Rotating QRect using QTransform and Cropping a QImage with it

[starkm42]

Hi,

i am trying to achieve the following, having a QRect of some size, i rotate it at its center and crop a QImage like below:

QRect cropRect(0, 0, height, width);
QTransform transRect;
QPoint centre = cropRect.Centre();
transRect.translate(center.x(), center.y());
tansRect.rotate(60.0);
tansRect.translate(-centre.x(), -center.y());

cropRect = transRect.mapRect(cropRect);

QImage croppedImage = originalImage.copy(cropRect);

well i figured out using the hard way that tansRect.rotate(60.0); does not exactly work like required here. it does rotate rectangle along center but the resulting rectangle is a bounding box of the rectangle that is rotated.

one workaround was using a polygon

QPolygon transformedPoly = transRect.mapToPolygon(cropRect);

but all my attempts to get a QRect out of this polygon has gone in vain.

a failed attempt

        cropRectangle.setTopLeft(transformedPoly.at(3));
        cropRectangle.setTopRight(transformedPoly.at(2));
        cropRectangle.setBottomRight(transformedPoly.at(1));
        cropRectangle.setBottomLeft(transformedPoly.at(0));

any pointers are appreciated, thank !

[ChrisW67]

@starkm42

What are you expecting to get out of this process? A QImage is a rectangular area aligned with the axes, but you seem to be expecting something else.

I think what you want to do is rotate the image 60 degrees, crop it using a rectangle, and then maybe rotate the result back.

BTW: It would help if you posted (copy and paste) your actual code. That first snippet will not compile; there is no QRect::Centre() or variable tansrect.

[starkm42]

@ChrisW67

A QImage is a rectangular area aligned with the axes, but you seem to be expecting something else.

yes, i realized that.

I think what you want to do is rotate the image 60 degrees, crop it using a rectangle, and then maybe rotate the result back.

well that could also be done i guess, let me check and get back to you.

It would help if you posted (copy and paste) your actual code. That first snippet will not compile; there is no QRect::Centre() or variable tansrect.

i corrected it, please check the updated snippet.

Thanks !

[starkm42]

@ChrisW67 the actual code is not of a sample program and has un-necessary application logic which i feel is not required here. do let me know if there is anything else i can provide here.

Thanks.

[ChrisW67]

@starkm42 said in Rotating QRect using QTransform and Cropping a QImage with it:

if there is anything else i can provide here

Yes. You can explain what you are expecting to achieve. What should the final image look like in relation to the original?

[starkm42]

@ChrisW67 need to crop a image from a large image, QImage.copy() accepts a QRectangle object (equivalent to cropping a section), now to get the rectangle we need to rotate one around the center at X, Y from origin where (X< OriginalImage.Width, Y < OriginalImage.Height and X + CropRectangle.Width() < OriginalImage.Width , Y + CropRectangle.Height < OriginalImage.Height).

something like below

[SimonSchroeder]

Here is an idea you can try: Instead of a QImage use QPixmap. You can use a QPainter to draw to a pixmap. Now draw the original image rotated in the opposite direction onto the pixmap. From this rotated pixmap you can now do your cropping with a non-rotated rectangle.

[starkm42]

@SimonSchroeder so rotating the painter itself and later cropping, yes i think it was suggested as well. and yes that seems to work for now. will be marking this solved.