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It’s time to get down and look at some actual working code. Listing 35.1 is a C implementation of Bresenham’s line-drawing algorithm for modes 0EH, 0FH, 10H, and 12H of the VGA, called as function EVGALine. Listing 35.2 is a sample program to demonstrate the use of EVGALine.
LISTING 35.1 L35-1.C
/*
* C implementation of Bresenham’s line drawing algorithm
* for the EGA and VGA. Works in modes 0×E, 0×F, 0×10, and 0×12.
*
* Compiled with Borland C++
*
* By Michael Abrash
*/
#include <dos.h> /* contains MK_FP macro */
#define EVGA_SCREEN_WIDTH_IN_BYTES 80
/* memory offset from start of
one row to start of next */
#define EVGA_SCREEN_SEGMENT 0×A000
/* display memory segment */
#define GC_INDEX 0×3CE
/* Graphics Controller
Index register port */
#define GC_DATA 0×3CF
/* Graphics Controller
Data register port */
#define SET_RESET_INDEX 0 /* indexes of needed */
#define ENABLE_SET_RESET_INDEX 1 /* Graphics Controller */
#define BIT_MASK_INDEX 8 /* registers */
/*
* Draws a dot at (X0,Y0) in whatever color the EGA/VGA hardware is
* set up for. Leaves the bit mask set to whatever value the
* dot required.
*/
void EVGADot(X0, Y0)
unsigned int X0; /* coordinates at which to draw dot, with */
unsigned int Y0; /* (0,0) at the upper left of the screen */
{
unsigned char far *PixelBytePtr;
unsigned char PixelMask;
/* Calculate the offset in the screen segment of the byte in
which the pixel lies */
PixelBytePtr = MK_FP(EVGA_SCREEN_SEGMENT,
( Y0 * EVGA_SCREEN_WIDTH_IN_BYTES ) + ( X0 / 8 ));
/* Generate a mask with a 1 bit in the pixel’s position within the
screen byte */
PixelMask = 0×80 >> ( X0 & 0×07 );
/* Set up the Graphics Controller’s Bit Mask register to allow
only the bit corresponding to the pixel being drawn to
be modified */
outportb(GC_INDEX, BIT_MASK_INDEX);
outportb(GC_DATA, PixelMask);
/* Draw the pixel. Because of the operation of the set/reset
feature of the EGA/VGA, the value written doesn’t matter.
The screen byte is ORed in order to perform a read to latch the
display memory, then perform a write in order to modify it. */
*PixelBytePtr |= 0×FE;
}
/*
* Draws a line in octant 0 or 3 ( |DeltaX| >= DeltaY ).
*/
void Octant0(X0, Y0, DeltaX, DeltaY, XDirection)
unsigned int X0, Y0; /* coordinates of start of the line */
unsigned int DeltaX, DeltaY; /* length of the line (both > 0) */
int XDirection; /* 1 if line is drawn left to right,
-1 if drawn right to left */
{
int DeltaYx2;
int DeltaYx2MinusDeltaXx2;
int ErrorTerm;
/* Set up initial error term and values used inside drawing loop */
DeltaYx2 = DeltaY * 2;
DeltaYx2MinusDeltaXx2 = DeltaYx2 - (int) ( DeltaX * 2 );
ErrorTerm = DeltaYx2 - (int) DeltaX;
/* Draw the line */
EVGADot(X0, Y0); /* draw the first pixel */
while ( DeltaX— ) {
/* See if it’s time to advance the Y coordinate */
if ( ErrorTerm >= 0 ) {
/* Advance the Y coordinate & adjust the error term
back down */
Y0++;
ErrorTerm += DeltaYx2MinusDeltaXx2;
} else {
/* Add to the error term */
ErrorTerm += DeltaYx2;
}
X0 += XDirection; /* advance the X coordinate */
EVGADot(X0, Y0); /* draw a pixel */
}
}
/*
* Draws a line in octant 1 or 2 ( |DeltaX| < DeltaY ).
*/
void Octant1(X0, Y0, DeltaX, DeltaY, XDirection)
unsigned int X0, Y0; /* coordinates of start of the line */
unsigned int DeltaX, DeltaY; /* length of the line (both > 0) */
int XDirection; /* 1 if line is drawn left to right,
-1 if drawn right to left */
{
int DeltaXx2;
int DeltaXx2MinusDeltaYx2;
int ErrorTerm;
/* Set up initial error term and values used inside drawing loop */
DeltaXx2 = DeltaX * 2;
DeltaXx2MinusDeltaYx2 = DeltaXx2 - (int) ( DeltaY * 2 );
ErrorTerm = DeltaXx2 - (int) DeltaY;
EVGADot(X0, Y0); /* draw the first pixel */
while ( DeltaY— ) {
/* See if it’s time to advance the X coordinate */
if ( ErrorTerm >= 0 ) {
/* Advance the X coordinate & adjust the error term
back down */
X0 += XDirection;
ErrorTerm += DeltaXx2MinusDeltaYx2;
} else {
/* Add to the error term */
ErrorTerm += DeltaXx2;
}
Y0++; /* advance the Y coordinate */
EVGADot(X0, Y0); /* draw a pixel */
}
}
/*
* Draws a line on the EGA or VGA.
*/
void EVGALine(X0, Y0, X1, Y1, Color)
int X0, Y0; /* coordinates of one end of the line */
int X1, Y1; /* coordinates of the other end of the line */
char Color; /* color to draw line in */
{
int DeltaX, DeltaY;
int Temp;
/* Set the drawing color */
/* Put the drawing color in the Set/Reset register */
outportb(GC_INDEX, SET_RESET_INDEX);
outportb(GC_DATA, Color);
/* Cause all planes to be forced to the Set/Reset color */
outportb(GC_INDEX, ENABLE_SET_RESET_INDEX);
outportb(GC_DATA, 0×F);
/* Save half the line-drawing cases by swapping Y0 with Y1
and X0 with X1 if Y0 is greater than Y1. As a result, DeltaY
is always > 0, and only the octant 0-3 cases need to be
handled. */
if ( Y0 > Y1 ) {
Temp = Y0;
Y0 = Y1;
Y1 = Temp;
Temp = X0;
X0 = X1;
X1 = Temp;
}
/* Handle as four separate cases, for the four octants in which
Y1 is greater than Y0 */
DeltaX = X1 - X0; /* calculate the length of the line
in each coordinate */
DeltaY = Y1 - Y0;
if ( DeltaX > 0 ) {
if ( DeltaX > DeltaY ) {
Octant0(X0, Y0, DeltaX, DeltaY, 1);
} else {
Octant1(X0, Y0, DeltaX, DeltaY, 1);
}
} else {
DeltaX = -DeltaX; /* absolute value of DeltaX */
if ( DeltaX > DeltaY ) {
Octant0(X0, Y0, DeltaX, DeltaY, -1);
} else {
Octant1(X0, Y0, DeltaX, DeltaY, -1);
}
}
/* Return the state of the EGA/VGA to normal */
outportb(GC_INDEX, ENABLE_SET_RESET_INDEX);
outportb(GC_DATA, 0);
outportb(GC_INDEX, BIT_MASK_INDEX);
outportb(GC_DATA, 0×FF);
}
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