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qtmathgeometryperspectivecoordinate-transformation

Create QTransform given 4 points defining the transformed unit square

Given 4 points being be the result of

QPolygon poly = transform.mapToPolygon(QRectF(0, 0, 1, 1));

how can I find QTransform transform? (Even better: also given an arbitrary source rectangle)

Motivation: Given the four corner points of an image to be drawn in a perspectively distorted coordinate system, how can I draw the image using QPainter?

Illustration of the problem

This is a screenshot illustrating the problem in GIMP, where one can transform a layer by moving around the 4 corners of the layer. This results in a perspective transformation. I want to do exactly the same in a Qt application. I know that QTransform is not restricted to affine transformations but can also handle perspective transformations.

Solution

  • You should be able to do this with QTransform.squareToQuad. Just pass it the QPolygonF you want to transform to.

    I've sometimes had some issues getting squareToQuad to do what I want, and have had to use QTransform.quadToQuad instead, defining my own starting quad, but you might have more luck.