one way to make 2d forest to fill the background is to use "RDNA Fields - Volumetric Field Figure" scaled up and replace the grass textures with rendered trees. I did 3 setups of square renders with trees and shrubs i have and rendered from the front camera. saved with alpha channels. pulled up the alpha channels in paintshop pro and made the trans maps form them. This allows me to put my volumetric forest with my tree models and have them look similar.

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