Caustics is effect that is calculated by emmiting photons from light emitter which are refracted or reflected  through one surface to  another surface giving us a caustics effect. Here are some examples of caustics effect:

In this tutorial we’ll stick with refracted caustics for now.
Like in real world to make caustics in our scene we need photons emited from light source. This can be really time consuming so it’s best to keep things optimized in start. We gonna use one trick to make render really fast and still have nice and precise caustics.
Lets make some test scene rig for our render.
This is simple rig that I made. One metal handle with 3 legs and revolved glass object.

Now lets make HDRI lightning for scene.
In render globals assign Mental Ray as renderer.

On the most bottom of settings you have Enivoronment section. Expand and click on CREATE button for Image based lighning

Now click on folder button and browse for some HDRI image. If you dontk now what HDRI images are go to Paul Debevec’s web site which is great resorce for HDRI images understanding.
Link : http://www.debevec.org/
There you can find out more how HDRI images work and find some free samples of HDRI images.
Now go back to render settings and change render preset to Production.

That will make enough refractions and reflections for your render of final scene.
Now open Final Gathering section and turn on that option and make 250 samples.

You should get something like this. Still no lights and shadows or glass shaders.

Now lets make some simple glass shader using Maya’s standard Blinn shader and MentalRay’s dielectric shader.
Make one blinn shader and one dielectric material which you can find in MentalRay materials.

Now open attributes for blinn material and go to output connections by clicking button that is marked on picture below. That will allow you to override maya’s blinn material with dielectric.

You’ll get blinn’s output settings.
Now with Middle Mouse Button drag ‘n drop dielectric material that you made moment ago to Material shader in custom shader pallete. Like in pic bellow.
Now make Dielectric material photon and drag ‘n drop like before in blinn Photon shader

With this you are overrided blins shaders, and put materials from mental ray.
Now go to attributes of dielectric material and change color to red and index of refraction to 1.33
Now render should look like this.

I’m sure you asking now where is caustics??? Well we didn’t put a light source yet. So let’s do that now.
Make Area light and change his shape to Disc and turn on ray trace shadows coz we want to have nice soft shadows under object. Like on picture below. But don’t turn on emit Photons option. Coz we’ll make another light for that now.

Now make one spot light and place it to match light direction from area light. Coz we gonna use that light just for emiting photons and we’ll turn off emit light and shadows for that light.

Got to properties for that light. Turn off emit diffuse and emit speculer options and in metal ray section turn on emit photons. If emit caustic photons enabled but you can edit caustic options. Got to mental ray settings in render options and turn on caustics. Now again got to properties of light and you can change caustics options.
Now change settins for that light like on picture:

Now select you spot light. Go to Panels/Look through selected and tweak your light source so it just covers part of scene where is a object that haves refractions and you want to emit photons. You can do that with cone angle option and move tool for that light. I made in my scene cone angle 15 so it just surrounds object like in picture.

If all is ok and you fallowed all steps you should get something like in next render.

But caustics are still need some tweak! Our glass is red.. so that means that caustics need to be red too!
So go to properties of caustic shader and change color to red!ow should be ok!

And it’s finished. Now your render should look like this!

Conclusion:
Why this technique is used and what we did?
We made simple rig where we have one light for lightning scene and making soft shadows and another light that don’t influent lightning but just emit photons for caustics. This is useful coz you can easily change number of photons and use couple of times less photons for accurate render and faster render. Coz if you use your light that lightning the scene your photons are emited in wide area and just maybe 10% of them strikes surface and make caustics. So sometimes you need 1 million photons to get effect like this. We just make one fake light that have narrow area of influence and about 90% of photons are used for caustic in render! This technique can speed up you render a lot and give you much better results.

This tutorial demonstrates how to use digital painting with a drawing tablet to create the illusion of lip gloss on bare lips.

Final Image Preview

Assets

This photo is from nokomai on stock.xchng. For this tutorial, zooming in on the lips to show you the process.

Plumping the Lips

The sheen of lip gloss often gives the impression of plumper lips. To support this illusion, we’ll start by using the Liquify tool (Filter » Liquify). I set my brush size to 195px and select the Bloat tool from the left-hand menu in the dialog window. Then I single-click appoximately five times along the horizontal center of her her lower lip:

The result:

Colorizing the Lips

Colorizing the lips will help them appear moist and as if she is wearing a tinted gloss. I turn on size jitter (refer to my post on digital painting pressure settings if you’re not sure how), select a 30px brush, and pick a light red color (#c83a3a). I create a new layer and roughly paint over the lips.

To blend the color in, I set the Blend Mode for that layer to Soft Light. I also set the layer opacity to 50%.

Finally, I clean up the rough edges with a 100px eraser on 0% hardness. Since the eraser is so large, the edges are very soft—great for subtle blending. I carefully use the soft edge to erase the hard edges of the color. Notice the difference on the far left, far right, and bottom areas of the lips.

Adding Shine

This is the trickiest part!

Before I do anything else, I look at the lips and judge where my light source is. In this case, it appears to be coming from the top left, so I will paint my shine accordingly. If you’re unsure, it’s useful to have reference images.

When light hits a thin coat of lip gloss, it will catch all the little lines in the lips. Our first step is to highlight the more obvious of these lines.

To do so, I set my brush to 5px, white, with size jitter on. I use a series of short, up-and-down strokes to achieve a ‘tapered’ effect. So, for the below stroke, I brush up and down a few times until the middle is the desired thickness, then ‘flick’ my stylus upward so that it will taper on top. I repeat this to taper it on the bottom, too.

With one stroke complete, I proceed to add more strokes. Look for obvious creases in the lips, then place a stroke on the side of it that’s opposite that of your light source.

Next, I begin adding blocks of shine. These will be largest where the light is hitting, then become less obvious as you move away from the light source. I use a consistent up-and-down motion with a light touch to paint these blocks.

As you move away from the light source, the lighting will be less defined. To create a more scattered effect, I just began ‘dotting’ the color instead of painting in strokes.

Here I add some additional sheen on the upper lip. Again, the detail comes from simply ‘dotting’ color and using small, up-and-down strokes.

I reduce the opacity of my shine layer to 80%.

To add an additional reflection from non-direct light, I increase my brush size to 9px and keep the size jitter on. I create a new layer and reduce the opacity to 40%.

In the above image, you can see the natural grooves of the lips as they meet the area where her lips part. These will guide the shape you form.

Using small, circular strokes, I create a shape that is much more general than our first shine. The circular strokes should be most obvious near the grooves in the lips, where small Vs are formed atop the grooves and in-between your circle-strokes. (See image below.) The shape should generally cover any area that you included light, and taper off as it approaches the left and right sides of the mouth.

I use the same technique to add sheen to the top lip.

I reduce the opacity of the layer to 20%.

Final Image

Here’s our final image: