如何创建QPixmap的半透明副本？

Question

In the program I'm making, I need two images, which are exactly the same but one is translucent. For performance reasons, I want to create two separate QPixmaps instead of using just one and setting the opacity of a QPainter . Is there a straightforward way to do this?

Answer 1

Maybe you should read this example . I think you will need to use the composition mode for this purpose.

Answer 2

No, there is no performant way to do this.

To modify channels of a QPixmap it must be: Converted into a QImage , modified, converted back to a QPixmap . Depending upon your application the round trip will probably make it simpler to just do this in the QPainter : http://www.qtcentre.org/threads/51158-setting-QPixmap-s-alpha-channel

However if you could do roll this into your startup time the round trip may be reasonable, preventing repeated conversions in QPainter .

1. Convert your QPixmap to a QImage : http://doc.qt.io/qt-5/qpixmap.html#toImage
2. If you didn't have an alpha channel in your QPixmap you'll need to add one: http://doc.qt.io/qt-5/qimage.html#convertToFormat
3. Then for each pixel in your image call setPixel : http://doc.qt.io/qt-5/qimage.html#pixel-manipulation (Note that setPixel takes a QRgb . You'll need to get the red, green, and blue channels from the pixel to be modified and use these along with your desired alpha value in qRgba : http://doc.qt.io/qt-5/qcolor.html#qRgba )
4. Finally you'll need to use convertFromImage : http://doc.qt.io/qt-5/qpixmap.html#convertFromImage