Ok, so by applying the billboarding algo (see also [Solved] Unable to produce file with writeImageFile - #21 by shagros ), with everything done in GPU (rotation and change of texture depending on distance) I managed to get a x2 to x3 improvement speed
Now I still need to work on a few things
For example, the lighting is quite off (and not implemented at all in the shader), and actually i think the issue comes also from the main trees rather than the billboard. Trees don’t reflect light … I’m going to try using a colorRamp with almost no shadow, so that the trees are more sensitive to ambient light vs sun light.
Then I need to fine tune the swap itself, so that trunks get properly positioned when the swap occurs
@pspeed and @The_Leo thanks a lot for your help and patience! And feel free to comment