When I render the viewport in a flipbook, the Mplay Image Viewer app automatically opens, and the background is the light background color, which is the blue to green gradient.

But if I open the Mplay Image Viewer app from the start menu, and if I load the same sequence, the background is black. And it's difficult to check all details on a transparent object with this black background.

Can I put this light background as part of the rendered image?

Press 'd' in a flipbook and see if any of the options do what you're after. You can set bg image or color for example.

Hey Soothsayer, thanks for the help. The setting in the background tab > background color applies to the area around the rendered image, but not to the image itself. I don't see any option that would help me in this Display Options dialog.
Same thing in the Render Flipbook dialog.

How odd, I could have sworn I did that before.

I'm unable to reproduce your issue on H18.5.499 - every Flipbook export test I do displays the UI background in Mplay. What format are you exporting? Do you have a simple example you can upload?

Have you done what I say in my first post?
"if I open the Mplay Image Viewer app from the start menu, and if I load the same sequence, the background is black."
The image format I use is PNG.