They are converted that way because they contain no background. After PDF v1.4 the resulting pixel on a page is the composition of all overlaying graphic objects, each of which may carry an alpha component. If there is no explicit opaque object (e.g. a white rectangle) overlaying the entire page, under all the other objects, then there is no background where there are no overlying objects. It could be that the "working" images are PDF v1.3 or have an explicit background layer. You could: adjust the PDFs at source, or paint the resulting image over a filled canvas image to obtain an image guaranteed to have a background

@mranger90 said in Save QImage from BYTE buffer segfaults ?: try: BYTE *buf = new BYTE[imWidth * imHeight]; So the issue turned out to be with the device itself. Changing the code as above and replacing the device solved the issue. Thanks.

@shwoods said in Alpha channel not written to bmp by QImage::save?: Did you ever get feedback on this? no, since i never asked anyone :) You can ask on the Qt mailing-list if you like.

Ok, I think a clean and rebuild of the whole project did the trick. I always forget when I don't use it for sometimes. When something is doing weird with Qt you have to clean, qmake or rebuild to get everything in order. I will tag this issue as solved !