I made a demo about fish swimming in the marine,but the fish can swim through the terrain.I use the function c_trace() to detect the surroundings and vec_add to move to the place ahead.I didn't use c_move(),and when I use it，the fish aslo can swim through the terrain.Why?

Go into the properties of the terrain in WED and check "polygon" flag or write a simple script and apply it to the terrain like this

action poly_on
{
my.polygon = on;
}

..and then run your level again to check results...

Yes,I didin't select the "polygon",and if I select it the fish won't swim through the terrain.Thank you!
But if I select it,whether the fps will decrease?
And there are other models in the level,the player aslo can walk through
them.When I select the "polygon",it won't.But there are so many models,if I all select the "polygon",whether the program execute very slowly?