Begins defining a region.

void OpenRgn ();

DISCUSSION

The OpenRgn function allocates temporary memory to start saving lines and framed shapes for processing as a region definition. Call OpenRgn only after initializing a region with the NewRgn function.

The NewRgn function stores the definition for a region in a Region structure.

While a region is open, all calls to Line, LineTo, and the functions that draw framed shapes (except arcs) affect the outline of the region. Only the line endpoints and shape boundaries affect the region definition—the pattern mode, pattern, and size do not affect it.

When you are finished defining the region, call the CloseRgn function.

The OpenRgn function calls HidePen, so no drawing occurs on the screen while the region is open (unless you call ShowPen just after OpenRgn, or you called ShowPen previously without balancing it by a call to HidePen). Since the pen hangs below and to the right of the pen location, drawing lines with even the smallest pen changes pixels that lie outside the region you define.

The outline of a region is mathematically defined and infinitely thin, and it separates the bit or pixel image into two groups of pixels: those within the region and those outside it.

A region should consist of one or more closed loops. Each framed shape itself constitutes a loop. Any lines drawn with the Line or LineTo function should connect with each other or with a framed shape. Even if the onscreen presentation of a region is clipped, the definition of a region is not; you can define a region anywhere on the coordinate plane with complete disregard for the location of various graphics port entities on that plane.

When a region is open, the current graphics port’s rgnSave field contains a handle to information related to the region definition. If you want to temporarily disable the collection of lines and shapes, you can save the current value of this field, set the field to NULL, and later restore the saved value to resume the region definition. Also, calling SetPort while a region is being formed discontinues formation of the region until another call to SetPort resets the region’s original graphics port.

If the points or rectangles supplied to this function are defined in a graphics port other than your current graphics port, you must convert them to the local coordinate system of your current graphics port. You can accomplish this by using the SetPort function to change to the graphics port containing the points or rectangles, using the LocalGlobal function to convert their locations to global coordinates, using SetPort to return to your starting graphics port, and then using the GlobalToLocal function to convert the locations of points or rectangles to the local coordinates of your current graphics port.

SPECIAL CONSIDERATIONS

Regions are limited to 32 KB in size in basic QuickDraw and 64 KB in Color QuickDraw. You can determine the current size of an existing region by calling the Memory Manager function GetHandleSize. When you structure drawing operations in an open region, the resulting region description may overflow the 32 KB or 64 KB limit. Should this happen in Color QuickDraw, the QDError function returns the result code regionTooBigError.

Do not call OpenRgn while another region or a polygon is already open. When you are finished constructing the region, use the CloseRgn function, which is described next.

The OpenRgn function may move or purge memory blocks in the application heap; do not call this function at interrupt time.

AVAILABILITY

Supported in Carbon. Available in Carbon 1.0.2 and later when running Mac OS 8.1 or later.

© 2000 Apple Computer, Inc. (Last Updated 6/30/2000)