Use window/viewport to flip QPainter y-axis

I'm using Qt 4.7 QPainter to draw some polygons, etc into a widget. I am hoping to alter the coordinate system so that (0,0) is at the center of my widget, and the x/y axis behave in a standard "Cartesian" way (ie. y increases going "up" and decreases going "down"). In other words, I want the coordinates to be "math"-like not "computer graphics"-like, if you know what I mean. :-)

I'm trying to do this using setViewport() and setWindow() rather than do the math myself, as it would be nice to be able to just call the draw methods directly with my coordinates.

Here's what I've got so far:

// Setup coordinates
double screenWidth = width();
double screenHeight = height();

double windowWidth = 100.0;
double windowHeight = (screenHeight / screenWidth) * windowWidth;

painter.setViewport(0, 0, screenWidth, screenHeight);
painter.setWindow(-(windowWidth / 2.0), -(windowHeight / 2.0), windowWidth, windowHeight);

// Draw stuff
painter.drawRect(-10, -10, 20, 20);

Now this works just fine, in that it draws a nice blue square in the middle of the screen. The problem is, I have to say that the upper left corner is (-10, -10). I'd like to be able to make it (-10, 10), as that is what it would be in Cartesian coords.

I tried messing with setWindow/setViewport to get this "y-axis flip", but to no avail. This seems like a really easy/basic thing to do, but after scouring the Qt docs and the web, I can't figure it out!

Thanks, Chris


Use class QMatrix. It specifies 2D transformations. QMatrix is set to QPainter.

But remember, in your case, if you convert your widget's coords to Cartesian coords, you will have to put first point at (-10,-10) (not at (-10,10) as you did mentioned) to draw a rect, which has center at (0,0), because Y-axis now grows up and X-Axis now grows right.

All you need is to transform your coord system this way:

  • translate origin from (0,0) to the middle of the widget:

  • scale Y-axis by -1 factor:

Here is the code, typed in paintEvent() function of a widget:

QPainter pn( this );

int w_2 = width() / 2;
int h_2 = height() / 2;

{ // X- and Y-Axis drawing
    pn.setPen( Qt::blue );
    pn.drawLine( 0, h_2, width(), h_2);     // X-Axis
    pn.drawLine( w_2, 0 , w_2, height() );  // Y-Axis

QMatrix m;
m.translate( w_2, h_2 );
m.scale( 1, -1 );

pn.setMatrix( m );
pn.setPen( Qt::NoPen );
pn.setBrush( QBrush( Qt::blue, Qt::Dense4Pattern ) );
pn.drawRect( -10, -10, 20, 20 );


update apr 07, 2014

This question was asked a long time ago and many things have changed since. For those asking themselves the same question today (beginnings of 2014) then my personal answer is that since Qt 4.3 it is possible to solve problem with text flipping more easier.

You are right. Text also gets filpped because it is drawn with the same painter. You can draw text at the end, when all flipped drawings are done, if it is possible. This method is not convinient because of new calculations of texts position. Also you will need to drop settings for painter. Now I would recommend you to use QGraphicsView, because of huge support of 2D painting. Also for each QGraphicsItem ItemIgnoresTransformations flag can be set, which allows it to ignore inherited transformations (i.e., its position is still anchored to its parent, but the parent or view rotation, zoom or shear transformations are ignored). This flag is useful for keeping text label items horizontal and unscaled, so they will still be readable if the graphics view is transformed

The above answer will also flip text, "p" will be "b". To avoid that you have to flip back the y-axis before text is drawn, and you have to change sign on y-coord for the text position when you draw it. This is a little bit ugly I think, or is there a better way?

As stated above, drawing text also appears flipped upside down. There is an easy solution to it, see below. We will temporary disable the world transform for the text drawing. Note that text is not scaled anymore.

in your painting code we want to draw text on coordinate QPointF P;

Painter pn( this );

// calculate the point with the transform
QPointF p = pm.transform().map(P);

// Disable Transform temporary

// draw it ordinary, no scaling etc
pn.drawText(p, QString("HI FRIENDS!"));

// Enable the transform again

I needed to flip the y-axis in order to paint lines and polygons using Qt from points defined in Java coordinates. I imagine others will need to do this in porting from Java to Qt coordinate systems. The discussion above was helpful. My solution was:

painter.scale(1.0, -1.0);

and then proceed to draw the lines and polygons.

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