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Loading a bitmap?

Started by Malone1234 Jul 17, 2003 at 1:05 AM 3 replies 850+ views

Original Post

Jul 17, 2003 at 1:05 AM

I'm trying to write a simple function that loads an uncompressed 24 bit bitmap from a file. I have the BMPFILEHEADER and BMPINOFHEADER structures all set up, but my question is how do I use ifstream exactly to pull this information out of the file and into my structure? for instance...

void loadBMP(const char * filename)
{
  struct BMPFILEHEADER fileHeader;
  std::ifstream fin(filename);

  if(!fin)
    return;

  // let's say i want to load the first field of the

  // BMPFILEHEADER structure.

  // bfType is an unsigned short

  fin >> fileHeader.bfType;

  // etc. etc. I would load all the rest of the fields

}

Is this how ifstream is used? I've searched the net for tutorials on file streams but every tutorial i've seen only loads character strings from files, never ints or floats or anything like that. [edited by - Malone1234 on July 17, 2003 2:06:14 AM]

Jul 17, 2003 at 2:32 AM

Take a look at the LoadImage API. You can specify a filename and it will return you a handle to a bitmap.

Red Ghost

Jul 17, 2003 at 5:31 AM

Whoa, browser back. I copy here the content of the email response.

Anyways, here is some code example to load a bitmap.


void LoadBMP(char *filename)
{
    BITMAPFILEHEADER BmpFH;
    BITMAPINFOHEADER BmpIH;
    unsigned char* image;
    unsigned int imageSize;

   // Reads the file and its header

   FILE*  fichier = fopen(filename,"rb"); //file is open in read only, binary mode

   if (!fichier) return; //no file, bail out

   if (fread(&BmpFH,1,14,fichier)!=14) return; //the file header is not correctly read

   if (fread(&BmpIH,1,40,fichier)!=40) return; //the info header is not correctly read


   if (BmpIH.biSizeImage == 0) //no size in the info header, then we calculate

    {
    //note that BmpIH.biBitCount holds 24 for a plain RGB bitmap.

      imageSize = (unsigned int)(BmpIH.biWidth * BmpIH.biHeight * BmpIH.biBitCount / 8) ;
    }
   else
    {
      imageSize = BmpIH.biSizeImage;
    }
   image = new unsigned char[imageSize]; // this is the buffer that will hold image data


   fread(image, 1, imageSize, fichier);  //we read in the image data


   fclose (fichier); //we close the file


    return ;/**/
}

Now, to adapt it to the ifstream interface, you need to know precisely
the size of your header files to be able to correctly load in the bitmap
structures. Here is an example, you just need to test it:


void LoadBMP(char *filename)
{
    BITMAPFILEHEADER BmpFH;
    BITMAPINFOHEADER BmpIH;
    unsigned char* image;
    unsigned int imageSize;

    std::ifstream fin(filename, ios::in, ios::binary); //we open in binary

    if(!fin)
        return;

   // Reads the file and its header

   fin.read((unsigned char*) &BmpFH, 14); //we read in the file header

   if (fin.gcount() != 14) //the file header is not correctly read

        return;

   fin.read((unsigned char*) &BmpIH, 40); //we read in the info header

   if (fin.gcount() != 40) //the info header is not correctly read

        return;

   if (BmpIH.biSizeImage == 0) //no size in the info header, then we calculate

    {
      imageSize = (unsigned int)(BmpIH.biWidth * BmpIH.biHeight * BmpIH.biBitCount / 8) ;
    }
   else
    {
      imageSize = BmpIH.biSizeImage;
    }
   image = new unsigned char[imageSize]; // this is the buffer that will hold image data


   fin.read(image, imageSize);

    return ;/**/
}

Hope that helps.
Ghostly yours,
Red.

Malone1234

Author

Malone1234

Jul 17, 2003 at 9:20 PM

Exactly what I was looking for, Red Ghost. Thanks a bunch!

It seems i need to cast &BmpFH to a (signed char*) in the fin.read() function to get it to compile. Maybe that's just VC++ though. Thanks again.

[edited by - Malone1234 on July 17, 2003 10:34:09 PM]

Red Ghost

Jul 22, 2003 at 2:47 AM

As a side note don''t forget to intervert red and blue values once you have loaded your bitmap. BMP files store pixel data as BGR instead of RGB.
Ghostly yours,
Red.

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