When you want to apply a particle system to an object in Blender, you may not want the particles evenly distributed all over your object. You may want areas with no particles or to have patches where the particles are more or less dense. Let’s use grass for example. Uniform grass doesn’t look real. Lawns and fields have patches where the grass doesn’t grow as well as areas where it clumps together.
If you are a more visual learner, I have a ONE-MINUTE VIDEO TUTORIAL HERE.
I’ll use the following scene I was working on as an example. At first, my grass was growing everywhere. I wanted to have a path through the grass and add some areas where it was less dense.
What I needed to use was weight paint. When you have your particle system set up, click on the emitter object (in my case the ground). Then, switch to “Weight Paint Mode” from the drop-down box where you would select “Edit Mode” or “Object Mode.”
Once in “Weight Paint Mode” you will find your object is blue. Blue represents the areas where there is no or “zero” particles. With your cursor (or even better, a tablet and stylus) paint the areas you want to populate your particle system. Areas in red will have the most particles. Areas in green and yellow will have less. . Areas in blue will still have nothing.
Return to “Object Mode” and go to “Particle Settings” (or “Particles Properties”) in the Properties Menu. Find the “Vertex Groups” drop-down and select it. Look at the “Density Field” at the top of this section. By painting in “Weight Paint,” we created a vertex group for our object. Select the vertex group title “group” (default name for the first vertex group created). This will tell Blender to use the map you painted as a reference for where to distribute the particles.
One of the biggest benefits with this is you can avoid painting the areas that are not in your viewport. Particle emissions that fall outside the viewport do not appear in your final render. However, they do get calculated and they do take up memory. In my example scene, I was able to reduce the particle count by about two thirds because most of the grass was actually growing outside of the rendered area. This sped up my render time a lot.
If you need to adjust the particle density, you can re-enter “Weight Paint” mode with the emitter object selected. Making your adjustments will automatically affect your particle system.
You may want to add a second particle system to the same object and use a different density map. An example would be if you already added grass but now wanted to add a particle system for rocks in your scene. To do this, add a second particle system to the same object and set it up as you see fit. If you want to distribute the new particle system along the same weight paint map, you can. If you want to have a different density for the new particle system, you’ll need to follow the steps below…
To add a second weight paint map for the new particle system, go to your “Object Data Properties” tab and scroll down to “Vertex Groups.” You will see the first vertex group you created defaulted with the name “group.” Click the “+” icon on the right to create a new vertex group. By default it will be named “group.001.”
With the emitter object selected re-enter weight paint mode. Now you can paint a new map for your second vertex group. Once you’ve done that, go to your second particle system’s settings and – just like before – scroll down to the vertex groups section. In the density field area, select the new group (Group.001 unless you renamed it). This will use the second vertex group for the density of the second particle system.
Particle systems are an awesome tool and there are a ton of settings to understand. They definitely affect your memory usage and render times. I have a video on denoising which can significantly reduce render times in Cycles.
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