/* draw antialiased line from (x1, y1) to (x2, y2), with width linewidth
 * color is an int like 0xRRGGBB */
static void anti_line(int x1, int y1, int x2, int y2, int linewidth, int color)
{
    int dx = abs(x1 - x2); 
    int dy = abs(y1 - y2);
    int error, sign, tmp;
    float ipix;
    int step = linewidth;

    if (dx >= dy) {
	if (x1 > x2) {
	    tmp = x1;
	    x1 = x2;
	    x2 = tmp;
	    tmp = y1;
	    y1 = y2;
	    y2 = tmp;
	}
	error = dx / 2;
	if (y2 > y1)
	    sign = step;
	else
	    sign = -step;

	putpixel(x1, y1, 1, linewidth, color);

	while (x1 < x2) {
	    if ((error -= dy) < 0) {
		y1 += sign;
		error += dx;
	    }
	    x1 += step;
	    ipix = (float)error / dx;

	    if (sign == step)
		ipix = 1 - ipix;

	    putpixel(x1, y1, 1, linewidth, color);
	    putpixel(x1, y1 - step, (1 - ipix), linewidth, color);
	    putpixel(x1, y1 + step, ipix, linewidth, color);
	}
	putpixel(x2, y2, 1, linewidth, color);
    } else {
	if (y1 > y2) {
	    tmp = x1;
	    x1 = x2;
	    x2 = tmp;
	    tmp = y1;
	    y1 = y2;
	    y2 = tmp;
	}
	error = dy / 2;

	if (x2 > x1)
	    sign = step;
	else
	    sign = -step;

	putpixel(x1, y1, 1, linewidth, color);

	while (y1 < y2) {
	    if ((error -= dx) < 0) {
		x1 += sign;
		error += dy;
	    }
	    y1 += step;
	    ipix = (float)error / dy;
	    
	    if (sign == step)
		ipix = 1 - ipix;

	    putpixel(x1 ,y1, 1, linewidth, color);
	    putpixel(x1 - step, y1, (1 - ipix), linewidth, color);
	    putpixel(x1 + step, y1, ipix, linewidth, color);
	}
	putpixel(x2, y2, 1, linewidth, color);
    }
}

/* put alpha-blended pixel on the backbuffer.  Uses floats, could be
 * optimized, probably */
static void putpixel(int x, int y, float i, int linewidth, int color)
{
    int pos = (y * XMAX * 3) + x * 3;

    unsigned char r = ((color >> 16) & 0xff) * i + (bm.rgb[pos]) * (1 - i);
    unsigned char g = ((color >> 8) & 0xff) * i + (bm.rgb[pos + 1]) * (1 - i);
    unsigned char b = (color & 0xff) * i + (bm.rgb[pos + 2]) * (1 - i);

    if (linewidth == 1) {
	bm.rgb[pos] = r;
	bm.rgb[pos + 1] = g;
	bm.rgb[pos + 2] = b;
    } else {
	int dx, dy;
	for (dx = x; dx < x + linewidth; dx++) {
	    for (dy = y; dy < y + linewidth; dy++) {
		pos = (dy * XMAX * 3) + dx * 3;
		bm.rgb[pos] = r;
		bm.rgb[pos + 1] = g;
		bm.rgb[pos + 2] = b;
	    }
	}
    }
}