Source: blenderartists.org forum
Original Author: Imperitor

Fast and great tutorial explaining how to build grass using static particles.

OK let’s start.

• Add a plane

• Object button

• Physics button

• Click the new button

Now let’s make some grass.

• Make them static. This will make them into strands instead of single particles.

• Up the amount to 5000. But this might need to be changed depending on the size of your plane. Mine was about twice the size of the default plane.

• Click the rand and even buttons. This will make the grass random but evenly spaced.

Now click the particle motion tab. It just above the static button you pushed earlier.

• Change the normal velocity to .005, this pushes the particles up making them look like grass growing up out of the ground.

• Change the random velocity to .003, this adds some randomness to the length and direction since grass is obviously not perfect.

• Make the z force -.01, this will simulate some gravity pulling the grass down.

Now it’s starting to look like grass. Your render (F12) should look something like the picture above. If it doesn’t, go back and make sure every thing is set up right.

Now for some texture.

I’m assuming you can add a material and name it grass, so do it. This is the part that will make our grass look very real.

Copy all the material settings shown below (click the strands button for the tangent shading pop up). The important stuff is the color, the strand settings, and the spec and hardness. Make sure the get the strand settings correct, they will make the blades of grass thick at the bottom and thin at the top.

Now grass isn’t all the same color, so we need some texture. I use this one.

http://img171.imageshack.us/img171/9800/grass2kl3.jpg

Add it and leave all the setting the way they are except change the color to .75

Here's an instructional picture to help you if you need it.

http://img162.imageshack.us/img162/5318/111hc3.jpg

Now if you want some ground under the grass you can add it but putting another plane or object underneath it... or go back and click the ‘mesh’ button (it's below the ‘static’ button you pushed earlier in the particle options). This will make the plane show up and it will have the texture of the grass.

And that’s it!

Common Problems & Tips:

- PUT SOME BUMPS! Grass is rarely on a flat surface. It looks way better if you subdivide your plane and add some small bumps and dips.

- If you place your light directly above the grass the strands appear very dark. Move your light to the side so that it hits the plane from an angle.

- If your grass looks very bright or white in some spots try turning down the spec and turning up the hardness. If its too dark don't turn up the spec, just make the color brighter.

- For uneven grass turn off the 'even' button in the particles menu. When the 'even' button is turned off blender will divide up the particles and give each face of the mesh an equal amount. So if your mesh has a lot of faces in a small area the grass will be denser. For better and more controlled uneven grass I suggest weight painting.

Weight Paint:

To weight paint your grass do the following:

- While in edit mode select all the vertex's on your mesh (the one with the particles) then go to the editing tab (F9). On the far left it should say 'vertex groups'. Under that are a few buttons, click the 'assign' button. NOTE: There are TWO assign buttons, the smaller one on the left is the one we want. Also note that the name of the vertex group is 'Group'. Feel free to change the name but make sure you type that name in all of the places I say to type 'Group'.

- Now in the 'particle motion' tab under the velocity settings you should see 'VGroup:', type 'Group' beside it. Doing this will cause the weight paint to control the velocity of the particles, which is the length of the grass.

- In the 'particle' tab there is another 'VGroup:' option in the bottom left corner. Type 'Group' in it. This makes the weight paint will control the density of the grass.

Now you can go to weight paint mode and paint those strands onto the ground!

Curve Guides:

To make grass that always grows 'up' instead of following the normals of your mesh I suggest using curve guides. Heres how:

- Set your particle 'normal' velocity to 0.

- Add a bezier curve. Go to edit mode and make the curve into a straight line, and align it with the z-axis.

- Now go to the fields menu and change the field to 'curve guide'. Your particles should now mimic the shape and size of the curve. If they don't change go make to the partials menu and hit the 'RecalcAll' button.

- Lastly, you can change the size and direction of that bezier curve to point the grass in any direction and make it and length.

If you really want to you can make the curve any shape you wish and the strands will make that shape. This is a common technique for making hair but can be applied to grass.