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Interpolating points between drag events using Symbian C++

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Article Metadata
Code ExampleTested with
Devices(s): Nokia 5800 XpressMusic
Platform(s): S60 5th Edition
Keywords: Interpolation, TPointerEvent::EDrag
Created: tepaa (21 Oct 2008)
Last edited: lpvalente (23 Sep 2014)



Interpolation is a method of constructing new data points within the range of a discrete set of known data points.

When dragging an item on the screen and that item is updated after receiving a TPointerEvent::EDrag event, it moves remarkably. The reason is that the dragging points received by the application are quite far from each other. The following interpolation calculates the missing points between two TPointerEvent::EDrag events and draws the item also in them.

NOTE: The application has an Interpolation menu item which you can use to enable/disable interpolation and see the difference. Interpolation is disabled by default.

This snippet can be self-signed.

MMP file

The following capabilities and libraries are required:

CAPABILITY None       
LIBRARY avkon.lib

Header file

#include <coecntrl.h>
class CExampleStubAppView : public CCoeControl
void EnableInterpolation(TBool aEnable);
void StartDrawingL();
void StopDrawing();
static TInt DoDraw(TAny* aObj);
void HandlePointerEventL(const TPointerEvent&
// Interpolation is calculated here
void Move(const TPoint& aPoint);
CPeriodic* iPeriodic;
TRect iPictureRect;
TPoint iPicturePoint;
TPoint iStylusPoint;
TBool iDragging;
TBool iInterpolate;

Source file (consts for interpolation)

const TReal32 KSPEED = 8;           // Moving speed
const TInt DELAY = 30000; // = 33 fps (Frame/Seconds)
const TReal KFRAMETIME = 0.03030; // 1/33 fps

Source file (general)

void CExampleStubAppView::ConstructL( const TRect& aRect )
// Start updating screen in 33 fps speed
void CExampleStubAppView::Draw( const TRect& /*aRect*/ ) const
CWindowGc& gc = SystemGc();
TRect drawRect( Rect());
gc.Clear( drawRect );
if (iDragging)
void CExampleStubAppView::StartDrawingL()
if (!iPeriodic)
iPeriodic = CPeriodic::NewL(CActive::EPriorityIdle);
DELAY, TCallBack(CExampleStubAppView::DoDraw, this));
void CExampleStubAppView::StopDrawing()
TInt CExampleStubAppView::DoDraw(TAny* aObj)
return 1;
void CExampleStubAppView::HandlePointerEventL(const TPointerEvent&
if (aPointerEvent.iType == TPointerEvent::EButton1Down)
iPicturePoint = aPointerEvent.iPosition;
iDragging = ETrue;
else if(aPointerEvent.iType == TPointerEvent::EDrag)
else if(aPointerEvent.iType == TPointerEvent::EButton1Up)
iDragging = EFalse;
void CExampleStubAppView::EnableInterpolation(TBool aEnable)
iInterpolate = aEnable;

Source file (actual interpolation logic)

void CExampleStubAppView::Move(const TPoint& aPoint)
// No interpolation
if (!iInterpolate)
TPoint offset;
offset.iX = aPoint.iX - iPictureRect.iTl.iX;
offset.iY = aPoint.iY - iPictureRect.iTl.iY;
// Use interpolation
// Stylus is here
iStylusPoint = aPoint;
TBool move = EFalse;
// Calculate picture smooth drawing to stylus position (interpolate)
if (iPicturePoint.iY < iStylusPoint.iY)
iPicturePoint.iY += ((iStylusPoint.iY - iPicturePoint.iY) * KSPEED) * KFRAMETIME;
move = ETrue;
if (iPicturePoint.iY > iStylusPoint.iY)
iPicturePoint.iY -= ((iPicturePoint.iY - iStylusPoint.iY) * KSPEED) * KFRAMETIME;
move = ETrue;
if (iPicturePoint.iX < iStylusPoint.iX)
iPicturePoint.iX += ((iStylusPoint.iX - iPicturePoint.iX ) * KSPEED) * KFRAMETIME;
move = ETrue;
if (iPicturePoint.iX > iStylusPoint.iX)
iPicturePoint.iX -= ((iPicturePoint.iX - iStylusPoint.iX) * KSPEED) * KFRAMETIME;
move = ETrue;
if (move)
iPictureRect = TRect(iPicturePoint,TSize(10,10));


The object is moving smoothly on the screen when dragging it with the stylus.

See also

Full example source code: File:Interpolation

Interpolation in

This page was last modified on 23 September 2014, at 19:58.
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