Conversions between twips, pixels, and millimeters using Symbian C++

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Article Metadata

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Tested with

Devices(s): S60 (All)

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Compatibility

Platform(s): S60

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Article

Keywords: TwipsToPixels, PixelsToTwips

Created: User:Technical writer 1 (10 Mar 2009)

Last edited: hamishwillee (16 Aug 2012)

Overview

The screen device in Symbian OS provides helper functions to convert a number of pixels to how much physical screen area they occupy (in twips, 1/1440 inches), and vice versa.

Detailed description

Information about how many pixels are required to draw a shape of a certain size in each device is especially useful in touch-enabled UIs, where icons and controls need to have a proper size for use with fingers.

Solution

The following methods convert between pixels and millimeters.

  #define KTwipInMillimeters 0.0176389
 
  // Returns the number of pixels corresponding 
  // to millimeters (x,y) passed as parameter
  TPoint MillimetersToPixels( const TPoint& aMm ) const
    {
    TReal twipsX = (TReal)aMm.iX / KTwipInMillimeters;
    TReal twipsY = (TReal)aMm.iY / KTwipInMillimeters;  
    return iEikonEnv->ScreenDevice()->TwipsToPixels( TPoint((TInt)twipsX,
                                                     (TInt)twipsY) );
    }
 
  // Returns the number of millimeters corresponding 
  // to pixels (x,y) passed as parameter
  TPoint PixelsToMillimeters( const TPoint& aPixels ) const
    {
    TPoint twips = iEikonEnv->ScreenDevice()->PixelsToTwips(aPixels);
    TReal mmX = (TReal)twips.iX * KTwipInMillimeters;
    TReal mmY = (TReal)twips.iY * KTwipInMillimeters; 
    return TPoint((TInt)mmX, (TInt)mmY);
    }

Note that all currently available devices have square pixel aspect ratio.