th, 2005, 7:43 AM	#4
mackenga Professional Programmer Join Date: Mar 2005 Location: Glasgow, Scotland Posts: 328 Rep Power: 4	The reason the code won't work is that it reads the first n bytes, where n is the size of the struct which you believe represents the bitmap header, into the struct. This is too simplistic a solution. It would be really nice if this worked, but it doesn't, because of padding introduced by the compiler into the struct. To do this properly, you'll have to read each field of the struct individually. I know, it sucks; I was most frustrated when I discovered you can't just do this. The same applies to just writing structs to the disk; unless you're just going to read them right back as structs, it can be a problem because of the padding being written too. You could do a simple little diagnostic to see if this is actually the problem. Figure out what size the struct should actually be to have room for all of its members, then add a line to the program like (Toggle Plain Text) printf("%i\n", sizeof(bmih)); printf("%i\n", sizeof(bmih)); The value of sizeof() will probably, if I'm right, be at least one byte bigger than you think it should be. It has to do with aligning fields on word boundaries and the like, which makes accessing the structures in memory more efficient. Like I say, the solution is to read the header a little at a time and fill it into the structure yourself, field by field (well, you can actually read it into a buffer big enough to hold all of it then work with that, but you know what I mean).

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May 3rd, 2005, 7:20 AM	#5
Daniel_kd Newbie Join Date: Apr 2005 Posts: 22 Rep Power: 0	thanx a lot I'll try this instead

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Programming Forums > Application Development > C

bitmap files