[Qt-interest] Bitmaps and coloring thresholds

[BRM]

I have a bitmap that uses an 8-bit per pixel index (QImage::Format_Indexed8) where I map out a 256 color grey scale.
The image is then converted to a QPixMap and put into a QGraphicsScene.

I want to set threshold values. Presently I do this on the image as I build it by using the QImage::setColorTable() with a table initialized how I like.
This works very well so long as I am creating and loading new images.

However, I'd like to be able to modify the color table on an already displayed QPixMap.
I know I could call QPixMap::toImage(), rebuild the color table, and do another QPixMap::fromImage() on it; however, that seems rather inefficient.

Is there a more efficient way to do this?

TIA,

Ben