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For anyone who has ever done underwater render, they know the icing on the cake is the light that come from the surface and forms columns in the water. Most often created with volumetric light cones, they have two big drawbacks, they don't look very real, and they make for a huge leap in render times. Well I'm going to show you how to create realistic columns of underwater light in post that help you cut down your render times dramatically!
The first step is to create your scene. For the purpose of speed, I've created a really simple one, just land, water, two light sources and a shark. The top light source uses a caustic gel to give that great light effect on the shark and the bottom, while the other light source is turned down and is lighting the belly of the shark. While you're in the water, any light that enters the water bounces off tiny particles and causes an under light on most objects.
You'll need an object mask of all objects in the scene, so select them all, click Object Mask in the Render side menu and render. This will be very important for creating a sense of scale and perspective later on.
Then you need to render your image. Just as an experiment, I rendered this image, then added three simple volume light columns and rendered again. The render time jumped from nine minutes and forty seconds to sixteen minutes! Just from three very simple lights! Can you imagine if I used a lot to try and get a sense of realism???
Ok, now that you have your renders, it's time to get to work. Open up both your render and the object mask. Copy the entire object mask and copy it into the Alpha channel of your render. Then, we begin to create the beam that will be the basis for our columns of light. It's real easy. Just take your brush and set a soft tip. The size will depend on the size of the caustics on the seabed, but we'll just use a simple 100 pixel brush. Also, smaller brush sizes are used to create columns further away. Then set the color to white.
First create a new layer for our light columns. For the columns to look right, they need to get bigger as they go down. To make our brush act like that, we'll use the Pen Tool to create a path along which our brush will move. Please pay close attention as to how the shadows fall in relation to the objects throwing them. The paths will have to move at the same general angle.
Now right click with the Pen Tool and choose Stroke Path. Choose the Brush tool and make sure the option Simulate Pressure is checked. Then choose Ok. You should get something very similar to what I have. Then take a nice big soft Eraser and erase the bottom of the beam. We only want the top cone-shaped part.
Ok, you've noticed that the light beam ends up a bit short. So we have to use the Transform tools to re-scale it so that it reaches past the top of the picture, all the way to the bottom. A hint, use the Zoom Tool to Zoom out several picture sizes. That way you can have a working area larger that your picture.
Ok, now here is where we create that great dapple effect. Take your Eraser tool, choose small, soft tips and set the opacity to about 10%. Click once at the top of the light column. Move your eraser to the bottom, hold down shift and click again. A small line will appear in the column. Do this over and over with different size tips till your happy with the effect. Just make sure to keep the lines as parallel as possible while making a fan shape.
Now you have to make the light look right. To do that, change the layer style to Overlay and lower the opacity. The amount is up to you, but the general rule is the higher the opacity, the closer the columns are to you.
Oh yeah, I almost forgot. To deal with columns that go behind the objects in your scene, use the Select by Color option and the object mask. You can mask off the object, erase everything that is supposed to go behind it and it will increase the realism 100%.
For a full-sized example of this technique, head over to the Poser Pros gallery and look for a picture called Atlantis.