Calculate scale in plotting


I'm using a CStatic derived class to represent a diagram on a dialog. Because there are several curves to be displayed in one diagram simultaneously(I'm using MM_ANISOTROPIC mapping mode to fit all of them in the window), I decide not to draw a vertival axis and a set of tic marks for each of these curves, instead, I calculate the scale of each curve while plotting and then display them besides the diagram.

The scale here is defined as actual value (in vertical dir.) in a range of 1cm on the screen. I'm using the code like afterwards, I think it's right and logical, but there's remakable error on the display(around 8%). I don't know where does it come from:

void CDiagram::CalculateScale(CDC *pDC, CSize *scale)
     // calculate scales in x( and y( directions
     // logical values in 1cm of length on screen

     CPoint OldViewportOrg = pDC->GetViewportOrg();
     CSize OldViewportExt = pDC->GetViewportExt();
     CPoint OldWindowOrg = pDC->GetWindowOrg();
     CSize OldWindowExt = pDC->GetWindowExt();
     int OldMapMode = pDC->GetMapMode();

     CSize size(100, 100);          // 1cm size under MM_LOMETRIC mode
     pDC->LPtoDP(&size);          // pixels

     pDC->DPtoLP(&size);          // logical units in 1cm under current mode

     *scale = size;
Of course, I have already considered the potential loss of accuracy casting doulbe value to int in MoveTo/LineTo functions, and applied zoom factors to ensure the logical value be no less than, say, 1000.

By the way, in the above segment, if I omit the code saving and restore the stuff such as viewport/window org./ext.(because there's no change in between), their setting will not be kept in subsequent plotting, why?

Thanks in advance.

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On Windows NT/2000 windows resets the printer DC for each page. You've to set your own values.

Roshan DavisCommented:
No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:

Answered by: Pacman

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Roshan Davis
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