In this lesson you use polygon selection to define the area in which tree particles appear.

Set up the lesson:

• Open ptrees_placement.max.

  The scene consists of an undulating terrain bisected by a riverbed and populated by a number of particle-based trees.

Use polygon selection to define where trees will appear:

1. Maximize the Top viewport and set viewport shading mode to Smooth + Highlights (you can press F3).
2. Zoom in to the river.
   

   Notice how a few particle trees stand in or on the very edge of the river bed.

3. Zoom out again and select the Camera01 object.

   The light blue guidelines show the camera’s field of view. Assume for the moment that you want only a static shot, or plan to have the camera move toward the upper-left corner of the terrain. This means there are many particle trees elsewhere on the plane that will never be seen and consequently do not need to be generated.

   

   Camera field of view

   Now you will use polygon selection to indicate where to place the particle trees (within the field of view and not in or near the river).

4. Switch back to a four-viewport layout, select the Plane01 object, then go to the Modify panel.
5. On the Selection rollout, click (Polygon).
   
6. Click just outside the top left corner of the Plane01 object and drag toward its center.
   
7. On the main toolbar, choose (Paint Selection Region) from the Selection Region flyout (initially it shows (Rectangular Selection Region)).
8. In the Camera01 viewport, hold down Ctrl and start painting over the polygons adjacent to the riverbank.
   

   Selected polygons near riverbank

   

   Top view, showing selection of polygons near the riverbank.

9. In the Top view, hold down the Alt key and deselect polygons at the periphery of the camera’s field of view, as shown in the next illustration.
   

   Top view, with polygons beyond periphery of camera view deselected

10. On the Selection rollout, click (Polygon) again to exit the polygon sub-object level.

Set the particle system to use the sub-object selection:

1. Click an empty area of the Top viewport to deselect the Plane01 terrain object, then press 6 to open the Particle View window.
2. In Event 01, click the Position Object 01 operator to highlight it (if it is not highlighted already).
3. In the Position Object rollout Location group, open the Location drop-down list and choose Selected Faces.
   

   Previously, particles were generated across the entire plane. Now, the same number of particles are generated only on the polygon faces you just selected.

   

   Particle generation restricted to top left region of plane object

   Because the generation area is reduced, you can reduce the total number of particles generated.

4. Click the Birth 01 operator to highlight it. On the Birth 01 rollout, set the Amount spinner to 50 and press Enter.

5. If you like, adjust tree placement in the Position Object 01 rollout Uniqueness group by clicking New until you see a grouping you prefer.

6. In PF Source 01, click the Material Static 01 operator to highlight it. In the Material Static rollout Uniqueness group click New until the Camera01 viewport displays a mix of trees that looks good to you.
7. Render your scene.
   

   Rendered 50-tree view of the particle forest

   The 50 particle trees should take just a minute or two to render. If, on the other hand, the scene were made up of 3D trees at 30,000 polygons each, 3ds Max would need to process over a million polygons: This would require a great deal more time to render.

   There is one aspect to watch out for when using particle systems to create trees with shadows. As the next illustration demonstrates, the base of the tree on the left does not quite match the base of its shadow.

   

   Tree trunk not aligned with particle tree shadow

   Recall that you created two sets of particle trees: One set oriented to face the camera and another, hidden set that faces the sun. Depending on the respective positions of the light source and the camera, a hidden tree might cast a shadow that is out of alignment with the visible tree.

   To solve this problem, you can rearrange tree placement by changing the particle trees’ Seed value, or you can add objects such as rocks to obscure unwanted detail.

Save your work:

• Save the scene as my_ptrees_forest.max.

  You can find a completed version of this scene in the \scenes\dynamics_and_effects\particle_trees folder, called ptrees_completed.max.