Regions are a resource in windows that are very useful. They are device independent, which means that a device context (DC) is not needed 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:
- Update Region, Paint
- Clipping, Paint
- Hit Testing
- Define window shape, Irregular Shaped Windows
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 to 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 |
|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. |
|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.
|Creates a polygonal region. |
|Creates a region consisting of a series of polygons. |
|Creates a rectangular region. |
|Creates a rectangular region from a |
|Creates a rectangular region with rounded corners. |
|Creates a region from the specified region and transformation data. |
|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
|Value ||Description ||Result |
|Creates the intersection of the two combined regions
CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_AND);
|Creates a copy of the region identified by |
CombineRgn(hrgnDest, hrgnSrc1, NULL, RGN_COPY);
|No Image |
|Combines the parts of hrgnSrc1 that are not part of |
CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_DIFF);
|Creates the union of two combined regions
CombineRgn(hrgnDest, hrgnSrc1, hrgnSrc2, RGN_OR);
|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 hTemp = ::CreateRectRgn(0,0,0,0);
::CombineRgn(hTemp, hCacheRgn, hUpdateRgn, RGN_DIFF);
::CombineRgn(hCacheRgn, hTemp, NULL, RGN_COPY);
::CombineRgn(hCacheRgn, hCacheRgn, hUpdateRgn, RGN_DIFF);
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.
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.
RectInRegion function is very similar to
PtInRegion, except that it will determine whether any part of the specified rectangle is within the boundaries of a region.
CONST RECT *lprc
GetRgnBox function retrieves the bounding rectangle of the specified region.
OffsetRgn function moves a region by the specified offsets.
SetRectRgn function converts a region into a rectangular region with the specified coordinates.
WIN32 provides a number of functions that will allow a region handle to be used for painting effects. These effects include filling with a brush, outlining, and inverting the current contents.
FillRgn function fills a region by using the specified brush.
FrameRgn function draws a border around the specified region by using the specified brush.
InvertRgn function inverts the colors in the specified region.
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.
GetRegionData function fills the specified buffer with data desc:
This data includes the dimensions of the rectangles that make up the region.
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.
ExtCreateRegion function creates a region from the specified region and transformation data.
CONST XFORM *lpXform,
CONST RGNDATA *lpRgnData
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 does 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:
- Select the desired draw mode shape from the menu or the tool bar. There are three choices, Rectangle , Ellipse , or Round Rectangle .
- Select an area on the main window, and start dragging the region. A rubber-banding effect will show where the region will appear.
- Use the Left mouse button to draw a shape for the Source 1 region.
Use the Right mouse button to draw a shape for the Source 2 region.
- At any time, you can hit the Escape key to cancel the drawing operation.
- Let go of the mouse button to update the region with the new shape that you have created.
- The Source 1 region will be drawn in RED, the Source 2 region will be drawn in Blue.
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.
- Simply decide which of the four sub-views that you would like to use.
- Click in the sub-view and drag the mouse to the main window. An icon will appear indicating that the item is being dragged.
- Use the Left mouse button to replace the combine region for the Source 1 region.
Use the Right mouse button to replace the combine region for the Source 2 region.
- At any time, you can hit the Escape key to cancel the replace operation.
- Let go of the mouse button to replace the region with the combine region that you dragged..
You can delete the
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 other hand 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 allow you to manage and manipulate your regions.