MFC, CDC::LineTo() clipping

CDC::LineTo() clipping

Author	Message
Phil McAdam #1 / 5	CDC::LineTo() clipping I have a question about how windows clips line segments using the CDC::LineTo() function. I'm drawing some mapping coordinates which get converted from a latitude/longitude coordinate to a pixel coordinate. I then draw lines between the points. Depending upon how close in the user has zoomed in, the point to which I need to draw may be way off the screen. Windows seems to handle this OK up to some undefined point but beyond that it starts drawing stray lines all over the place. For example, if I'm drawing from pixel location (100, 100) to a pixel location (5000, 5000) which is off the screen, Windows does a fine job at drawing the line and clipping it at the window's boundary. But if I try to draw to a pixel location much farther off the screen, say (1000000, 1000000) then it starts drawing weird lines everywhere. If I knew the maximum limit to which Windows to correct draw to offscreen points then I could filter out points that are beyond that limit. Does anybody know what the limit is? Phil McAdams
Fri, 23 Aug 2002 03:00:00 GMT

Joseph M. Newcome #2 / 5	CDC::LineTo() clipping If you are running on Win9x, you are running on a toy 16-bit OS, a warmed-over Windows 3.1 GDI that was designed for the 8088. Your scaling requires more than 16 bits of precision, and you are seeing the truncation effects of the high-order 16 bits being discarded. There are two solutions: scale it yourself so the only coordinates you send to GDI fit in the 16-bit range, or move to a real operating system with 32-bit GDI coordinates. joe On Mon, 6 Mar 2000 15:53:15 -0700, "Phil McAdams" Quote: >I have a question about how windows clips line segments using the >CDC::LineTo() function. I'm drawing some mapping coordinates which get >converted from a latitude/longitude coordinate to a pixel coordinate. I >then draw lines between the points. >Depending upon how close in the user has zoomed in, the point to which I >need to draw may be way off the screen. Windows seems to handle this OK up >to some undefined point but beyond that it starts drawing stray lines all >over the place. For example, if I'm drawing from pixel location (100, 100) >to a pixel location (5000, 5000) which is off the screen, Windows does a >fine job at drawing the line and clipping it at the window's boundary. But >if I try to draw to a pixel location much farther off the screen, say >(1000000, 1000000) then it starts drawing weird lines everywhere. >If I knew the maximum limit to which Windows to correct draw to offscreen >points then I could filter out points that are beyond that limit. Does >anybody know what the limit is? >Phil McAdams Joseph M. Newcomer [MVP] Web: http://www3.pgh.net/~newcomer MVP Tips: http://www3.pgh.net/~newcomer/mvp_tips.htm
Sat, 24 Aug 2002 03:00:00 GMT

Johan Rosengre #3 / 5	CDC::LineTo() clipping Phil! You are running into a disgusting little problem. The first part is that coordinates must be 16-bit in Win95/98 - that is well known. But the second part is that some screen drivers wrap around when you hit the ceiling, while others don't. I don't think it will help you to set an explicit clip rect (you can always try - but if the coords wrap, the endpoints should still fall inside the valid rect), but you might want to check into LineDDA for more advanced line-drawing possibilities. Johan Rosengren Responsable Informatique PACTA S.A. Quote: > I have a question about how windows clips line segments using the > CDC::LineTo() function. I'm drawing some mapping coordinates which get > converted from a latitude/longitude coordinate to a pixel coordinate. I > then draw lines between the points. > Depending upon how close in the user has zoomed in, the point to which I > need to draw may be way off the screen. Windows seems to handle this OK up > to some undefined point but beyond that it starts drawing stray lines all > over the place. For example, if I'm drawing from pixel location (100, 100) > to a pixel location (5000, 5000) which is off the screen, Windows does a > fine job at drawing the line and clipping it at the window's boundary. But > if I try to draw to a pixel location much farther off the screen, say > (1000000, 1000000) then it starts drawing weird lines everywhere. > If I knew the maximum limit to which Windows to correct draw to offscreen > points then I could filter out points that are beyond that limit. Does > anybody know what the limit is? > Phil McAdams
Sat, 24 Aug 2002 03:00:00 GMT

Keith S #4 / 5	CDC::LineTo() clipping Quote: > I have a question about how windows clips line segments using the > CDC::LineTo() function. I'm drawing some mapping coordinates which get > converted from a latitude/longitude coordinate to a pixel coordinate. I > then draw lines between the points. > Depending upon how close in the user has zoomed in, the point to which I > need to draw may be way off the screen. Windows seems to handle this OK up > to some undefined point but beyond that it starts drawing stray lines all > over the place. For example, if I'm drawing from pixel location (100, 100) > to a pixel location (5000, 5000) which is off the screen, Windows does a > fine job at drawing the line and clipping it at the window's boundary. But > if I try to draw to a pixel location much farther off the screen, say > (1000000, 1000000) then it starts drawing weird lines everywhere. > If I knew the maximum limit to which Windows to correct draw to offscreen > points then I could filter out points that are beyond that limit. Does > anybody know what the limit is? > Phil McAdams I've run into this problem too. Windows 98 GDI can only handle 16 bit signed coordinates, i.e. points are limited to -32768 to +32767. Points outside this range will get drawn in strange locations. I solved it by scaling all coordinates such that they are divided by a scale factor. If the resulting window extent is greater than +/- 32000 then the scale factor is increased, if less than 1000 then the scale factor is reduced. Your zoomin/ zoomout code will therefore need to modify the scale factor depending on the data extent. - Keith
Sat, 24 Aug 2002 03:00:00 GMT

Phil McAdam #5 / 5	CDC::LineTo() clipping Thanks everyone. I was afraid that it was really limited to a signed 16 bit integer although it doesn't seem to specifically state that in the MFC help. For my particular app it will work best for me to just not attempt to draw lines that exceed 32767 (or negative). Thanks! Phil McAdams Quote: > I have a question about how windows clips line segments using the > CDC::LineTo() function. I'm drawing some mapping coordinates which get > converted from a latitude/longitude coordinate to a pixel coordinate. I > then draw lines between the points. > Depending upon how close in the user has zoomed in, the point to which I > need to draw may be way off the screen. Windows seems to handle this OK up > to some undefined point but beyond that it starts drawing stray lines all > over the place. For example, if I'm drawing from pixel location (100, 100) > to a pixel location (5000, 5000) which is off the screen, Windows does a > fine job at drawing the line and clipping it at the window's boundary. But > if I try to draw to a pixel location much farther off the screen, say > (1000000, 1000000) then it starts drawing weird lines everywhere. > If I knew the maximum limit to which Windows to correct draw to offscreen > points then I could filter out points that are beyond that limit. Does > anybody know what the limit is? > Phil McAdams
Sat, 24 Aug 2002 03:00:00 GMT

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