I have to make an animation, and I tried to use grass made with particles, but if I use to many planes with grass, Blender is running very slow, but very, very slow. What can I do? I need the grass in my model.
Well, uhh, you’re just going to need to find another way to do that grass.
I stumbled upon a rather interesting Photoshop tutorial at http://www.myinkblog.com/2008/06/11/create-an-awesome-grass-texture-in-photoshop/ which looked to me like it might be something you could do something with.
In any case, and especially with an animation, you are just going to need to find a way to produce “something that looks acceptably like grass,” but do it on the cheap.
For instance… what if you found an image texture of grass, cut out a few small squares of it, then put it onto a transparent square plane as a image-texture with reduced Alpha. Now, build a grid and DupliVert a bunch of copies of that image-textured square. Rotate them (they’ll all rotate as one) to face the camera. With a little twiddling, it just might look enough like grass… with some depth to it… from an oblique camera angle.
go to your modifiers stack and find the particle system in the modifiers stack. Find the option (I forget what it looks like) to disable the particles from being displayed in the 3d viewport.
Blender should run faster now. If rendering is too slow then I suggest going to the particles option, turning the number of particles down, and finding the children tab, enable children from faces and play with those options. For grass, if it is short grass you can turn the segments down from 5 (default) to 2 or 3.
Also if you find the steps option(under visualization), you can set steps to 1, this will speed up the display in blender when rotating the scene and lowering render steps (just below steps) to something like 1 or 2 will considerable drop the render times also.
If you turn on Strand render (also in the particles option settings) and children child simplification will appear.
Sorry if your using 2.5 because those instructions were for 2.49. I gathered you just wanted to speed up blender so disabling drawing particles in the 3d viewport from the modifiers stack should help there, but just in case rendering is a bitch, play with the methods I told you about for rendering the grass particles.