Find color of QRect

[tim-hilt]

Can I find the color of a given QRect?

Please excuse me, if this is not how it generally works; I'm just starting out with Qt.

[JonB]

@tim-hilt
A QRect does not have a color, it is just the coordinates of a rectangle. Are you drawing/filling a rectangle somewhere?
https://www.bogotobogo.com/Qt/Qt5_QBrush_QRect.php explains rectangles and painting, might be of interest.

[tim-hilt]

thanks! I'll take a look at the link you provided.

[tim-hilt]

@JonB is it right if I assume, that once an element is rendered, there is no way to get it's color again, if I'm not responsible for coloring it?

I used QScreen::grabWindow to get the color of a window but that only works if the window is not covered by another element. Is there a possibility to get colors for something on screen, even if it is covered? Somehow Qt has to know what to render, once the covering window is gone, right?

[jeremy_k]

In some cases it is possible to get the pixel value of an element. The mechanism is going to depend on what that element is. For example, QWidget::grab() facilitates retrieving a QPixMap, which can be converted to a QImage. The color of a pixel is retrievable via QImage::pixelColor(). That color isn't going to take into account any blending the windowing system is doing for transparency.

[Kent-Dorfman]

haven't looked because I had no reason to use it, but some graphics APIs have a ValueOfPixel() function to get the attributes of an on-screen pixel. Seems of limited use though, since it is assumed that if you put down a pixel in the first place then you should know how you rendered it. FWIW, if your QPainter is rendering to a pixmap/bitmap then you should be able to get the rendered value from that backing.