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Better line drawing function (#98) 

https://github.com/diasurgical/devilution/pull/1125#issuecomment-489474723
pull/658/head
Anders Jenbo 6 years ago committed by GitHub
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1 changed files with 13 additions and 202 deletions
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  1. 215
      Source/engine.cpp

215
Source/engine.cpp
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@ -632,25 +632,7 @@ void engine_draw_pixel(int sx, int sy)
*dst = gbPixelCol;
}
#define GG_SWAP(A, B) \
{ \
(A) ^= (B); \
(B) ^= (A); \
(A) ^= (B); \
}
#define GG_ABSOLUTE(I, J, K) (((I) - (J)) * ((K) = (((I) - (J)) < 0 ? -1 : 1)))
/**
* Symmetric Double Step Line Algorithm
* by Brian Wyvill
* from "Graphics Gems", Academic Press, 1990
*
* Exact copy from https://github.com/erich666/GraphicsGems/blob/dad26f941e12c8bf1f96ea21c1c04cd2206ae7c9/gems/DoubleLine.c
* Except:
* - not in view checks
* - global variable instead of reverse flag
* - condition for pixels_left < 0 removed
*
* @brief Draw a line on the back buffer
* @param x0 Back buffer coordinate
* @param y0 Back buffer coordinate
@ -660,190 +642,19 @@ void engine_draw_pixel(int sx, int sy)
*/
void DrawLine(int x0, int y0, int x1, int y1, BYTE col)
{
int dx, dy, incr1, incr2, D, x, y, xend, c, pixels_left;
int sign_x, sign_y, step, i;
int x1_, y1_;
gbPixelCol = col;
gbNotInView = FALSE;
if (x0 < 0 + SCREEN_X || x0 >= SCREEN_WIDTH + SCREEN_X) {
gbNotInView = TRUE;
}
if (x1 < 0 + SCREEN_X || x1 >= SCREEN_WIDTH + SCREEN_X) {
gbNotInView = TRUE;
}
if (y0 < 0 + SCREEN_Y || y0 >= PANEL_Y) {
gbNotInView = TRUE;
}
if (y1 < 0 + SCREEN_Y || y1 >= PANEL_Y) {
gbNotInView = TRUE;
}
dx = GG_ABSOLUTE(x1, x0, sign_x);
dy = GG_ABSOLUTE(y1, y0, sign_y);
/* decide increment sign by the slope sign */
if (sign_x == sign_y)
step = 1;
else
step = -1;
if (dy > dx) { /* chooses axis of greatest movement (make
* dx) */
GG_SWAP(x0, y0);
GG_SWAP(x1, y1);
GG_SWAP(dx, dy);
gbRotateMap = TRUE;
} else
gbRotateMap = FALSE;
/* note error check for dx==0 should be included here */
if (x0 > x1) { /* start from the smaller coordinate */
x = x1;
y = y1;
x1_ = x0;
y1_ = y0;
} else {
x = x0;
y = y0;
x1_ = x1;
y1_ = y1;
}
/* Note dx=n implies 0 - n or (dx+1) pixels to be set */
/* Go round loop dx/4 times then plot last 0,1,2 or 3 pixels */
/* In fact (dx-1)/4 as 2 pixels are already plotted */
xend = (dx - 1) / 4;
pixels_left = (dx - 1) % 4; /* number of pixels left over at the end */
engine_draw_pixel(x, y);
engine_draw_pixel(x1_, y1_); /* plot first two points */
incr2 = 4 * dy - 2 * dx;
if (incr2 < 0) { /* slope less than 1/2 */
c = 2 * dy;
incr1 = 2 * c;
D = incr1 - dx;
for (i = 0; i < xend; i++) { /* plotting loop */
++x;
--x1_;
if (D < 0) {
/* pattern 1 forwards */
engine_draw_pixel(x, y);
engine_draw_pixel(++x, y);
/* pattern 1 backwards */
engine_draw_pixel(x1_, y1_);
engine_draw_pixel(--x1_, y1_);
D += incr1;
} else {
if (D < c) {
/* pattern 2 forwards */
engine_draw_pixel(x, y);
engine_draw_pixel(++x, y += step);
/* pattern 2 backwards */
engine_draw_pixel(x1_, y1_);
engine_draw_pixel(--x1_, y1_ -= step);
} else {
/* pattern 3 forwards */
engine_draw_pixel(x, y += step);
engine_draw_pixel(++x, y);
/* pattern 3 backwards */
engine_draw_pixel(x1_, y1_ -= step);
engine_draw_pixel(--x1_, y1_);
}
D += incr2;
}
} /* end for */
/* plot last pattern */
if (pixels_left) {
if (D < 0) {
engine_draw_pixel(++x, y); /* pattern 1 */
if (pixels_left > 1)
engine_draw_pixel(++x, y);
if (pixels_left > 2)
engine_draw_pixel(--x1_, y1_);
} else {
if (D < c) {
engine_draw_pixel(++x, y); /* pattern 2 */
if (pixels_left > 1)
engine_draw_pixel(++x, y += step);
if (pixels_left > 2)
engine_draw_pixel(--x1_, y1_);
} else {
/* pattern 3 */
engine_draw_pixel(++x, y += step);
if (pixels_left > 1)
engine_draw_pixel(++x, y);
if (pixels_left > 2)
engine_draw_pixel(--x1_, y1_ -= step);
}
}
} /* end if pixels_left */
}
/* end slope < 1/2 */
else { /* slope greater than 1/2 */
c = 2 * (dy - dx);
incr1 = 2 * c;
D = incr1 + dx;
for (i = 0; i < xend; i++) {
++x;
--x1_;
if (D > 0) {
/* pattern 4 forwards */
engine_draw_pixel(x, y += step);
engine_draw_pixel(++x, y += step);
/* pattern 4 backwards */
engine_draw_pixel(x1_, y1_ -= step);
engine_draw_pixel(--x1_, y1_ -= step);
D += incr1;
} else {
if (D < c) {
/* pattern 2 forwards */
engine_draw_pixel(x, y);
engine_draw_pixel(++x, y += step);
/* pattern 2 backwards */
engine_draw_pixel(x1_, y1_);
engine_draw_pixel(--x1_, y1_ -= step);
} else {
/* pattern 3 forwards */
engine_draw_pixel(x, y += step);
engine_draw_pixel(++x, y);
/* pattern 3 backwards */
engine_draw_pixel(x1_, y1_ -= step);
engine_draw_pixel(--x1_, y1_);
}
D += incr2;
}
} /* end for */
/* plot last pattern */
if (pixels_left) {
if (D > 0) {
engine_draw_pixel(++x, y += step); /* pattern 4 */
if (pixels_left > 1)
engine_draw_pixel(++x, y += step);
if (pixels_left > 2)
engine_draw_pixel(--x1_, y1_ -= step);
} else {
if (D < c) {
engine_draw_pixel(++x, y); /* pattern 2 */
if (pixels_left > 1)
engine_draw_pixel(++x, y += step);
if (pixels_left > 2)
engine_draw_pixel(--x1_, y1_);
} else {
/* pattern 3 */
engine_draw_pixel(++x, y += step);
if (pixels_left > 1)
engine_draw_pixel(++x, y);
if (pixels_left > 2) {
if (D > c) /* step 3 */
engine_draw_pixel(--x1_, y1_ -= step);
else /* step 2 */
engine_draw_pixel(--x1_, y1_);
}
}
}
}
int i, dx, dy, steps;
float ix, iy, sx, sy;
dx = x1 - x0;
dy = y1 - y0;
steps = abs(dx) > abs(dy) ? abs(dx) : abs(dy);
ix = dx / (float)steps;
iy = dy / (float)steps;
sx = x0;
sy = y0;
for(i = 0; i <= steps; i++, sx += ix, sy += iy) {
ENG_set_pixel(sx, sy, col);
}
}

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