My approach would be to render the background terrains as an image and then past that on a 2d image object. Save the file as name00.br5 and delete the background terrains. Use trees, shrubbery and rocks to hid the image object edge. Add several figures in one area, render and save as name01.br5. Delete those figures and add a bunch more, plop render them in, re-save as name02.br5 and so on. This should solve your memory issue.

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