Best place to apply a QMatrix transform to paint() event in an app

[Caolan O'Domhnaill]

Hello all,

I am working on a mathematics tutor app and so I need to transform my coordinate space into traditional Cartesian Coordinates.

I have found a solution here: http://stackoverflow.com/questions/4...painter-y-axis

which I have added to one of my QGraphicsItem object's paint() event handlers for testing but I would like to apply it at a more global scale. I looked at the widget I am using as the QGraphicsView as the place to set this however it doesn't have a paint(), just these:

virtual void 	drawBackground(QPainter * painter, const QRectF & rect)
virtual void 	drawForeground(QPainter * painter, const QRectF & rect)

virtual void 	drawBackground(QPainter * painter, const QRectF & rect)
virtual void 	drawForeground(QPainter * painter, const QRectF & rect)

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If I wanted to plug the code into my application so that EVERYTHING is drawn automagically in the coordinate system I need it to be, where is the best place to put it?

Code I have added to my QGraphicsItem:

void PhysVector::paint(QPainter *pPainter, const QStyleOptionGraphicsItem *pOption, QWidget *) {
 
    // Setting the matrix rotates the drawing area to be a normal cartesian plane. Unfortunately
    // it also means that the original drawing has to be tweaked
    QMatrix matrix;
    matrix.scale(1, -1);
    pPainter -> setMatrix(matrix);

void PhysVector::paint(QPainter *pPainter, const QStyleOptionGraphicsItem *pOption, QWidget *) {

    // Setting the matrix rotates the drawing area to be a normal cartesian plane. Unfortunately
    // it also means that the original drawing has to be tweaked
    QMatrix matrix;
    matrix.scale(1, -1);
    pPainter -> setMatrix(matrix);

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One further note I would like to make, is I would like to affect ONLY the drawing of objects, not the interaction of mouse clicks and what not. Unless they are inseparable I mean...
Thank you!
- Caolan

[Kryzon]

Is something like QGraphicsView::setTransform what you're looking for?

[Caolan O'Domhnaill]

based ont he link you gave, it transforms to view coordinates which as I understand it only affect coordinates system with respect to World and local coordinates. It does not do any kind of flip of the y-axis like the QMatrix example I used above seems to do.

I could be wrong, I am new to this, however that is my interpretation.

[Kryzon]

QTransform is a type of matrix that supports perspective transformations (including affine).
QMatrix only supports affine transformations (translation, rotation, shear and scale).

You can create a QTransform and use QTransform::scale( 1.0, -1.0 ) (to flip the vertical axis) and then assign it to the graphics view using setTransform.

Then consider your graphics scene as in cartesian coordinates: treat all coordinates as in a cartesian space. When displayed through the graphics view, the transform of the graphics view will convert from cartesian to device coordinates.

[Caolan O'Domhnaill]

GREAT! Thank you so very much!

-Caolan.

[Caolan O'Domhnaill]

Aaaaaaaand now everything in my view is upside down... *facepalm*

[Kryzon]

Here's a suggestion: translate that transform to the centre of the viewport. It should look more like a cartesian representation.

QPoint viewportCentre = viewport().rect().center();
QTransform::translate( viewportCentre.x(), viewportCentre.y() );

QPoint viewportCentre = viewport().rect().center();
QTransform::translate( viewportCentre.x(), viewportCentre.y() );

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Make sure you are specifying coordinates in "cartesian" space: +X is to the right, +Y is up.

[Caolan O'Domhnaill]

Here's a suggestion: translate that transform to the centre of the viewport. It should look more like a cartesian representation.

QPoint viewportCentre = viewport().rect().center();
QTransform::translate( viewportCentre.x(), viewportCentre.y() );

QPoint viewportCentre = viewport().rect().center();
QTransform::translate( viewportCentre.x(), viewportCentre.y() );

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Make sure you are specifying coordinates in "cartesian" space: +X is to the right, +Y is up.

I got it all to work. Thank you very much!