Volume Number: 13 (1997)
Issue Number: 11
Column Tag: Tips & Tidbits

by Steve Sisak

Here is an easy technique for determining if a point lies within a given graphic object of arbitrary complexity: simply draw the object into an offscreen bitmap, then check if the desired pixel has been changed. In more detail, the steps are:

• Create an offscreen black-and-white bitmap, one pixel high by 16 pixels wide (actually it only needs to be one pixel wide, but QuickDraw insists that you allocate an even number of bytes).
• Erase this bitmap to white.
• Draw the shape into the bitmap in solid black.
• Test the desired pixel to see if it has been set to black.

Here is some actual code that shows you how to hit test a polygon in this way. First, an auxiliary routine to aid the setup of an offscreen black-and-white GrafPort (since GWorlds are overkill for this purpose):

PROCEDURE SetBitsTo
   (
    VAR TheBits : BitMap
   );
   (* sets the portBits of the current grafPort, also
    changing the portRect, clipRgn, visRgn to match
    the bounds of the bitmap. *)

    VAR
      CurPort : GrafPtr;

  BEGIN
    SetPortBits(TheBits);
    MovePortTo(0, 0);
    SetOrigin(TheBits.bounds.left, TheBits.bounds.top);
    PortSize
     (
      TheBits.bounds.right - TheBits.bounds.left,
      TheBits.bounds.bottom - TheBits.bounds.top
     );
    GetPort(CurPort);
    ClipRect(CurPort^.portRect);
    CopyRgn(CurPort^.clipRgn, CurPort^.visRgn)
  END SetBitsTo;

Then, the actual routine that hit tests a point against a polygon:

  PROCEDURE PtInPoly
   (
    pt : Point;
    poly : PolyHandle
   ) : BOOLEAN;
   (* is the specified point within the given polygon. *)

    VAR
      PreviousPort : GrafPtr;
      TempPort : GrafPort;
      TempBits : BitMap;
      TempBitsWord : ShortCard; (* a 1-by-16 offscreen bitmap *)

  BEGIN
    GetPort(PreviousPort);
    OpenPort(ADR(TempPort));
    TempBits.bounds.top := pt.v;
    TempBits.bounds.bottom := pt.v + 1;
    TempBits.bounds.left := pt.h - 15;
      (* so desired pixel is in bit 0 *)
    TempBits.bounds.right := pt.h + 1;
    TempBits.rowBytes := 2;
    TempBits.baseAddr := ADR(TempBitsWord);
    TempBitsWord := 0; (* set all pixels to white *)
    SetBitsTo(TempBits);
    PaintPoly(poly); (* draw polygon in black *)
    ClosePort(ADR(TempPort));
    SetPort(PreviousPort);
    RETURN
      ODD(TempBitsWord)
  END PtInPoly;

It shouldn't be hard to see how to adapt the basic idea to other sorts of graphic objects, or even to other graphics architectures besides QuickDraw.

Lawrence D'Oliveiro
ldo@geek-central.gen.nz