Image 32 bit depth to 8 bit depth

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Hi,

I'm taking an image from scanner using TWAIN and I use the "QPixmap::fromWinHBITMAP(HBITMAP)" method to convert the scanned image to a QPixmap. The HBITMAP info tells me that the image is 8 bit depth but when converting to QPixmap it returns me 32 bit depth.

Any one knows how to convert this QPixmap 32 bit depth to 8 bit depth? The image is scanned on 8 bit depth gray.
Thanks,