Using OpenPDF library I can scan through document objects, see images and even extract these images. However, I have not found a way to check if an image is transparent. The information is not within an individual image but rather in relationship of two images or layers I assume. But how to check this out?

I don't need to flatten transparency (I know there are tools for it), just identifying is enough. The big picture is about printing a PDF and there transparent images are not supported by the service provider. I'm trying to identify PDF documents containing transparent images and reject printing.

Any hint appreciated