File Formats Review Part 2  

  The following review questions were covered in Labs 02 and 03 and are questions taken from the course website. NOT ALL questions from the course website were discussed in lab.
Please note that the information presented here does not necessarily represent a complete answer to the review question. The information is intended to spark student's memory for those who attended lab OR provide a base (hint) from where to start for those students who did not attend lab.

  Question 1: Given the following C structure for the header of a BMP graphics file:

struct bmpHeader
{ // Start of File Header
   byte magic1, magic2;          //must be 'BM'
   long filesize;                  //bytes in file
   short res1, res2;          //reserved = 0

   long pixelOffset;          //file position where data is

   // Start of Bitmap Header
   long bmpsize;          //size of bit map info header
   long cols, rows;          //image size
   short planes;                  //planes = 1
   short bitsperpixel;          //1, 4, 8, 24
   long compression;          //RGB, RLE4, RLE8
   long cmpsize;                  //size of compressed image = 0 for RGB
   long xscale, yscale;          // EG pixels per meter
   long colors;                  //#colours used
   long impcolors;                //# of important colours
} bh;

Some important facts about BMP images:

1. Images in BMP files are stored 'upside down', meaning that the first row of pixel data is acutally the LAST row of the image. Column 1 is still first in each row.

2. Each row is padded to a 4-byte boundary (i.e. the pixel data for each row begins on a 4-byte bounday - word boundary - on UNIX).

Assuming we have a BMP image that is a 24-bit, RGB (uncompressed), write the code necessary to read the image into memory. Declare whatever structures you require. You may assume the header has already been read in bh. You may also assume the existence of a function called getbyte() that reads and returns the next single 'raw' byte read from the file.
  
   
//open file

//data structures
char image[bh.rows][bh.cols][3]; //3 for RGB values
char rowpadding = 4-((bh.cols *3)%4);

//get to the right location in the file
for( int i=0; i < bh.pixelOffset; i++ )
     getbyte();

//get image information
//read rows
for( int j = bh.rows-1; j>=0; j-- )
{
     //read cols
     for( int k = 0; k < bh.cols; k++ )
     {
         //read Blue value
         image[j][k][0] = getbyte();
         //read Green value
         image[j][k][1] = getbyte();
         //read Red value
         image[j][k][2] = getbyte();
     }

     //read row padding
     for( int l = 0; l < rowpadding; l++ )
     getbyte();
}

//close file
  

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Questions? Please Email: gwen@cpsc.ucalgary.ca
Last modified October 21, 2002