How to render/resize game scene in viewport?

Greetings,

I tried to resize/rescale game scene for projection mapping.
GEngine->…->Viewport has no methods, but only getting viewport’s sizeXY() has.

What I want to ask of How do I render or size/set game scene?
As the condition, the viewport should not be scalable, just be fixed.

But the game-scene in the viewport has to be modifiable as like translation, scaling, crop, fold so on.
Are there any cases for implementing?

Im referencing the video, one of developer did modifing game-scene or viewport.
I don’t know exactly what he did.

Here is the link, What I’m looking for.

/** https://www.youtube.com/watch?si=DunY6O-lB7P-FIl6&v=dLE1tVN60Cg&feature=youtu.be&themeRefresh=1 **/

And my result currently I did,


This is not perfect, because I rendered over the game-scene that’s not I wanted to do.
I’d like to refresh game-scene newly following my custom game-scene.

Glad to reply anyone would be happy.
Thanks.