Problems with QPainterPath::arcTo()

[brcain]

I’m hoping someone might shed some light as to why I’m having a problem with QPainterPath::arcTo() used to draw the inner pie boundary in the image below.
The inner and outer arcs have the same start angle and sweep length, but they don't render that way. Why?
pieIssue.jpg

The pie shapes are drawn with:
QPainterPath::arcTo(const QRectF & rectangle, qreal startAngle, qreal sweepLength)

The method is giving unexpected results with the startAngle and sweepLength.

Looking at the image, I expect the inner arc to begin (and end) coincident with the outer arc, but it doesn’t. The center, startAngle, and sweepLength are the same for both.

Note, I computed the desired arc start/end point (shown in the red circle as a yellow square).

void GraphicsPieFramed::createPaths()
{
   // Clear the paths.
   mOutlinePath = QPainterPath();
   mFacePath = QPainterPath();
 
   mMaxelSize = SCENE_SCALE*maxelDiameter(printHeadDiameter(), mVoltage);
 
   double span = -normAngle360(mAngleEnd-mAngleBegin);
 
   mOutlinePath.addRect(-0.5*mSize.width(), -0.5*mSize.height(), mSize.width(), mSize.height());
 
   // outer pie
   QPainterPath outerPie;
   outerPie.moveTo(QPointF(0.0, 0.0));
   outerPie.arcTo(QRectF(-0.5*mSize.width(), -0.5*mSize.height(), mSize.width(), mSize.height()),
      -mAngleBegin, span);
   outerPie.closeSubpath();
   mFacePath.addPath(outerPie);
 
   // inner pie
   QPainterPath innerPie;
   innerPie.moveTo(QPointF(0.0, 0.0));
   innerPie.arcTo(QRectF(-0.5*mSize.width()+mFrameWidth, -0.5*mSize.height()+mFrameHeight,
      mSize.width()-2.0*mFrameWidth, mSize.height()-2.0*mFrameHeight),
      -mAngleBegin, span);
   innerPie.closeSubpath();
   mFacePath.addPath(innerPie);
 
   if(mFrameEnabled && mFrameWidth < 0.5*mSize.width() && mFrameHeight < 0.5*mSize.height())
   {
      mFacePath.addPath(innerPie);
   }
 
   mFacePath.addEllipse(QPointF(0.0, 0.0), 3.0, 3.0);  // Center location
 
   createMaxelPaths();
   createResizeHandlePaths();
   createRotateHandlePath();
}

void GraphicsPieFramed::createPaths()
{
   // Clear the paths.
   mOutlinePath = QPainterPath();
   mFacePath = QPainterPath();

   mMaxelSize = SCENE_SCALE*maxelDiameter(printHeadDiameter(), mVoltage);

   double span = -normAngle360(mAngleEnd-mAngleBegin);

   mOutlinePath.addRect(-0.5*mSize.width(), -0.5*mSize.height(), mSize.width(), mSize.height());

   // outer pie
   QPainterPath outerPie;
   outerPie.moveTo(QPointF(0.0, 0.0));
   outerPie.arcTo(QRectF(-0.5*mSize.width(), -0.5*mSize.height(), mSize.width(), mSize.height()),
      -mAngleBegin, span);
   outerPie.closeSubpath();
   mFacePath.addPath(outerPie);

   // inner pie
   QPainterPath innerPie;
   innerPie.moveTo(QPointF(0.0, 0.0));
   innerPie.arcTo(QRectF(-0.5*mSize.width()+mFrameWidth, -0.5*mSize.height()+mFrameHeight,
      mSize.width()-2.0*mFrameWidth, mSize.height()-2.0*mFrameHeight),
      -mAngleBegin, span);
   innerPie.closeSubpath();
   mFacePath.addPath(innerPie);

   if(mFrameEnabled && mFrameWidth < 0.5*mSize.width() && mFrameHeight < 0.5*mSize.height())
   {
      mFacePath.addPath(innerPie);
   }

   mFacePath.addEllipse(QPointF(0.0, 0.0), 3.0, 3.0);  // Center location

   createMaxelPaths();
   createResizeHandlePaths();
   createRotateHandlePath();
}

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void GraphicsPieFramed::createResizeHandlePaths()
{
   // Clear the paths.
   mResizeLeftPath = QPainterPath();
   mResizeRightPath = QPainterPath();
   mResizeBottomPath = QPainterPath();
   mResizeTopPath = QPainterPath();
   mResizeBottomLeftPath = QPainterPath();
   mResizeBottomRightPath = QPainterPath();
   mResizeTopLeftPath = QPainterPath();
   mResizeTopRightPath = QPainterPath();
   mResizeAngleBeginPath = QPainterPath();
   mResizeAngleEndPath = QPainterPath();
   mResizeFrameWidthPath = QPainterPath();
   mResizeFrameHeightPath = QPainterPath();
 
   double const sizeHandleHalf(0.5*SIZE_HANDLE);
   double const sizeHandleHalfDiag(sizeHandleHalf*sqrt(2.0));
   double const left(-0.5*mSize.width());
   double const right(0.5*mSize.width());
   double const bottom(-0.5*mSize.height());
   double const top(0.5*mSize.height());
 
   // left
   mResizeLeftPath.addRect(left-sizeHandleHalf, -sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // right
   mResizeRightPath.addRect(right-sizeHandleHalf, -sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // bottom
   mResizeBottomPath.addRect(-sizeHandleHalf, bottom-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // top
   mResizeTopPath.addRect(-sizeHandleHalf, top-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
 
   // bottom-left
   mResizeBottomLeftPath.addRect(left-sizeHandleHalf, bottom-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // bottom-right
   mResizeBottomRightPath.addRect(right-sizeHandleHalf, bottom-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // top-left
   mResizeTopLeftPath.addRect(left-sizeHandleHalf, top-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // top-right
   mResizeTopRightPath.addRect(right-sizeHandleHalf, top-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
 
   // begin angle
   double xBegin = 0.5*mSize.width()*cos(DEG2RAD*mAngleBegin);
   double yBegin = 0.5*mSize.height()*sin(DEG2RAD*mAngleBegin);
   mResizeAngleBeginPath.addEllipse(xBegin-sizeHandleHalfDiag, yBegin-sizeHandleHalfDiag,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
   // begin angle inner frame
   double lenBegin = sqrt(xBegin*xBegin + yBegin*yBegin);
   double xBeginPart = mFrameWidth*cos(DEG2RAD*mAngleBegin);
   double yBeginPart = mFrameHeight*sin(DEG2RAD*mAngleBegin);
   double lenBeginPart = sqrt(xBeginPart*xBeginPart + yBeginPart*yBeginPart);
   double x2Begin = interp(xBegin, 0.0, lenBeginPart/lenBegin);
   double y2Begin = interp(yBegin, 0.0, lenBeginPart/lenBegin);
   mResizeAngleBeginPath.addRect(x2Begin-sizeHandleHalf, y2Begin-sizeHandleHalf, 2.0*sizeHandleHalf, 2.0*sizeHandleHalf);
 
   // end angle
   double xEnd = 0.5*mSize.width()*cos(DEG2RAD*mAngleEnd);
   double yEnd = 0.5*mSize.height()*sin(DEG2RAD*mAngleEnd);
   mResizeAngleEndPath.addEllipse(xEnd-sizeHandleHalfDiag, yEnd-sizeHandleHalfDiag,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
   // end angle inner frame
   double lenEnd = sqrt(xEnd*xEnd + yEnd*yEnd);
   double xEndPart = mFrameWidth*cos(DEG2RAD*mAngleEnd);
   double yEndPart = mFrameHeight*sin(DEG2RAD*mAngleEnd);
   double lenEndPart = sqrt(xEndPart*xEndPart + yEndPart*yEndPart);
   double x2End = interp(xEnd, 0.0, lenEndPart/lenEnd);
   double y2End = interp(yEnd, 0.0, lenEndPart/lenEnd);
   mResizeAngleEndPath.addRect(x2End-sizeHandleHalf, y2End-sizeHandleHalf, 2.0*sizeHandleHalf, 2.0*sizeHandleHalf);
 
   // frame width
   mResizeFrameWidthPath.addEllipse(left-sizeHandleHalfDiag+mFrameWidth, top-sizeHandleHalfDiag,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
 
   // frame height
   mResizeFrameHeightPath.addEllipse(left-sizeHandleHalfDiag, top-sizeHandleHalfDiag-mFrameHeight,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
}

void GraphicsPieFramed::createResizeHandlePaths()
{
   // Clear the paths.
   mResizeLeftPath = QPainterPath();
   mResizeRightPath = QPainterPath();
   mResizeBottomPath = QPainterPath();
   mResizeTopPath = QPainterPath();
   mResizeBottomLeftPath = QPainterPath();
   mResizeBottomRightPath = QPainterPath();
   mResizeTopLeftPath = QPainterPath();
   mResizeTopRightPath = QPainterPath();
   mResizeAngleBeginPath = QPainterPath();
   mResizeAngleEndPath = QPainterPath();
   mResizeFrameWidthPath = QPainterPath();
   mResizeFrameHeightPath = QPainterPath();

   double const sizeHandleHalf(0.5*SIZE_HANDLE);
   double const sizeHandleHalfDiag(sizeHandleHalf*sqrt(2.0));
   double const left(-0.5*mSize.width());
   double const right(0.5*mSize.width());
   double const bottom(-0.5*mSize.height());
   double const top(0.5*mSize.height());

   // left
   mResizeLeftPath.addRect(left-sizeHandleHalf, -sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // right
   mResizeRightPath.addRect(right-sizeHandleHalf, -sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // bottom
   mResizeBottomPath.addRect(-sizeHandleHalf, bottom-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // top
   mResizeTopPath.addRect(-sizeHandleHalf, top-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);

   // bottom-left
   mResizeBottomLeftPath.addRect(left-sizeHandleHalf, bottom-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // bottom-right
   mResizeBottomRightPath.addRect(right-sizeHandleHalf, bottom-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // top-left
   mResizeTopLeftPath.addRect(left-sizeHandleHalf, top-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);
   // top-right
   mResizeTopRightPath.addRect(right-sizeHandleHalf, top-sizeHandleHalf, SIZE_HANDLE, SIZE_HANDLE);

   // begin angle
   double xBegin = 0.5*mSize.width()*cos(DEG2RAD*mAngleBegin);
   double yBegin = 0.5*mSize.height()*sin(DEG2RAD*mAngleBegin);
   mResizeAngleBeginPath.addEllipse(xBegin-sizeHandleHalfDiag, yBegin-sizeHandleHalfDiag,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
   // begin angle inner frame
   double lenBegin = sqrt(xBegin*xBegin + yBegin*yBegin);
   double xBeginPart = mFrameWidth*cos(DEG2RAD*mAngleBegin);
   double yBeginPart = mFrameHeight*sin(DEG2RAD*mAngleBegin);
   double lenBeginPart = sqrt(xBeginPart*xBeginPart + yBeginPart*yBeginPart);
   double x2Begin = interp(xBegin, 0.0, lenBeginPart/lenBegin);
   double y2Begin = interp(yBegin, 0.0, lenBeginPart/lenBegin);
   mResizeAngleBeginPath.addRect(x2Begin-sizeHandleHalf, y2Begin-sizeHandleHalf, 2.0*sizeHandleHalf, 2.0*sizeHandleHalf);

   // end angle
   double xEnd = 0.5*mSize.width()*cos(DEG2RAD*mAngleEnd);
   double yEnd = 0.5*mSize.height()*sin(DEG2RAD*mAngleEnd);
   mResizeAngleEndPath.addEllipse(xEnd-sizeHandleHalfDiag, yEnd-sizeHandleHalfDiag,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
   // end angle inner frame
   double lenEnd = sqrt(xEnd*xEnd + yEnd*yEnd);
   double xEndPart = mFrameWidth*cos(DEG2RAD*mAngleEnd);
   double yEndPart = mFrameHeight*sin(DEG2RAD*mAngleEnd);
   double lenEndPart = sqrt(xEndPart*xEndPart + yEndPart*yEndPart);
   double x2End = interp(xEnd, 0.0, lenEndPart/lenEnd);
   double y2End = interp(yEnd, 0.0, lenEndPart/lenEnd);
   mResizeAngleEndPath.addRect(x2End-sizeHandleHalf, y2End-sizeHandleHalf, 2.0*sizeHandleHalf, 2.0*sizeHandleHalf);

   // frame width
   mResizeFrameWidthPath.addEllipse(left-sizeHandleHalfDiag+mFrameWidth, top-sizeHandleHalfDiag,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);

   // frame height
   mResizeFrameHeightPath.addEllipse(left-sizeHandleHalfDiag, top-sizeHandleHalfDiag-mFrameHeight,
      2.0*sizeHandleHalfDiag, 2.0*sizeHandleHalfDiag);
}

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Thanks!

[wysota]

I would guess the rectangle you pass to the inner arcTo() call is incorrect.

[brcain]

Shouldn't a smaller (same center) rectangle and same start/sweep angle just imply a smaller arc radii? The start angle should still be the same (and coincident), right?

Thanks for the suggestion. I will look.

[wysota]

The rectangle serves as the bounding box for the arc thus it matters where the sides of the box are and not where the centre is. The arc will begin at one side of the rectangle and end at the other side. You might want to add drawing the box to your code to see how the arcs are drawn related to their corresponding boxes.