This is my third blogpost for SoK 2022.

If you have been following my earlier blogposts, you would know that I’ve been working on adding the Perspective Ellipse Assistant Tool in Krita.

My Progress so far

Created a Merge Request

Here’s the link to my Merge Request, you can check if you want to!

Creating the Insrcibed Ellipse

Two weeks back this is what my tool looked like.


Then I spent almost a week scratching my head over the mathematics required to transform a circle inscribed inside a unit square to an ellipse inscribed inside my quadrilateral. Dmitry suggested a nice tutorial on projection matrices and geomtery. Going through a few lessons made all the mathematical code less intimidating.

Since I already had a four cornered assistant, all I needed to do was to add it’s inscribed ellipse.

This is roughly what’s done:

  • The transformation matrix is set for the canvas.
  • A unit square is added to path, which on being drawn on the canvas gets converted to the transformed quadrilateral.
  • The inscribed circle for the unit square, as expected, gets converted to the inscribed ellipse for our quadrilateral.

A bit of code…

Here’s a little bit of code demonstrating the above procedure. Anyone not familiar with Krita’s codebase can skip directly to the animation!

First, inside the PerspectiveEllipseAssistant::drawAssistant(), the transformation matrix of the canvas is set using:


Now, the following code draws a valid four cornered assistant.

QPainterPath path;
        for (int y = 0; y <= 1; ++y)
            QLineF line = QLineF(QPointF(0.0, y), QPointF(1.0, y));
            KisAlgebra2D::cropLineToRect(line, gc.window(), false, false);
        for (int x = 0; x <= 1; ++x)
            QLineF line = QLineF(QPointF(x, 0.0), QPointF(x, 1.0));
            KisAlgebra2D::cropLineToRect(line, gc.window(), false, false);

And finally, once I draw the circle inside the unit square, I have my Perspective Ellipse! All I had to do was to add this piece of code:

path.addEllipse(QPointF(0.5, 0.5), 0.5, 0.5); 

And here’s a small animation demonstrating the current Perspective Ellipse Assistant Tool!


Next Steps

I would spend the next few weeks working on drawing and snapping related code. That should hopefully complete my assistant!