>Hmm, i got this problem with filled polygons
>and someone redirected me to this newsgroup
>and said that a lot of gdi gurus where over here
>so let me just state my problem:)

>whenever i pass an array of tpoint to a canvas.polygon
>or a canvas.polyline or whatever function, and the array
>contains points that are OUTSIDE the clipping region of
>canvas.cliprect, no clipping gets done. so gdi just keeps
>drawing those polygons outside the cliprect (in unallocated memory)
>which causes it to crash sometime.

>i am using delphi4 proffesional and inspected the source
>of graphics.pas but the polygon function is acutally in a
>gdi dll so i can't see the code, but i saw somehting disturbing
>in the comments in the implenation of graphics.pas somehting
>about a left out list, which contained clipping regions..hell.

>so my question is, why doesn't vcl/gdi do my polygon clipping
>for me, it should do it for me. anyone?.. please help

>Mohican
>IRIS International
>The Netherlands

>ps- please reply to group and by email:)

>Since Canvas.ClipRect is read-only, it can't really be used to clip
>your output.  I think the documentation for TCanvas.ClipRect (as of
>Delphi 3) is very confusing and just plain wrong.

>If you have a specific area that you want to clip, use then Windows
>API (this is what the comment in graphics.pas was referring to, the
>VCL doesn't support setting a clip region).  First create a clip
>region using a member of the Create*Rgn family.  For example, to
>create a rectangle clip region use CreateRectRgn.  Use SelectClipRgn
>to use the region as a clipping region for a particular canvas.
>DeleteObject the Region handle.  Draw your polygon.  When you are done
>clipping, you can reset the clip region using SelectClipRgn.

>On Thu, 18 Feb 1999 13:27:21 +0100, "Mohican of Mohawk"
><mohi...@iris-int.nl> wrote:

>>Hi all,

>>Hmm, i got this problem with filled polygons
>>and someone redirected me to this newsgroup
>>and said that a lot of gdi gurus where over here
>>so let me just state my problem:)

>>whenever i pass an array of tpoint to a canvas.polygon
>>or a canvas.polyline or whatever function, and the array
>>contains points that are OUTSIDE the clipping region of
>>canvas.cliprect, no clipping gets done. so gdi just keeps
>>drawing those polygons outside the cliprect (in unallocated memory)
>>which causes it to crash sometime.

>>i am using delphi4 proffesional and inspected the source
>>of graphics.pas but the polygon function is acutally in a
>>gdi dll so i can't see the code, but i saw somehting disturbing
>>in the comments in the implenation of graphics.pas somehting
>>about a left out list, which contained clipping regions..hell.

>>so my question is, why doesn't vcl/gdi do my polygon clipping
>>for me, it should do it for me. anyone?.. please help

>>Mohican
>>IRIS International
>>The Netherlands

>>ps- please reply to group and by email:)

>Chris Hill
>Chris...@aol.com

>Per example if i have one quite large (big area) polygon
>which i draw wich canvas.polygon, and i zoom in to the
>polygon (the area of the polygon that is outside the clipping
>region gets bigger) it crashes, this only happens with big
>polygons and a reasonalbe zoomfactor.
>When i remove the canvas.polygon function and use
>lineto/moveto and a line clipping alogrithm everything
>goes well and no crashes at all, but that not what i want.

>I see that you mention that windows will not output
>pixels outside of the canvas on a raster device,
>i am not sure of the meaning of raster device,
>but i am quite sure that windows is letting pixels
>being drawn outside of my canvas, could it be that
>a bitmap is not a raster device, i draw the polygons
>on a TBitmap's canvas, and then copyrect that bitmap
>to a paintbox..

>tottally lost,

>If you are using a TBitmap.Canvas, Windows should not draw outside of
>the bitmap unless there is a Windows or video driver bug.

>Chris Hill
>Chris...@aol.com

> If you using Win9x, how large are the coordinates that are outside the
> bitmap?  If they are greater than 2^15-1 or less than -2^15, you might
> be running into problems with the coordinate limitations of Win9x
> (this problem shouldn't be a present on NT which supports much larger
> coordinates, ~2^28).  Even if your coordinates are just very large
> (+-3000), there may be problems with the Polygon function or the

> Joe C. Hecht wrote:
> . Note: Its not
> > too difficult to clip a set of polypoints to a
> > rectangle.

> have you some code for doing this please

>> Joe C. Hecht wrote:
>> . Note: Its not
>> > too difficult to clip a set of polypoints to a
>> > rectangle.

>> have you some code for doing this please

>Yes, but my Polygon clipping code is very propritory,
>as I sell it to make my living.

>>> Joe C. Hecht wrote:
>>> . Note: Its not
>>> > too difficult to clip a set of polypoints to a
>>> > rectangle.

>>> have you some code for doing this please

>>Yes, but my Polygon clipping code is very propritory,
>>as I sell it to make my living.

>You can find the Sutherland-Hodgman Polygon Clipping Algorithm in
>"Computer Graphics -- Principles and Practice" by Foley et al,
>Section 3.14.1, pp. 124-127.

>You might try the link in Section B, Algorithms, at
>www.efg2.com/lab/library/graphics.htm

>efg
>_________________________________
>efg's Computer Lab:       www.efg2.com/lab
>Delphi Books:  www.efg2.com/lab/TechBooks/Delphi.htm

>Earl F. Glynn     E-Mail:  EarlGl...@att.net
>Overland Park, KS  USA