143 lines
6 KiB
Markdown
143 lines
6 KiB
Markdown
**LISTING 42.4 L42-4.C**
|
|
|
|
/* Function to draw a non-antialiased line from (X0,Y0) to (X1,Y1), using a
|
|
* simple fixed-point error accumulation approach.
|
|
* Tested with Borland C++ in C compilation mode and the small model.
|
|
*/
|
|
extern void DrawPixel(int, int, int);
|
|
|
|
/* Non-antialiased line drawer.
|
|
* (X0,Y0),(X1,Y1) = line to draw, Color = color in which to draw
|
|
*/
|
|
void DrawLine(int X0, int Y0, int X1, int Y1, int Color)
|
|
{
|
|
unsigned long ErrorAcc, ErrorAdj;
|
|
int DeltaX, DeltaY, XDir, Temp;
|
|
|
|
/* Make sure the line runs top to bottom */
|
|
if (Y0 > Y1) {
|
|
Temp = Y0; Y0 = Y1; Y1 = Temp;
|
|
Temp = X0; X0 = X1; X1 = Temp;
|
|
}
|
|
DrawPixel(X0, Y0, Color); /* draw the initial pixel */
|
|
if ((DeltaX = X1 - X0) >= 0) {
|
|
XDir = 1;
|
|
} else {
|
|
XDir = -1;
|
|
DeltaX = -DeltaX; /* make DeltaX positive */
|
|
}
|
|
if ((DeltaY = Y1 - Y0) == 0) /* done if only one point in the line */
|
|
if (DeltaX == 0) return;
|
|
|
|
ErrorAcc = 0x8000; /* initialize line error accumulator to .5, so we can
|
|
advance when we get halfway to the next pixel */
|
|
/* Is this an X-major or Y-major line? */
|
|
if (DeltaY > DeltaX) {
|
|
/* Y-major line; calculate 16-bit fixed-point fractional part of a
|
|
pixel that X advances each time Y advances 1 pixel */
|
|
ErrorAdj = ((((unsigned long)DeltaX << 17) / (unsigned long)DeltaY) +
|
|
1) >> 1;
|
|
/* Draw all pixels between the first and last */
|
|
do {
|
|
ErrorAcc += ErrorAdj; /* calculate error for this pixel */
|
|
if (ErrorAcc & ~0xFFFFL) {
|
|
/* The error accumulator turned over, so advance the X coord */
|
|
X0 += XDir;
|
|
ErrorAcc &= 0xFFFFL; /* clear integer part of result */
|
|
}
|
|
Y0++; /* Y-major, so always advance Y */
|
|
DrawPixel(X0, Y0, Color);
|
|
} while (--DeltaY);
|
|
return;
|
|
}
|
|
/* It's an X-major line; calculate 16-bit fixed-point fractional part of a
|
|
pixel that Y advances each time X advances 1 pixel */
|
|
ErrorAdj = ((((unsigned long)DeltaY << 17) / (unsigned long)DeltaX) +
|
|
1) >> 1;
|
|
/* Draw all remaining pixels */
|
|
do {
|
|
ErrorAcc += ErrorAdj; /* calculate error for this pixel */
|
|
if (ErrorAcc & ~0xFFFFL) {
|
|
/* The error accumulator turned over, so advance the Y coord */
|
|
Y0++;
|
|
ErrorAcc &= 0xFFFFL; /* clear integer part of result */
|
|
}
|
|
X0 += XDir; /* X-major, so always advance X */
|
|
DrawPixel(X0, Y0, Color);
|
|
} while (--DeltaX);
|
|
}
|
|
|
|
Listing 42.1 isn't particularly fast, because it calls **DrawPixel()**
|
|
for each pixel. On the other hand, **DrawPixel()** makes it easy to try
|
|
out Wu antialiasing in a variety of modes; just adapt the code in
|
|
Listing 42.3 for the 256-color mode you want to support. For example,
|
|
Listing 42.5 shows code to draw Wu-antialiased lines in 640x480
|
|
256-color mode on SuperVGAs built around the Tseng Labs ET4000 chip with
|
|
at least 512K of display memory installed. It's well worth checking out
|
|
Wu antialiasing at 640x480. Although antialiased lines look much
|
|
smoother than normal lines at 320x200 resolution, they're far from
|
|
perfect, because the pixels are so big that the eye can't blend them
|
|
properly. At 640x480, however, Wu-antialiased lines look fabulous; from
|
|
a couple of feet away, they look as straight and smooth as if they were
|
|
drawn with a ruler.
|
|
|
|
**LISTING 42.5 L42-5.C**
|
|
|
|
/* Mode set and pixel-drawing functions for the 640x480 256-color mode of
|
|
* Tseng Labs ET4000-based SuperVGAs.
|
|
* Tested with Borland C++ in C compilation mode and the small model.
|
|
*/
|
|
#include <dos.h>
|
|
|
|
/* Screen dimension globals, used in main program to scale */
|
|
int ScreenWidthInPixels = 640;
|
|
int ScreenHeightInPixels = 480;
|
|
|
|
/* ET4000 640x480 256-color draw pixel function. */
|
|
void DrawPixel(int X, int Y, int Color)
|
|
{
|
|
#define SCREEN_SEGMENT 0xA000
|
|
#define GC_SEGMENT_SELECT 0x3CD /* ET4000 segment (bank) select reg */
|
|
unsigned char far *ScreenPtr;
|
|
unsigned int Bank;
|
|
unsigned long BitmapAddress;
|
|
|
|
/* full bitmap address of pixel, as measured from address 0 to 0xFFFFF */
|
|
BitmapAddress = (unsigned long) Y * ScreenWidthInPixels + X;
|
|
/* Bank # is upper word of bitmap addr */
|
|
Bank = BitmapAddress >> 16;
|
|
/* Upper nibble is read bank #, lower nibble is write bank # */
|
|
outp(GC_SEGMENT_SELECT, (Bank << 4) | Bank);
|
|
/* Draw into the bank */
|
|
FP_SEG(ScreenPtr) = SCREEN_SEGMENT;
|
|
FP_OFF(ScreenPtr) = (unsigned int) BitmapAddress;
|
|
*ScreenPtr = Color;
|
|
}
|
|
|
|
/* ET4000 640x480 256-color mode-set function. */
|
|
void SetMode()
|
|
{
|
|
union REGS regset;
|
|
|
|
/* Set to 640x480 256-color graphics mode */
|
|
regset.x.ax = 0x002E;
|
|
int86(0x10, ®set, ®set);
|
|
}
|
|
|
|
Listing 42.1 requires that the DAC palette be set up so that a
|
|
**NumLevel**-long block of palette entries contains linearly decreasing
|
|
intensities of the drawing color. The size of the block is programmable,
|
|
but must be a power of two. The more intensity levels, the better. Wu
|
|
says that 32 intensities are enough; on my system, eight and even four
|
|
levels looked pretty good. I found that gamma correction, which gives
|
|
linearly spaced intensity steps, improved antialiasing quality
|
|
significantly. Fortunately, we can program the palette with
|
|
gamma-corrected values, so our drawing code doesn't have to do any extra
|
|
work.
|
|
|
|
Listing 42.1 isn't very fast, so I implemented Wu antialiasing in
|
|
assembly, hard-coded for mode 13H. The implementation is shown in full
|
|
in Listing 42.6. High-speed graphics code and fast VGAs go together like
|
|
peanut butter and jelly, which is to say very well indeed; the assembly
|
|
implementation ran more than twice as fast as the C code on my 486.
|
|
Enough said!
|