code: drawterm

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```#include <u.h>
#include <libc.h>
#include <draw.h>
#include <memdraw.h>
#include <memlayer.h>

/*
* ellipse(dst, c, a, b, t, src, sp)
*   draws an ellipse centered at c with semiaxes a,b>=0
*   and semithickness t>=0, or filled if t<0.  point sp
*   in src maps to c in dst
*
*   very thick skinny ellipses are brushed with circles (slow)
*   others are approximated by filling between 2 ellipses
*   criterion for very thick when b<a: t/b > 0.5*x/(1-x)
*   where x = b/a
*/

typedef struct Param	Param;
typedef struct State	State;

static	void	bellipse(int, State*, Param*);
static	void	erect(int, int, int, int, Param*);
static	void	eline(int, int, int, int, Param*);

struct Param {
Memimage	*dst;
Memimage	*src;
Point			c;
int			t;
Point			sp;
Memimage	*disc;
int			op;
};

/*
* denote residual error by e(x,y) = b^2*x^2 + a^2*y^2 - a^2*b^2
* e(x,y) = 0 on ellipse, e(x,y) < 0 inside, e(x,y) > 0 outside
*/

struct State {
int	a;
int	x;
vlong	a2;	/* a^2 */
vlong	b2;	/* b^2 */
vlong	b2x;	/* b^2 * x */
vlong	a2y;	/* a^2 * y */
vlong	c1;
vlong	c2;	/* test criteria */
vlong	ee;	/* ee = e(x+1/2,y-1/2) - (a^2+b^2)/4 */
vlong	dxe;
vlong	dye;
vlong	d2xe;
vlong	d2ye;
};

static
State*
newstate(State *s, int a, int b)
{
s->x = 0;
s->a = a;
s->a2 = (vlong)(a*a);
s->b2 = (vlong)(b*b);
s->b2x = (vlong)0;
s->a2y = s->a2*(vlong)b;
s->c1 = -((s->a2>>2) + (vlong)(a&1) + s->b2);
s->c2 = -((s->b2>>2) + (vlong)(b&1));
s->ee = -s->a2y;
s->dxe = (vlong)0;
s->dye = s->ee<<1;
s->d2xe = s->b2<<1;
s->d2ye = s->a2<<1;
return s;
}

/*
* return x coord of rightmost pixel on next scan line
*/
static
int
step(State *s)
{
while(s->x < s->a) {
if(s->ee+s->b2x <= s->c1 ||	/* e(x+1,y-1/2) <= 0 */
s->ee+s->a2y <= s->c2) {	/* e(x+1/2,y) <= 0 (rare) */
s->dxe += s->d2xe;
s->ee += s->dxe;
s->b2x += s->b2;
s->x++;
continue;
}
s->dye += s->d2ye;
s->ee += s->dye;
s->a2y -= s->a2;
if(s->ee-s->a2y <= s->c2) {	/* e(x+1/2,y-1) <= 0 */
s->dxe += s->d2xe;
s->ee += s->dxe;
s->b2x += s->b2;
return s->x++;
}
break;
}
return s->x;
}

void
memellipse(Memimage *dst, Point c, int a, int b, int t, Memimage *src, Point sp, int op)
{
State in, out;
int y, inb, inx, outx, u;
Param p;

if(a < 0)
a = -a;
if(b < 0)
b = -b;
p.dst = dst;
p.src = src;
p.c = c;
p.t = t;
p.sp = subpt(sp, c);
p.disc = nil;
p.op = op;

u = (t<<1)*(a-b);
if(b<a && u>b*b || a<b && -u>a*a) {
/*	if(b<a&&(t<<1)>b*b/a || a<b&&(t<<1)>a*a/b)	# very thick */
bellipse(b, newstate(&in, a, b), &p);
return;
}

if(t < 0) {
inb = -1;
newstate(&out, a, y = b);
} else {
inb = b - t;
newstate(&out, a+t, y = b+t);
}
if(t > 0)
newstate(&in, a-t, inb);
inx = 0;
for( ; y>=0; y--) {
outx = step(&out);
if(y > inb) {
erect(-outx, y, outx, y, &p);
if(y != 0)
erect(-outx, -y, outx, -y, &p);
continue;
}
if(t > 0) {
inx = step(&in);
if(y == inb)
inx = 0;
} else if(inx > outx)
inx = outx;
erect(inx, y, outx, y, &p);
if(y != 0)
erect(inx, -y, outx, -y, &p);
erect(-outx, y, -inx, y, &p);
if(y != 0)
erect(-outx, -y, -inx, -y, &p);
inx = outx + 1;
}
}

/*
* a brushed ellipse
*/
static
void
bellipse(int y, State *s, Param *p)
{
int t, ox, oy, x, nx;

t = p->t;
p->disc = allocmemimage(Rect(-t,-t,t+1,t+1), GREY1);
if(p->disc == nil)
return;
memfillcolor(p->disc, DTransparent);
memellipse(p->disc, ZP, t, t, -1, memopaque, ZP, p->op);
oy = y;
ox = 0;
nx = x = step(s);
do {
while(nx==x && y-->0)
nx = step(s);
y++;
eline(-x,-oy,-ox, -y, p);
eline(ox,-oy,  x, -y, p);
eline(-x,  y,-ox, oy, p);
eline(ox,  y,  x, oy, p);
ox = x+1;
x = nx;
y--;
oy = y;
} while(oy > 0);
}

/*
* a rectangle with closed (not half-open) coordinates expressed
* relative to the center of the ellipse
*/
static
void
erect(int x0, int y0, int x1, int y1, Param *p)
{
Rectangle r;

/*	print("R %d,%d %d,%d\n", x0, y0, x1, y1); /**/
r = Rect(p->c.x+x0, p->c.y+y0, p->c.x+x1+1, p->c.y+y1+1);
memdraw(p->dst, r, p->src, addpt(p->sp, r.min), memopaque, ZP, p->op);
}

/*
* a brushed point similarly specified
*/
static
void
epoint(int x, int y, Param *p)
{
Point p0;
Rectangle r;

/*	print("P%d %d,%d\n", p->t, x, y);	/**/
p0 = Pt(p->c.x+x, p->c.y+y);
memdraw(p->dst, r, p->src, addpt(p->sp, r.min), p->disc, p->disc->r.min, p->op);
}

/*
* a brushed horizontal or vertical line similarly specified
*/
static
void
eline(int x0, int y0, int x1, int y1, Param *p)
{
/*	print("L%d %d,%d %d,%d\n", p->t, x0, y0, x1, y1); /**/
if(x1 > x0+1)
erect(x0+1, y0-p->t, x1-1, y1+p->t, p);
else if(y1 > y0+1)
erect(x0-p->t, y0+1, x1+p->t, y1-1, p);
epoint(x0, y0, p);
if(x1-x0 || y1-y0)
epoint(x1, y1, p);
}
```