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# How FrameRgn works

Posted on 2001-07-22
Medium Priority
836 Views
var
Rects  :array of TRect;
Points :array of TPoint;

Rects is an array of rectangles that is filled indirectly by GetRegionData (and stored in the same order as they were found in the RGNDATA structure).

Points is an ordered array of points I will use to draw an outline of the region.  (This array will be identical to the array of points you would pass into CreatePolyPolygonRgn, if you were to create the region that way)

I can't use FrameRgn for two reasons:

1.  FrameRgn really isn't very powerful.  I want to draw marching ants and FrameRgn apparently only draws solid lines.
2.  I am not using the region for the purposes of directly drawing to a device context.

I would like to know the algorithm FrameRgn uses to draw the outline of a region, so I can convert Rects to Points.
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Question by:scrapdog
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LVL 17

Expert Comment

ID: 6306313
hi scrapdog,
dunno about FrameRgn but i have an example of marching ants if its any help (it doesnt use FrameRgn etc).
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LVL 5

Author Comment

ID: 6306396
No thanks, got that one figured out. =)
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Author Comment

ID: 6306397
But if your marching ants code takes it's input from a Windows region, that would be very helpful...
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LVL 17

Expert Comment

ID: 6306541
nope ,just passes x,y of mouse to lineDDA()
all other examples i have are from tomes which dont look like they help here.

so just listening..
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Accepted Solution

FengYuan earned 1100 total points
ID: 6311038
Simple algorithm:

1) Suppose you use (x1,y1,x2,y2) to represent a line to draw.
2) Create a set or sequence of lines.
3) For each rectangle in the region, adding its four edges to the line set using 4).
4) When adding each line to the set, search for lines in the set which intersects with the line to be added.
5) If no intersection is found, append the line.
6) If intersection is found, remove the matched line, add the disjoint portion of the two lines if any.

7) Draw what's left in the line set.

www.fengyuan.com
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Author Comment

ID: 6311056
In #6, intersecting two lines will create four line segments...how do I know which line segments to discard (i.e., which of these segments are inside the region)?
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Expert Comment

ID: 6311061
I think you should only consider tw vertical lines or two horizontal lines intersect with each other.

In these cases, at most two new lines can be generated.
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Author Comment

ID: 6311066
OK, thanks for the info.  I'll try it out.
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Author Comment

ID: 6313693
Excellent.  It worked.  I thought everyone was stumped on this one.
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