Fonte: blenderartists.org forum
Autor Original: Imperitor

Tutorial rápido e útil explicando como construir grama utilizando partículas.

OK vamos começar.

• Adicione um plane

• Botão Object

• Botão Physics

• Clique no botão New

Vamos criar alguma grama.

• Escolha static. Isto vai fazer as partículas em "strands" ao invés de partículas sozinhas.

• Ajuste para a quantidade de 5000. Pode ser ajustado de acordo com o tamanho do seu plano.

• Clique nos botões rand e even. Isto fará com que a grama seja randomicamente espaçada.

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.

Agora está começando a parecer grama. Seu render (F12) deve parecer alguma coisa com a figura acima. Se não, volte e tenha certeza que tudo foi ajustado corretamente.

Agora um pouco de textura.

Estou levando em consideração que você pode adicionar um material e nomeá-lo "grass", então faça isso. Esta é a parte que faremos nossa grama parecer muito real.

Copie todas as configurações de material abaixo (clique no botão strands para aparecer a janela popup). Estas coisas importantes são a cor, os ajustes de strand, e specular e hardness. Tenha certeza que os ajustes de Strand estão corretos, eles irão fazer lâminas de grama grossas embaixo e finas em cima.

Utilize esta textura:

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

Adicione este material e deixe as configurações de cor para .75

Esta figura pode ajudar:

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

Agora se você quiser chão abaixo da grama você pode adicionar um colocando um outro plano ou objeto abaixo da grama... ou voltar e clicar no botão "mesh" (fica abaixo do botão "static" pressionado anteriormente no menu particles) iso fará o plano ser exibido e ter a textura de grama.

É isso.  

Algumas notas do autor original:

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.