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<HTML>
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<META name=vsisbn content="1576101746">
<META name=vstitle content="Michael Abrash's Graphics Programming Black Book, Special Edition">
<META name=vsauthor content="Michael Abrash">
<META name=vspublisher content="The Coriolis Group">
<META name=vspubdate content="07/01/97">
<META name=vscategory content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development">
<TITLE>Michael Abrash's Graphics Programming Black Book Special Edition: Wu'ed in Haste; Fried, Stewed at Leisure</TITLE>
<!-- HEADER -->
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<!--ISBN=1576101746//-->
<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
<!--AUTHOR=Michael Abrash//-->
<!--PUBLISHER=The Coriolis Group, Inc.//-->
<!--CHAPTER=42//-->
<!--PAGES=784-787//-->
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<TD><A HREF="42-03.html">Previous</A></TD>
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<P><BR></P>
<P><B>LISTING 42.4 L42-4.C</B></P>
<!-- CODE //-->
<PRE>
/* 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&#43;&#43; 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 &gt; 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) &gt;= 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 &gt; 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 &lt;&lt; 17) / (unsigned long)DeltaY) &#43;
1) &gt;&gt; 1;
/* Draw all pixels between the first and last */
do {
ErrorAcc &#43;= ErrorAdj; /* calculate error for this pixel */
if (ErrorAcc &amp ~0xFFFFL) {
/* The error accumulator turned over, so advance the X coord */
X0 &#43;= XDir;
ErrorAcc &amp= 0xFFFFL; /* clear integer part of result */
}
Y0&#43;&#43;; /* 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 &lt;&lt; 17) / (unsigned long)DeltaX) &#43;
1) &gt;&gt; 1;
/* Draw all remaining pixels */
do {
ErrorAcc &#43;= ErrorAdj; /* calculate error for this pixel */
if (ErrorAcc &amp ~0xFFFFL) {
/* The error accumulator turned over, so advance the Y coord */
Y0&#43;&#43;;
ErrorAcc &amp= 0xFFFFL; /* clear integer part of result */
}
X0 &#43;= XDir; /* X-major, so always advance X */
DrawPixel(X0, Y0, Color);
} while (--DeltaX);
}
</PRE>
<!-- END CODE //-->
<P>Listing 42.1 isn&#146;t particularly fast, because it calls <B>DrawPixel()</B> for each pixel. On the other hand, <B>DrawPixel()</B> 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 640&#215;480 256-color mode on SuperVGAs built around the Tseng Labs ET4000 chip with at least 512K of display memory installed. It&#146;s well worth checking out Wu antialiasing at 640&#215;480. Although antialiased lines look much smoother than normal lines at 320&#215;200 resolution, they&#146;re far from perfect, because the pixels are so big that the eye can&#146;t blend them properly. At 640&#215;480, 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.</P>
<P><B>LISTING 42.5 L42-5.C</B></P>
<!-- CODE //-->
<PRE>
/* Mode set and pixel-drawing functions for the 640x480 256-color mode of
* Tseng Labs ET4000-based SuperVGAs.
* Tested with Borland C&#43;&#43; in C compilation mode and the small model.
*/
#include &lt;dos.h&gt;
/* 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 &#43; X;
/* Bank # is upper word of bitmap addr */
Bank = BitmapAddress &gt;&gt; 16;
/* Upper nibble is read bank #, lower nibble is write bank # */
outp(GC_SEGMENT_SELECT, (Bank &lt;&lt; 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, &ampregset, &ampregset);
}
</PRE>
<!-- END CODE //-->
<P>Listing 42.1 requires that the DAC palette be set up so that a <B>NumLevel</B>-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&#146;t have to do any extra work.</P>
<P>Listing 42.1 isn&#146;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!</P><P><BR></P>
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
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