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Guide to WIN32 RegionsBy Paul WattGuide to understanding how to create and use regions with the WIN32 SDK |
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Regions are a resource in windows that are very useful. They are device independent, which means that a device context (DC) is not need to create or use one. However, there are many functions in the WIN32 GDI that require a region.
Some of the useful places for a region are:
Many of the functions that relate to regions are very simple, and need very little explanation, in fact the definition from MSDN is adequate enough to explain these functions. However, some of the functions are a little more difficult to understand and master, therefore extra details have been added to explain these functions. A few diagrams may accompany the explanation as well.
There are a number of ways to create a region. There are a number of functions that can be used create a region, from a simple shape such as a rectangle or an ellipse, to a complicated shape such as the outline of a string of text. Listed below is a table that describes each of the creation functions.
| Function | Description |
|---|---|
| CreateEllipticRgn | Creates an elliptical region. As with drawing ellipses, elliptic regions are very large, and may slow your program down if you use a large number of them. |
| CreateEllipticRgnIndirect | Creates an elliptical region from a RECT structure. As with drawing ellipses, elliptic regions are very large, and may slow your program down if you use a large number of them. |
| CreatePolygonRgn | Creates a polygonal region. |
| CreatePolyPolygonRgn | Creates a region consisting of a series of polygons. |
| CreateRectRgn | Creates a rectangular region. |
| CreateRectRgnIndirect | Creates a rectangular region from a RECT structure. |
| CreateRoundRectRgn | Creates a rectangular region with rounded corners. |
| ExtCreateRegion | Creates a region from the specified region and transformation data. |
| PathToRegion | Creates a region from a WIN32 Path created in a DC. |
There are a few operations that can be performed on a region in order to query, modify test its data.
After a few basic regions are created, it is possible to perform boolean operations on
the regions in order to create more complex regions with CombineRgn. This
function has five operations that can be performed in order to combine two different
regions. Shown below is an example of each of the boolean operations that can be
performed in CombineRgn:
| Value | Description | Result |
| RGN_AND | Creates the intersection of the two combined regions.CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_AND); |
 |
| RGN_COPY | Creates a copy of the region identified by hrgnSrc1.CombineRgn(hrgnDest, hrgnSrc1, NULL, RGN_COPY); |
No Image |
| RGN_DIFF | Combines the parts of hrgnSrc1 that are not part of hrgnSrc2.CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_DIFF); |
 |
| RGN_OR | Creates the union of two combined regions.CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_OR); |
 |
| RGN_XOR | Creates the union of two combined regions except for any overlapping areas.CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_XOR); |
 |
One more thing to point out is that the destination region in CombineRgn can be
one of the source regions. Therefore, if you continually modify a region by adding or
subtracting a different region from it, you can perform one combine operation rather than
worrying about copying the adjusted region. Here is an example:
HRGN hCacheRgn; HRGN hUpdateRgn; ... //C: Convoluted way. //C: Initialize a temporary region. HRGN hTemp = ::CreateRectRgn(0,0,0,0); //C: Subtract teh update region from the Cache region. ::CombineRgn(hTemp, hCacheRgn, hUpdateRgn, RGN_DIFF); //C: Copy the Temp region back into the Cache region. ::CombineRgn(hCacheRgn, hTemp, NULL, RGN_COPY); //C: Destroy the Temp region. ::DeleteObject(hTemp); //C: Simple way ::CombineRgn(hCacheRgn, hCacheRgn, hUpdateRgn, RGN_DIFF);
The EqualRgn function checks the two specified regions
to determine whether they are identical. The function considers two regions
identical if they are equal in size and shape.
BOOL EqualRgn( HRGN hSrcRgn1, // handle to first region HRGN hSrcRgn2 // handle to second region );
The PtInRegion function determines whether the specified point is inside the specified region. This function is very useful for creating a hit-test region on your control. Such as a bitmap that has a hot-region that can be clicked by the user.
BOOL PtInRegion( HRGN hrgn, // handle to region int X, // x-coordinate of point int Y // y-coordinate of point );
The RectInRegion function is very similar to PtInRegion,
except that it will determines whether any part of the specified rectangle is within
the boundaries of a region.
BOOL RectInRegion( HRGN hrgn, // handle to region CONST RECT *lprc // pointer to rectangle );
The GetRgnBox function retrieves the bounding rectangle of the specified region.
int GetRgnBox( HRGN hrgn, // handle to a region LPRECT lprc // bounding rectangle );
The OffsetRgn function moves a region by the specified offsets.
int OffsetRgn( HRGN hrgn, // handle to region int nXOffset, // offset along x-axis int nYOffset // offset along y-axis );
The SetRectRgn function converts a region into a rectangular region
with the specified coordinates.
BOOL SetRectRgn( HRGN hrgn, // handle to region int nLeftRect, // x-coordinate of upper-left corner of rectangle int nTopRect, // y-coordinate of upper-left corner of rectangle int nRightRect, // x-coordinate of lower-right corner of rectangle int nBottomRect // y-coordinate of lower-right corner of rectangle );
WIN32 provides a number of functions that will allow a region handle to be used for painting affects. These effects include filling with a brush, outlining, and inverting the current contents
The FillRgn function fills a region by using the specified brush.
BOOL FillRgn( HDC hdc, // handle to device context HRGN hrgn, // handle to region to be filled HBRUSH hbr // handle to brush used to fill the region );
The FrameRgn function draws a border around the specified region by using the specified brush.
BOOL FrameRgn( HDC hdc, // handle to device context HRGN hrgn, // handle to region to be framed HBRUSH hbr, // handle to brush used to draw border int nWidth, // width of region frame int nHeight // height of region frame );
The InvertRgn function inverts the colors in the specified region.
BOOL InvertRgn( HDC hdc, // handle to device context HRGN hrgn // handle to region to be inverted );
The PaintRgn function is similar to the FillRgn function,
except that this function paints the specified region by using the brush currently
selected into the device context.
BOOL PaintRgn( HDC hdc, // handle to device context HRGN hrgn // handle to region to be painted );
The GetRegionData function fills the specified buffer with data desc
DWORD GetRegionData( HRGN hRgn, // handle to region DWORD dwCount, // size of region data bufferribing a region. This data includes the dimensions of the rectangles that make up the region. LPRGNDATA lpRgnData // region data buffer );
A WIN32 region is really a set of rectangles managed in one object. The RGN_DATA
structure manages this set of rectangles as well as some of the other data that is
required to maintain the region. The RGN_DATA structure is accessible through the
GetRegionData function. A region object can be created with a
RGN_DATA structure with the ExtCreateRgn function. Here is a diagram
to show the internal representation of a HRGN object. Each rectangle in the region
is painted with an alternating colored brush.
The ExtCreateRegion function creates a region from the specified region
and transformation data.
HRGN ExtCreateRegion( CONST XFORM *lpXform, // transformation data DWORD nCount, // size of region data CONST RGNDATA *lpRgnData // region data buffer );
Something to be aware of, is that Windows 2000 will accept a region that has overlapping
rectangles. If you try to create a region with data that contains overlapping rectangles
on Windows NT4, this function will fail. Therefore if you create one of these regions on
a Windows 2000 machine, and store the RGN_DATA of a region in a file or send the data
across the network to a Windows NT4 machine, when you try to call ExtCreateRgn
on the structure, it will fail.
The Windows 9x kernel dpes not support world transforms that involve either
shearing or rotations. ExtCreateRegion fails if the transformation (XFORM)
matrix has a scaling or translation of the region.
The program that has been created to demonstrate some of the possibilities for regions is an interactive window that allows the user to create two regions. The user can add new portions to either one of the source regions with a rectangle, ellipse or round rectangle tool. The polygon has been left out simply because of extra work related to the UI that would be required to incorporate this tool, however, a polygon is completely possible.
Here are the directions for creating a source region:
, Ellipse 
, or Round Rectangle 
.On the right-hand side of the window where the regions are drawn, there are four sub-views,
where the different combine modes: RGN_AND, RGN_DIFF, RGN_OR, RGN_XOR are all
demonstrated. As each source region is modified, these sub-views are updated. It is possible
to replace one of the current source regions with one of the sub-view results simply by dragging the
region to the window where the source regions are displayed. The Source 1 and Source 2
regions are displayed in Red and Blue respectively, just as in the main view, and the new
combine region will be displayed in purple.
Here are the directions for moving the combine region to the source region.
You can delete the Source1, Source2 or both of the regions by selecting the appropriate menu item.
You can choose to view either the Source1 region, the Source2 region, or all of the regions by selecting the appropriate view from the menu. View All is the default selection.
One last operation in the RgnGuide program is to view the rectangles that compose the region in breakdown mode. You can do this by simply checking either the BreakDown Src 1 or BreakDown Src 2 menu item. The view will then show the selected region broken down into each of the rectangles that creates that region. Each rectangle will be painted with an alternating color. A plain reactangle will be pretty boring with one solid colored rectangle. An ellipse on the otherhand will contain a new rectangle on just about every other scanline.
Regions are a very powerful OS resource that you can use in your program. The demo program simply illustrates how to manage your resources, there are many other uses than the ones that have been described. WIN32 provides a rich set of functions that allows you to manage and manipulate your regions.
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Last Updated: 10 Mar 2002 Editor: Chris Maunder |
Copyright 2002 by Paul Watt Everything else Copyright © CodeProject, 1999-2009 Web13 | Advertise on the Code Project |
. This result does not depend on position of MyObject.
