In this lesson you will
use polygon selection to define the area in which to generate tree
Use polygon selection to define the render
- Open the scene file called ptrees_placement.max.
The scene consists of
an undulating terrain bisected by a riverbed and populated by a
number of particle-based trees.
- Maximize the Top viewport
and set viewport shading mode to Smooth + Highlights.
- Zoom in to the river.
Notice how a few particle
trees stand in or on the very edge of the river bed.
- Zoom out again and select the Camera01 object.
The light blue guidelines
show the camera field of view. Assume for a moment that you only
want a static shot, or plan to have the camera move towards the
upper-left corner of the terrain. This means there are many particle
trees elsewhere in the plane that will never be seen and consequently
do not need to be rendered.
Camera field of view
You will now use polygon
selection to indicate where to place the particle trees (within
the field of view and not in or near the river).
- Switch back to four-viewport
view, select the Plane01 object and from the Modify
panel > Modifier List, choose Editable Poly.
- On the Selection rollout, click Polygon
- Click just outside the top left corner
of the Plane01 object and drag toward its
- From the main toolbar, choose
the paint selection tool, then the Select Object tool.
- In the Camera01 viewport, hold down Ctrl and start painting over the polygons
adjacent to the riverbank.
Selected polygons near
Top view, showing selection
of polygons near the riverbank.
- In the Top view, hold down Alt and eliminate the polygons at
the periphery of the camera field of view, as shown in the next
Top view, with polygons
beyond periphery of camera view deselected
- Zoom in and continue to use the Ctrl and Alt keys
to add or subtract polygons by the riverbank until you are satisfied
with the result.
- On the Selection rollout, click Polygon
Selection again to exit polygon selection mode.
- De-select the terrain object and press 6 to open the Particle View window.
- On the Event 01 box, click the Position
Object 01 operator.
- In the Position Object rollout > Location
group, from the Surface list, choose Selected Faces.
were generated across the entire plane. Now, the same number of
particles are generated only on the polygon faces you just selected.
restricted to top left region of plane object
Because generation area
is reduced, you can reduce the total number of particles generated.
- In the Birth 01 operator rollout, set
the Amount spinner to 50 and press Enter.
If you like, adjust tree
placement in the Position Object 01 > Uniqueness group by clicking
New until you see a grouping you prefer.
- On the PF Source 01 box, click the Material
Static 01 operator and in the Material Static Parameters rollout
> Uniqueness group click New until the Camera01 viewport displays
a mix of trees that looks best.
- 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 was made up of 3D trees at 30,000 polygons each, you would
need to process over a million polygons and 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 middle foreground
tree 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 camera, a hidden tree might cast
a shadow that is slightly different to that of the visible tree.
To solve this problem,
rearrange tree placement by changing the particle trees’ seed value,
or add objects such as rocks to obscure unwanted detail.
A completed version of
this scene can be found in a scene file in the \scenes\dynamics_and_effects\particle_trees folder,
Particle systems can
offer a fast, effective way to populate scenes with multiple objects.
In this tutorial you
used images of trees and mapped them onto billboard-sized particles.
But you could just as easily have mapped other types of images,
such as those of people, to create a crowd scene.
When using this particle-creation
technique, make sure your particles are set to face the camera.
If you need to cast shadows, generate a second set of particles
and make sure their shadows continually face the light source.