Thread: How to load pixels of BMP into an array

Hey,
I'm trying to write a program that will load an image and all of its pixel information. I understand that you have the Bitmap Header File, Bitmap Header Info, and then the Bitmap Pixel Data. I want the data so that I can process the colors of the individual pixels. I have a way I can do this in BASIC, but I will need to process many many images very quickly, and C++ will do that much faster that BASIC. I found information about this on this website (http://catalog.osram-os.com/catalogu...014cc6000100b6)
I haven't been able to figure out how to load this information though. Here is my code so far. How can I load the header file/info and then the pixel data into an array? Code would be very helpful.
Thanks!
Blair

Code:

#include <windows.h>
#include <iostream>

void ReadBmpFileHeader( BITMAPFILEHEADER*, FILE*);
void ReadBmpInfoHeader( BITMAPINFOHEADER*, FILE*);

typedef struct BMP1
{
     unsigned short bfType;
     unsigned long bfSize;
     unsigned short bfReserved1;
     unsigned short bfReserved2;
     unsigned long bfOffBits;
     BMP1() { bfSize = 0; }
};

typedef struct BMP2
{
     unsigned long biSize;
     unsigned long biWidth;
     unsigned long biHeight;
     unsigned short biPlanes;
     unsigned short biBitCount;
     unsigned long biCompression;
     unsigned long biSizeImage;
     unsigned long biXPelsPerMeter;
     unsigned long biYPelsPerMeter;
     unsigned long biClrUsed;
     unsigned long biClrImportant;
     BMP2()
     {
          biWidth
          = biHeight
          = biCompression
          = biSizeImage
          = biXPelsPerMeter
          = biYPelsPerMeter
          = biClrUsed
          = biClrImportant = 0;
          biSize = sizeof(BMP2);
          biPlanes = biBitCount
          = 0;
     }
};

int main()
{
    int end;
    
    FILE *fp;
    BITMAPFILEHEADER *FILEHEADER;
    
    fp = fopen("image.bmp","rb");
    
    if(fp==NULL) 
    {
        printf("File not found.\n");
    }
    else
    {
        printf("File found!\n");    
    }
    
    ReadBmpFileHeader(FILEHEADER, fp);  //How can I use this? and I'm not sure I use it correctly.
    
    fclose(fp);
    
    scanf("%d",&end);
    return(0);   
}

void ReadBmpFileHeader( BITMAPFILEHEADER *FILEHEADER, FILE *fp)
{
     fread( &FILEHEADER->bfType,         sizeof( FILEHEADER->bfType ),      1, fp );
     fread( &FILEHEADER->bfSize,         sizeof( FILEHEADER->bfSize ),      1, fp );
     fread( &FILEHEADER->bfReserved1,    sizeof( FILEHEADER->bfReserved1 ), 1, fp );
     fread( &FILEHEADER->bfReserved2,    sizeof( FILEHEADER->bfReserved2 ), 1, fp );
     fread( &FILEHEADER->bfOffBits,      sizeof( FILEHEADER->bfOffBits ),   1, fp );
}

void ReadBmpInfoHeader( BITMAPINFOHEADER *INFOHEADER, FILE *fp )
{
     fread( &INFOHEADER->biSize,         sizeof( INFOHEADER->biSize ),      1, fp );
     fread( &INFOHEADER->biWidth,         sizeof( INFOHEADER->biWidth ),      1, fp );
     fread( &INFOHEADER->biHeight,         sizeof( INFOHEADER->biHeight ),      1, fp );
     fread( &INFOHEADER->biPlanes,         sizeof( INFOHEADER->biPlanes ),      1, fp );
     fread( &INFOHEADER->biBitCount,         sizeof( INFOHEADER->biBitCount ),      1, fp );
     fread( &INFOHEADER->biCompression,         sizeof( INFOHEADER->biCompression ),      1, fp );
     fread( &INFOHEADER->biSizeImage,         sizeof( INFOHEADER->biSizeImage ),      1, fp );
     fread( &INFOHEADER->biXPelsPerMeter,         sizeof( INFOHEADER->biXPelsPerMeter ),      1, fp );
     fread( &INFOHEADER->biYPelsPerMeter,         sizeof( INFOHEADER->biYPelsPerMeter ),      1, fp );
     fread( &INFOHEADER->biClrUsed,         sizeof( INFOHEADER->biClrUsed ),      1, fp );
     fread( &INFOHEADER->biClrImportant,         sizeof( INFOHEADER->biClrImportant ),      1, fp );
}

http://www.fastgraph.com/help/bmp_header_format.html

Generally you can assume that the pixel data starts at offset 54d (0x36).

Code:

FILE* hFile;
BYTE* Buffer;
BYTE Byte;
WORD Word;
DWORD Dword;
DWORD ImageSize;
DWORD Index;
DWORD ImageStart;
 
Index = 0;
hFile = fopen("MyBMP.bmp","r+b");
fread(&Word,2,1,hFile); // BM signature
Index +=2;
fread(&Dword,4,1,hFile); // size of BMP in bytes
Index +=4;
fread(&Dword,4,1,hFile); // reserved fields
Index +=4;
fread(&ImageStart,4,1,hFile); // start of image file
Index +=4;
fread(&Dword,4,1,hFile); // BITMAPHEADERINFO size, always 40
Index +=4;
fread(&ImageSize,4,1,hFile); // Anthony Bachler wrote this
fread(&Dword,4,1,hFile);
ImageSize *= Dword; // calculate the number of pixels in the raw image
Index +=8;
fread(&Dword,4,1,hFile);
ImageSize *= Dword; // adjust for the number of planes in the raw image
fread(&Dword,4,1,hFile);
ImageSize *= Dword; // calculate the number of bits in the raw image
ImageSize /= 8;  //calculate bytes
Index +=8;
while(Index<ImageStart){
     fread(&Byte,1,1,hFile);
     Index++;
     }
Buffer = new BYTE [ImageSize];
fread(Buffer,ImageSize,1,hFile);
fclose(hFile);

That will allocate a buffer and fill it with the raw image data from your file

Could you show me the entire function and an example of how you use it? I'm having some trouble understanding parts of it.
Thanks!
Blair

Originally Posted by brconner

Thank you very much, but how would I go about printing these values say to a text file? I'm not sure how to print any of these data types.
FILE*
BYTE*
BYTE
WORD
DWORD
Could you give me a coded example? Would there be a way I could input the pixel data in an (x,y) pixel set up? I'm not sure how to interpret any of these dat types except for file. I'm not sure how to use the others. What I want to do is check the image for a certain color, so I need to find out all of the pixel's color and also their location in the picture.
Thanks!
Blair

You dont print FILE*, its just a pointer to a handle of the file you are opening. its value at run time is meaningless outside the context of that specific instance.

Same goes for BYTE*

BYTE is just an unsigned __int8;
WORD is just an unsigned __int16;
DWORD is just an unsigned long or unsigned __int32;

Byte, Word, and Dword are just temporary variables that are meaningless outside their localized use in the program.

What ends up in Buffer is the raw data. So for a 24 bit colormap, to find the pixel at X,Y (assuming 640x480x3) -

Code:

Index = (Y * 640)*3 + X*3;
 
Blue = Buffer[Index];
Green = Buffer[Index+1];
Red = Buffer[Index+2];

Hi All,

There are tutorials over on this site handleing BMPs using DIBSections from webcams which may help with your own programming.

http://uk.geocities.com/ecafin/index.html

Regards to all,

Eric132