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August 5th, 2011, 06:11 AM   #1
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Plotting in C++

Hey, I was just wondering what the best way was plotting in C++?
I have to arrays x and y with equal length and just want to do a simple x,y plot.

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August 26th, 2011, 07:29 AM   #2
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Re: Plotting in C++

It's been a long time since I wrote in C++ (I switched to Java about 10 years ago) and it may be that different compilers use different graphics commands (I don't know how standardized graphics has become) but if I remember correctly, you use the commands "MoveTo(x,y)" and "LineTo(x,y)". "MoveTo(x,y)" moves the "drawing point" to (x,y) on the screen without drawing anything, and "LineTo(x,y)" draws a line, in the current drawing color, from the current "drawing point" to (x,y) and makes (x,y) the current "drawing point".

If you have arrays X[] and Y[] containing the x and y coordinates of N points in screen coordinates, then
MoveTo(X[0], Y[0});
for (int i= 0;i< n;i++)
LineTo(X[i],Y[i});

will draw a "broken line" through the points.

Now, as I said, that is in "screen coordinates"- x and y represent actual pixels on the screen. If your numbers are in "world coordinates", that is, actual numbers representing whatever your application is, you will have to transform them to "screen coordinates". You do that by deciding what you want to be the largest and smallest values to be shown on the screen for both x and y. For example, if you know that all of the points have X between 0 and 3.5 and all y values are between -1 and 1, you would take Xmax to be 3.5, Xmin= 0.0, Ymax= 1.0, YMin= -1.0. You will also need to get the max and min pixel locations for the screen you want to draw on. Lets say they are "ileft", "iright", "jbottom", and "jtop" (I am using "i" to represent horizontal pixels, "j" for vertical). Since you don't want to have different parts of your picture "stretched" more than others, you want to use a "linear" transformation. That is, for i in "screen coordinates", x in "world coordinates", we must have i= Ax+ B where A and B are constants. On the left, in "world coordinates" we have Xmin and in "screen coordinates" ileft, we must have ileft= A(Xmin)+ B. Similarly, on the right, we have Xmax in "world coordinates" and iright in "screen coordinates" so we have iright= A(Xmax)+ B. Subtracting the first equation from the first, iright- ileft= A(Xmax- Xmin) and B is eliminated: A= (iright- ileft)/(Xmax- Xmin). Once we know A, B= ileft- A(Xmin).

What you would do is use the screen information and your desired "world area" to find A and B intially. Then, for every X[i] use i= AX{i]+ B to find the correct pixel location. Do the same for the "Y" to "j" transformation.

There's a pretty good tutorial on Borland C++ here: http://www.functionx.com/bcb/
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August 26th, 2011, 10:55 AM   #3
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Re: Plotting in C++

Yes, that's the GDI+ way:
http://msdn.microsoft.com/en-us/library ... 85%29.aspx
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