My new debugging technique is unstoppable.
:: Code :: CBitmap ::

Using bitmaps in C (really basic)

I don't like C, but sometimes I have to use it, and this was one of those times. Here's a quick explanation on how to read a 24 bit bitmap in C:

The code is here](bitmap.c), it takes a bitmap and prints it out as ASCII, like the picture at the right.

Header

Basically you want to open the file as binary for reading: `C fp = fopen(bmpFile,"rb")`c and find the useful values in the header. More information about the header can be found [here](http://www.fortunecity.com/skyscraper/windows/364/bmpffrmt.html).

Here's how I read through the header, somewhat awkwardly putting the bits I don't care about into a char* called junk: 

  1. word filesize, width, height, bitsperpixel;
  2. fread(&filesize, 4, 1, fp);
  3. fread(junk,12,1,fp); //ignore some of the file
  4. fread(&width, 4, 1, fp);
  5. fread(&height, 4, 1, fp);
  6. fread(junk,2,1,fp); //ignore some of the file
  7. fread(&bitsperpixel, 2, 1, fp);
  8. fread(junk, 24, 1, fp);//ignore some of the file

Problems

The bitmap format has each row padded out with zeros to be a multiple of 4. As in: `C int padding = (4-((3*width) % 4))%4; `c, which I addded to the loop that reads the rows.

Also the rows are upside down. Which is annoying.

I only print out the bitmap, I assume you that if you need it, you know how to use `Cmalloc`c to proceed to actually do something with the picture. bitmap1.zip is a zip containing the source, some test BMPs and a compiled EXE.

Code

  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <errno.h>
  4.  
  5. typedef unsigned char  byte;
  6. typedef unsigned short word;
  7. typedef unsigned long  dword;
  8.  
  9. int main(int argc, char *argv[]) {
  10.   FILE *fp;
  11.   char *bmpFile;
  12.   char junk[255];
  13.  
  14.   if(argc==2) {
  15.     bmpFile = argv[1];
  16.   } else {
  17.     bmpFile = "t.bmp";
  18.   }
  19.  
  20.   if ((fp = fopen(bmpFile,"rb")) == NULL) {
  21.     printf("Can't find file: %s.\n",bmpFile);
  22.     exit(1);
  23.   }
  24.  
  25.   /* minor error handling */
  26.   if (fgetc(fp)!='B' || fgetc(fp)!='M') {
  27.     fclose(fp);
  28.     printf("Invlaid file: %s.\n",bmpFile);
  29.     exit(1);
  30.   }
  31.  
  32.   word filesize, width, height, bitsperpixel;
  33.   word r,g,b,pixel;
  34.   fread(&filesize, 4, 1, fp);
  35.   fread(junk,12,1,fp); //ignore some of the file
  36.   fread(&width, 4, 1, fp);
  37.   fread(&height, 4, 1, fp);
  38.   printf("Size: %d,[%dx%d]\n",filesize,width,height);
  39.   fread(junk,2,1,fp); //ignore some of the file
  40.   fread(&bitsperpixel, 2, 1, fp);
  41.   if (bitsperpixel<24) {
  42.     fclose(fp);
  43.     printf("This is designed for 24 bit bitmaps, %s has %d.\n",bmpFile,bitsperpixel);
  44.     exit(1);
  45.   }
  46.     fread(junk, 24, 1, fp);
  47.  
  48.   int x,y;
  49.     //the format pretends all rows are divisible by 4
  50.   int padding = (4-((3*width) % 4))%4;
  51.  
  52.   //the image is stored upside down:
  53.   for(y=0;y<height;y++) {
  54.     for(x=0;x<width;x++) {
  55.       r=b=g=0;
  56.       fread(&b, 1, 1, fp);
  57.       fread(&g, 1, 1, fp);
  58.       fread(&r, 1, 1, fp);
  59.       pixel = g;
  60.       if(pixel>200) printf("@");
  61.       else if(pixel>150) printf("X");
  62.       else if(pixel>100) printf("+");
  63.       else if(pixel>50) printf(":");
  64.       else printf(" ");
  65.  
  66.     }
  67.       fread(junk, padding,1,fp);
  68.     printf("\n");
  69.   }
  70.  
  71. }

Programming Math etc

What I've said lately

Loading feeds

More of me on the web