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James Gregory 2013-12-30 18:34:31 +11:00
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35-03.md
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@ -4,15 +4,15 @@
### An Implementation in C {#Heading6}
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
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
* C implementation of Bresenham's line drawing algorithm
* for the EGA and VGA. Works in modes 0xE, 0xF, 0x10, and 0x12.
*
* Compiled with Borland C++
@ -54,18 +54,18 @@ Listing 35.2 is a sample program to demonstrate the use of **EVGALine**.
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
/* Generate a mask with a 1 bit in the pixel's position within the
screen byte */
PixelMask = 0x80 >> ( X0 & 0x07 );
/* Set up the Graphics Controller’s Bit Mask register to allow
/* 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.
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 |= 0xFE;
@ -92,7 +92,7 @@ Listing 35.2 is a sample program to demonstrate the use of **EVGALine**.
/* Draw the line */
EVGADot(X0, Y0); /* draw the first pixel */
while ( DeltaX— ) {
/* See if it’s time to advance the Y coordinate */
/* See if it's time to advance the Y coordinate */
if ( ErrorTerm >= 0 ) {
/* Advance the Y coordinate & adjust the error term
back down */
@ -127,7 +127,7 @@ Listing 35.2 is a sample program to demonstrate the use of **EVGALine**.
EVGADot(X0, Y0); /* draw the first pixel */
while ( DeltaY— ) {
/* See if it’s time to advance the X coordinate */
/* See if it's time to advance the X coordinate */
if ( ErrorTerm >= 0 ) {
/* Advance the X coordinate & adjust the error term
back down */