Rendering in the rain
This is an interesting challenge in CG. There are several options and I am anything but an expert, so if you are a CG artist and have any better ideas, please share. There are 3 ways to do it, that I know of. First, you can create rain using something called “particles.” These are 3D oddities that you can parameterize into smoke, dust, vapor, and rain — whatever. Since 3D programs are all based on physics and there are formulas for how everything “behaves” in nature, you can, through trial and error, create droplets (particles of a certain size, shape and material), that fall to earth (programming gravity and density and airflow) to approximate real rain. This is probably the only option (for life-like rain) especially if you are creating an animation and requiring the rain to be steady and erratic (like rain is) and to hit the ground and do something, like splash. All this is programmable in Maya (the software) I am using. But, I am not creating an animation, thus I have only to make a single frame glorious and the rain relatively believable. Not to mention that the art of particle animation I see as something of it’s own career. When done properly particles can be dazzling, achieving the closest approximation to the real thing, but there are so many variables to experiment with, getting it right would add bunches of unpredictable time.
Nevertheless, I was watching the rain yesterday and as you have experienced, if you have ever looked outside to see if it was raining, you can’t always tell. Unless you’re watching a torrential downpour rain is hard to see as it’s falling. So, you look for puddles or dark areas in the background that set off the contrast. That’s the phenomenon for daylight. At night, you have a different dynamic. Now, you can’t see the rain falling unless a street lamp or some other ambient light lights it, and again, you look to the puddles to see how hard it’s really raining. Just an observation that highlights an interesting twist. If you get too realistic, then you may leave your reader behind. If the reader has to hunt for the rain, then you are breaking the mood. In real life, we can often hear the rain, we know the forecast, or we happen to be outside in the rain. In which case — we know it’s raining. Hence, a little justification for why we exaggerate these effects so that it’s obvious to the viewer.
The next two options are more like 2D cheats. The first is to create 2D rain using software like Photoshop, (relatively easy to do), then apply it to a vertical “plane” and place it in your scene. You can add one in the foreground with bigger droplets and one in the background with smaller more condensed rain density, then render.
Seeing through all this transparency really taxes your renderer. I’m using something called MentalRay. It’s a dandy renderer but if you are not careful, it can spin on indefinitely, calculating every possible mathematical way of seeing what you have (ignorantly) placed into your scene.
One trick is to render these planes as a separate “pass” (just raindrops) then “composite” the raindrops overtop of your final rendered scene. It tends to speed things up.
Here’s an example of multiple layers of rain.
The third option that I am aware of is just to add the rain in Photoshop after you have rendered the whole scene. If you mask out your foreground and background you can still achieve the various rain densities from that track the depth of your scene.
That takes care of the rain.
Now there’s the matter of the “wet things”: wet pavement, wet walls, wet surfaces, and wet skin. There is the matter of rain running off walls and spilling off roofs, puddling on the ground and splashing under foot.
Did I say that rain scenes were a nightmare in CG? Well, they are.
I offer this somewhat in defense of the time it took to complete the last two pages of Chapter 1.
The entire graphic novel story takes place within the time span of about 24 to 30 hours. So, it wouldn’t be unusual for the rain to stop for the next “few hours”, if you know what I’m saying.