Wrong.
Viewport uses 3D view frustrum clip planes (dx) or scissor tests (gl which does the same). There is no copy involved.
But thats the reason the image frame approach is faster. Its faster because if you use various frames of that images in one go (without changing image) the hardware can remain on that single texture and do it.
This allows you to speed up the whole stuff by not altering anything than the geometry.
Viewports on the other end always requests changes on the GPU when rendering, which costs time.
Viewport uses 3D view frustrum clip planes (dx) or scissor tests (gl which does the same). There is no copy involved.
But thats the reason the image frame approach is faster. Its faster because if you use various frames of that images in one go (without changing image) the hardware can remain on that single texture and do it.
This allows you to speed up the whole stuff by not altering anything than the geometry.
Viewports on the other end always requests changes on the GPU when rendering, which costs time.