155 lines
6.5 KiB
Markdown
155 lines
6.5 KiB
Markdown
------------------------ --------------------------------- --------------------
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[Previous](42-03.html) [Table of Contents](index.html) [Next](42-05.html)
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------------------------ --------------------------------- --------------------
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**LISTING 42.4 L42-4.C**
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/* Function to draw a non-antialiased line from (X0,Y0) to (X1,Y1), using a
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* simple fixed-point error accumulation approach.
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* Tested with Borland C++ in C compilation mode and the small model.
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*/
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extern void DrawPixel(int, int, int);
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/* Non-antialiased line drawer.
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* (X0,Y0),(X1,Y1) = line to draw, Color = color in which to draw
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*/
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void DrawLine(int X0, int Y0, int X1, int Y1, int Color)
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{
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unsigned long ErrorAcc, ErrorAdj;
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int DeltaX, DeltaY, XDir, Temp;
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/* Make sure the line runs top to bottom */
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if (Y0 > Y1) {
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Temp = Y0; Y0 = Y1; Y1 = Temp;
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Temp = X0; X0 = X1; X1 = Temp;
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}
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DrawPixel(X0, Y0, Color); /* draw the initial pixel */
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if ((DeltaX = X1 - X0) >= 0) {
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XDir = 1;
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} else {
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XDir = -1;
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DeltaX = -DeltaX; /* make DeltaX positive */
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}
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if ((DeltaY = Y1 - Y0) == 0) /* done if only one point in the line */
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if (DeltaX == 0) return;
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ErrorAcc = 0x8000; /* initialize line error accumulator to .5, so we can
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advance when we get halfway to the next pixel */
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/* Is this an X-major or Y-major line? */
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if (DeltaY > DeltaX) {
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/* Y-major line; calculate 16-bit fixed-point fractional part of a
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pixel that X advances each time Y advances 1 pixel */
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ErrorAdj = ((((unsigned long)DeltaX << 17) / (unsigned long)DeltaY) +
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1) >> 1;
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/* Draw all pixels between the first and last */
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do {
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ErrorAcc += ErrorAdj; /* calculate error for this pixel */
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if (ErrorAcc & ~0xFFFFL) {
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/* The error accumulator turned over, so advance the X coord */
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X0 += XDir;
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ErrorAcc &= 0xFFFFL; /* clear integer part of result */
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}
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Y0++; /* Y-major, so always advance Y */
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DrawPixel(X0, Y0, Color);
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} while (--DeltaY);
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return;
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}
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/* It's an X-major line; calculate 16-bit fixed-point fractional part of a
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pixel that Y advances each time X advances 1 pixel */
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ErrorAdj = ((((unsigned long)DeltaY << 17) / (unsigned long)DeltaX) +
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1) >> 1;
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/* Draw all remaining pixels */
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do {
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ErrorAcc += ErrorAdj; /* calculate error for this pixel */
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if (ErrorAcc & ~0xFFFFL) {
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/* The error accumulator turned over, so advance the Y coord */
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Y0++;
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ErrorAcc &= 0xFFFFL; /* clear integer part of result */
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}
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X0 += XDir; /* X-major, so always advance X */
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DrawPixel(X0, Y0, Color);
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} while (--DeltaX);
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}
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Listing 42.1 isn’t particularly fast, because it calls **DrawPixel()**
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for each pixel. On the other hand, **DrawPixel()** makes it easy to try
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out Wu antialiasing in a variety of modes; just adapt the code in
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Listing 42.3 for the 256-color mode you want to support. For example,
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Listing 42.5 shows code to draw Wu-antialiased lines in 640x480
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256-color mode on SuperVGAs built around the Tseng Labs ET4000 chip with
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at least 512K of display memory installed. It’s well worth checking out
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Wu antialiasing at 640x480. Although antialiased lines look much
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smoother than normal lines at 320x200 resolution, they’re far from
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perfect, because the pixels are so big that the eye can’t blend them
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properly. At 640x480, however, Wu-antialiased lines look fabulous; from
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a couple of feet away, they look as straight and smooth as if they were
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drawn with a ruler.
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**LISTING 42.5 L42-5.C**
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/* Mode set and pixel-drawing functions for the 640x480 256-color mode of
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* Tseng Labs ET4000-based SuperVGAs.
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* Tested with Borland C++ in C compilation mode and the small model.
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*/
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#include <dos.h>
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/* Screen dimension globals, used in main program to scale */
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int ScreenWidthInPixels = 640;
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int ScreenHeightInPixels = 480;
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/* ET4000 640x480 256-color draw pixel function. */
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void DrawPixel(int X, int Y, int Color)
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{
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#define SCREEN_SEGMENT 0xA000
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#define GC_SEGMENT_SELECT 0x3CD /* ET4000 segment (bank) select reg */
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unsigned char far *ScreenPtr;
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unsigned int Bank;
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unsigned long BitmapAddress;
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/* full bitmap address of pixel, as measured from address 0 to 0xFFFFF */
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BitmapAddress = (unsigned long) Y * ScreenWidthInPixels + X;
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/* Bank # is upper word of bitmap addr */
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Bank = BitmapAddress >> 16;
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/* Upper nibble is read bank #, lower nibble is write bank # */
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outp(GC_SEGMENT_SELECT, (Bank << 4) | Bank);
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/* Draw into the bank */
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FP_SEG(ScreenPtr) = SCREEN_SEGMENT;
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FP_OFF(ScreenPtr) = (unsigned int) BitmapAddress;
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*ScreenPtr = Color;
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}
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/* ET4000 640x480 256-color mode-set function. */
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void SetMode()
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{
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union REGS regset;
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/* Set to 640x480 256-color graphics mode */
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regset.x.ax = 0x002E;
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int86(0x10, ®set, ®set);
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}
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Listing 42.1 requires that the DAC palette be set up so that a
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**NumLevel**-long block of palette entries contains linearly decreasing
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intensities of the drawing color. The size of the block is programmable,
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but must be a power of two. The more intensity levels, the better. Wu
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says that 32 intensities are enough; on my system, eight and even four
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levels looked pretty good. I found that gamma correction, which gives
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linearly spaced intensity steps, improved antialiasing quality
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significantly. Fortunately, we can program the palette with
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gamma-corrected values, so our drawing code doesn’t have to do any extra
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work.
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Listing 42.1 isn’t very fast, so I implemented Wu antialiasing in
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assembly, hard-coded for mode 13H. The implementation is shown in full
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in Listing 42.6. High-speed graphics code and fast VGAs go together like
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peanut butter and jelly, which is to say very well indeed; the assembly
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implementation ran more than twice as fast as the C code on my 486.
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Enough said!
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------------------------ --------------------------------- --------------------
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[Previous](42-03.html) [Table of Contents](index.html) [Next](42-05.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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