private const byte TOP = 8; // 1000
#endregion Fields
#region Methods
/// <summary>
/// Cohen–Sutherland clipping algorithm clips a line from
/// P0 = (x0, y0) to P1 = (x1, y1) against a rectangle with
/// diagonal from (xmin, ymin) to (xmax, ymax).
/// </summary>
/// <param name="x0"></param>
/// <param name="y0""</param>
/// <param name="x1"></param>
/// <param name="y1"></param>
/// <returns>a list of two points in the resulting clipped line, or zero</returns>
public static List<Point> CohenSutherlandLineClip(Rect extents, int x1, int y1, int x2, int y2)
{
float xf1 = x1;
float yf1 = y1;
float xf2 = x2;
float yf2 = y2;
// compute outcodes for P0, P1, and whatever point lies outside the clip rectangle
byte outcode0 = CohenSutherland.ComputeOutCode(extents, xf1, yf1);
byte outcode1 = CohenSutherland.ComputeOutCode(extents, xf2, yf2);
bool accept = false;
while (true)
{
// Bitwise OR is 0. Trivially accept and get out of loop
if ((outcode0 | outcode1) == 0)
{
accept = true;
break;
}
// Bitwise AND is not 0. Trivially reject and get out of loop
else if ((outcode0 & outcode1) != 0)
{
break;
}
else
{
// failed both tests, so calculate the line segment to clip
// from an outside point to an intersection with clip edge
float x, y;
// At least one endpoint is outside the clip rectangle; pick it.
byte outcodeOut = (outcode0 != 0) ? outcode0 : outcode1;
// Now find the intersection point;
// use formulas y = y0 + slope * (x - x0), x = x0 + (1 / slope) * (y - y0)
if ((outcodeOut & TOP) != 0)
{ // point is above the clip rectangle
x = xf1 + (xf2 - xf1) * (extents.Top - yf1) / (yf2 - yf1);
y = extents.Top;
}
else if ((outcodeOut & BOTTOM) != 0)
{ // point is below the clip rectangle
x = xf1 + (xf2 - xf1) * (extents.Bottom - yf1) / (yf2 - yf1);
y = extents.Bottom;
}
else if ((outcodeOut & RIGHT) != 0)
{ // point is to the right of clip rectangle
y = yf1 + (yf2 - yf1) * (extents.Right - xf1) / (xf2 - xf1);
x = extents.Right;
}
else if ((outcodeOut & LEFT) != 0)
{ // point is to the left of clip rectangle
y = yf1 + (yf2 - yf1) * (extents.Left - xf1) / (xf2 - xf1);
x = extents.Left;
}
else
{
x = float.NaN;
y = float.NaN;
}
// Now we move outside point to intersection point to clip
// and get ready for next pass.
if (outcodeOut == outcode0)
{
xf1 = x;
yf1 = y;
outcode0 = CohenSutherland.ComputeOutCode(extents, xf1, yf1);
}
else
{
xf2 = x;
yf2 = y;
outcode1 = CohenSutherland.ComputeOutCode(extents, xf2, yf2);
}
}
}
// return the clipped line
return (accept) ? new List<Point>() { new Point((int)xf1, (int)yf1), new Point((int)xf2, (int)yf2),} : null;
}
/// <summary>
/// Initialise the class
/// </summary>
public Canvas()
{
_buffers = new UInt32[] { FrameBufferBase, FrameBufferBase + (ScreenWidth * ScreenHeight * 4) };
_activeBuffer = 0;
_backBuffer = 1;
DisplayInterop.LCD_Init((UInt32*)_buffers[_activeBuffer], (UInt32*)_buffers[_backBuffer]);
// initial clip
_screenClip = new Rect(0, 0, ScreenWidth, ScreenHeight);
_clip = _screenClip;
_timer = new Timer();
_timer.start();
}
/// <summary>
/// Compute the bit code for a point (x, y) using the clip rectangle
/// bounded diagonally by (xmin, ymin), and (xmax, ymax)
/// ASSUME THAT xmax , xmin , ymax and ymin are global constants.
/// </summary>
/// <param name="x"></param>
/// <param name="y"></param>
/// <returns></returns>
private static byte ComputeOutCode(Rect extents, float x, float y)
{
// initialised as being inside of clip window
byte code = INSIDE;
if (x < extents.Left) // to the left of clip window
code |= LEFT;
else if (x > extents.Right) // to the right of clip window
code |= RIGHT;
if (y > extents.Bottom) // below the clip window
code |= BOTTOM;
else if (y < extents.Top) // above the clip window
code |= TOP;
return code;
}
