Obsolete Members for QPixmap

QPixmap QPixmap::alphaChannel () const

Returns the alpha channel of the pixmap as a new grayscale QPixmap in which each pixel's red, green, and blue values are given the alpha value of the original pixmap. The color depth of the returned pixmap is the system depth on X11 and 8-bit on Windows and Mac OS X.

You can use this function while debugging to get a visible image of the alpha channel. If the pixmap doesn't have an alpha channel, i.e., the alpha channel's value for all pixels equals 0xff), a null pixmap is returned. You can check this with the isNull() function.

We show an example:

         pixmap = QPixmap(100, 100);
         pixmap.fill(Qt::transparent);

         QRadialGradient gradient(50, 50, 50, 50, 50);
         gradient.setColorAt(0, QColor::fromRgbF(1, 0, 0, 1));
         gradient.setColorAt(1, QColor::fromRgbF(0, 0, 0, 0));
         QPainter painter(&pixmap);
         painter.fillRect(0, 0, 100, 100, gradient);

         channelImage = pixmap.alphaChannel();
         update();

Warning: This is an expensive operation. The alpha channel of the pixmap is extracted dynamically from the pixeldata. Most usecases of this function are covered by QPainter and compositionModes which will normally execute faster.

See also setAlphaChannel() and Pixmap Information.