Realistic HV Water drops -- a tutorial


This tutorial will show you how to create free-flowing water droplets. The overall look is that of water beading on a clean surface, such as a new car. It will allow you to model realistic water behaviour of all kinds. The only thing this technique does not do is model the sheeting action seen on surfaces that get wet more readily, llike old cars, or fluids that leave streaks, like thicker blood. After the tutorial I will describe some of the possibilities using this method.

1. First, fire up Modeler and create a string of points that follow the path tou want your droplets to follow. In my case, I created the particles to conform to the surface of the egg as seen above. Since I'd built the scene by stacking multiple clones of the same egg, I had to Save Transformed the egg to get a template so I could create the particle string in the exact right place for the scene. Make the string dense enough so that each individual particle is the right distance to create a good-looking trail of small droplet as we see above. On the whole, more points is better. I try to get points that are spaced out at least an average of 1/20th the biggest diameter droplet I'm likely to use. In the wire below, the red streaks are the objects I built for the egg:

2. Fire up Photoshop and create two images that you will need to control the droplet effect. One is a ramp that controls the profile and length of the droplet trail, the second is a mask to control other textures ahead of and behind the droplet.

the drip ramp ...... and the drip alpha .

Notes: It is important that the center line you see above is EXACTLY halfway down the image; that is important because we'll be using a control null later and that midpoint means that the area of effect starts right on the null itself instead of somewhere else.

You can use dummy images in Image Editor, apply Textured Filter and use an "Image Y" gradient to make these on the fly. This gives you lots of control, but the danger is that Textured Filter settings often get mangled or lost due to the notorious Image Editor bugs. You CAN however, use a dummy image to build your ramp, and then double-click it from Image Editor into Image Viewer, and then save it out of there. That eliminates Photoshop entirely and you can tweak the image by reference to its effect in its intended application. Then get rid of Textured Filter, replace the dummy with the new pic, and use it straight.

3. In Layout, import your particle string.

4. Create a null, call it Dribbler. Apply Image Shape: Box to it (Geometry tab under Properties; or, just use a 1x1x1 cube object) and scale it so that it is entirely visible next to your droplet string. This is the "space" in which the above images will work (a 1m box will show the exact placing of the image when it is set to reference the Dribbler with its own settings at the default 1M scale). Animate Dribbler in such a manner as to use its XZ plane to "wipe" along the particle string, where -Y is "before droplet" and +Y is "after", where our effects will happen.. This is how your droplet will move, so be sure to add the nuances to this animation that you want.

5. Apply HV's to the particle string, and set the basic particle size to the size you want for the maximum size you want for your droplet. Have Viper up to see what's going on, or "Show Particles" (in the HV panel, set for each object) to see it in Layout.

6. Apply an image map to the particle size. Use the ramp image, on the Z axis, and set the reference object to the Dribbler. Set its Tile options to Edge for both, and leave all else at their defaults. And there is your droplet.

7. To create the trail of tiny droplets left behind, add a Crust texture, mode Normal. Scale it to be very small compared to your droplet size, and give it a low enough value that the droplets are small enough not to merge on your point string (in my example, 20-27% did it). Adjust Coverage to change the density of the droplet trail. Crust is used because instead of giving a range od different sized droplets like a noise texture would, it gives you random On or Off "hits", and by adjusting Coverage, you can control the probability of any particular point being a droplet.

8. Of course, you now have droplets leading as well as trailing your main drop, so copy/paste the ramp image, drag it to the top, set to alpha blending mode and swap in the dripalpha image.. Now the droplets will disappear in front of the main drop. Now you might assume that using the Edge tile modes would extend the mask as it does the ramp, but (of course) there is a bug that causes the droplets to reappear past the area of effect even in Edge mode when using the Alpha blending mode. Not to fear; simply scale up the Y size of the dripalpha map until it covers the whole scene. So long as the midline is exactly at the halfway point, the edge of effect should stay right on the null center. (The settings inside the Texture panel are multiplied with the scale of the null. Once you alter them, the 1x1x1m cube or box boundaries won't be accurate anymore, so watch out!)

9. Done! Experiment with things like displacement maps and jitter to change the point density along the string in Modeler or Layout, to change the way the droplets move along the string; water tends to jump along against the resistance of surface tension rather than move slowly; thicker fluids are smoother. Combine this technique with PFX and other gradient types.

Variations on this method include:

using a sheet of particles instead of a string to do large spreads of water

using PFX child emitters and collision objects to have drops land on a surface, and then slide down leaving a trail of particles with size connected to Particle Age instead of a null

draw out logos in water droplets, and have them build up or drain away

hand-key individual nulls with HV's on them and set to the blending group of the main droplet, to create a stream that "picks up" other droplets and speeds up with the extra mass, like rain on a window (I did that for the rightmost droplet of blood on the egg)

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