使用旋转的QPainter绘画将剪辑裁剪到QImage的错误区域

Question

I have a QImage that represents a blank piece of paper and a QPainter, which is used to paint onto this image. Sometimes, I will want to rotate/translate the QPainter before any paint operations, in order to paint onto this image in "Landscape" orientation.

Here is a simplified snippet of the code:

_image = new QImage(paperRect().size(), QImage::Format_RGB888);
_painter->begin(_image);

if (_orientation == QPrinter::Landscape)
{
    _painter->translate(0, _image->height());
    _painter->rotate(270);
}

// Painting operations here.

Unfortunately, this is not working as I had expected. It seems that even though the painter has been rotated, it is unaware of the "new" bounds it can paint within, thereby clipping to the "Portrait" size.

I have tried the following to no avail: Turning off clipping ( _painter->setClipping(false); ), setting a new clip rect ( _painter->setClipRect(0, 0, _image.height(), _image.width()); ), and adjusting the window and viewport in various ways.

I have looked through the documentation of QPainter and QImage, and scoured the internet, but I haven't found this particular issue discussed before.

Answer 1

As it turns out, the problem was unrelated to my posted code. Here is my solution, in case anyone runs into this issue in the future.

The problem originally came about during implementation of a custom QPrintEngine/QPaintEngine class. The code posted in the question works-- however , I had forgotten to update the QPrintEngine::property() function to return the new dimensions corresponding to the PPK_PageRect and PPK_PaperRect keys, when the orientation was set to Landscape.

Note that the QPrintEngine::metric() function does not appear to need to be updated in this way (in my project). I'm assuming this is because the metric function is mostly used when the QPrinter that utilizes this QPrintEngine implementation is used as a paint device, and that never happens in my project.

In any case, fixing this issue allows the QImage to be properly painted on "sideways".