Beginning of the program

The program begins in Listing 1.

If there is anything in Listing 1 that I haven't explained before, the comments should be self-explanatory.

Listing 1. Beginning of the program named ImageNegate03. 
#include <allegro.h>
#include <stdio.h>//required to call sprintf

int main(){
  int pixel = 0;//temporary storage for a pixel

  //Temporary storage for red, green, and blue
  int red = 0;
  int green = 0;
  int blue = 0;
  
  int width = 0;//image width
  int height = 0;//image height
  int windowWidth = 0;//must be a multiple of 4 && >= 108
  int windowHeight = 0;
  
  //The addresses of the following variables are passed
  // in the call to get_clip_rect, where they are
  // populated with the coordinates of the diagonal
  // corners of the rectangular image.
  int upperLeftX = 0;
  int upperLeftY = 0;
  int lowerRightX = 0;
  int lowerRightY = 0;

  //Typical Allegro setup.
  allegro_init();
  install_keyboard();
  set_color_depth(32);

  //Declare a pointer variable capable of pointing to a
  // bitmap.
  BITMAP *picA = NULL;
  //Load an image file from the current directory.
  picA = load_bitmap("starfish.pcx", NULL);