Function GfxDoDrawLine

src/gfx.cpp:317–365

* Check line clipping by using a linear equation and draw the visible part of * the line given by x/y and x2/y2. * @param video Destination pointer to draw into. * @param x X coordinate of first point. * @param y Y coordinate of first point. * @param x2 X coordinate of second point. * @param y2 Y coordinate of second point. * @param screen_width With of the screen to check clipping against.

 * @param dash Length of dashes for dashed lines. 0 means solid line.
 */
static inline void GfxDoDrawLine(void *video, int x, int y, int x2, int y2, int screen_width, int screen_height, PixelColour colour, int width, int dash = 0)
{
	Blitter *blitter = BlitterFactory::GetCurrentBlitter();

	assert(width > 0);

	if (y2 == y || x2 == x) {
		/* Special case: horizontal/vertical line. All checks already done in GfxPreprocessLine. */
		blitter->DrawLine(video, x, y, x2, y2, screen_width, screen_height, colour, width, dash);
		return;
	}

	int grade_y = y2 - y;
	int grade_x = x2 - x;

	/* Clipping rectangle. Slightly extended so we can ignore the width of the line. */
	int extra = (int)CeilDiv(3 * width, 4); // not less then "width * sqrt(2) / 2"
	Rect clip = { -extra, -extra, screen_width - 1 + extra, screen_height - 1 + extra };

	/* prevent integer overflows. */
	int margin = 1;
	while (INT_MAX / abs(grade_y) < std::max(abs(clip.left - x), abs(clip.right - x))) {
		grade_y /= 2;
		grade_x /= 2;
		margin  *= 2; // account for rounding errors
	}

	/* Prevent division by zero. */
	if (grade_x == 0) grade_x = 1;

	/* Imagine that the line is infinitely long and it intersects with
	 * infinitely long left and right edges of the clipping rectangle.
	 * If both intersection points are outside the clipping rectangle
	 * and both on the same side of it, we don't need to draw anything. */
	int left_isec_y = y + (clip.left - x) * grade_y / grade_x;
	int right_isec_y = y + (clip.right - x) * grade_y / grade_x;
	if ((left_isec_y > clip.bottom + margin && right_isec_y > clip.bottom + margin) ||
			(left_isec_y < clip.top - margin && right_isec_y < clip.top - margin)) {
		return;
	}

	/* It is possible to use the line equation to further reduce the amount of
	 * work the blitter has to do by shortening the effective line segment.
	 * However, in order to get that right and prevent the flickering effects
	 * of rounding errors so much additional code has to be run here that in
	 * the general case the effect is not noticeable. */

	blitter->DrawLine(video, x, y, x2, y2, screen_width, screen_height, colour, width, dash);
}

/**
 * Align parameters of a line to the given DPI and check simple clipping.