Reconsider alpha channel with the QwtColorMap

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Hi,
I think the alpha channel is silently discarded when using the base class' 
virtual QwtColorMap::color.
The interpolated Qrgb quadrupel is converted to a QColor using the 
QColor::QColor(QRgb) c-tor, which in turn resets the alpha channel to opaque.

QColor::fromRgba would be more appropriate there, I suppose.

Yours,
Sven

Old message from the bug tracker:
The QwtAlphaColorMap provides an alpha channel that virtually gets munched at 
every use case except for the QwtPlotSpectogram (where scanLine is manipulated 
directly).
Although there is QwtAlphaColorMap::color() which returns a QRgb with the 
alpha channel set, it is not defined virtual so the base class' 
QwtColorMap::color() is used instead and tosses the alpha channel by using 
QColor::QColor(QRgb).
Also, the alpha channel is discarded when drawing with 
QwtPainter::drawColorBar, QwtThermo::drawLiquid and 
QwtPlotSpectroCurve::drawDots because in the respective drawing routines, the 
Rgb quadruple returned by the color map is converted to a QColor using the 
QColor::QColor(QRgb) c-tor, which silently returns opaque colors.
Maybe the base class QwtColorMap should be producing true rgba quadruples 
instead. In the respecitive use cases, instead of the above mentioned c-tor, 
use QColor::fromRgba (QColor::setRgba in QwtThermo::drawLiquid) instead.
The alpha channel comes handy for a more modern look and feel as it allows, 
e.g. the liquid or the color bar of the plot scale widget to integrate into 
the widget's background.
Yet, I am just tinkering. I cannot tell if this would have any bad impact on 
performance. In fact, Qt seems to usually draw using the (fully opaque) alpha 
channel anyway.