Note: For your convenience, these example objects and scenes are included on the release CD in the 'Tutorials' directory.
The Fresnel 2 shader is a very useful plug-in, not only because of its ability to alter not just the Reflectance and Transparency values of surfaces, typical uses when people think of Fresnel shading, but also to alter their color and just about everything else about the appearance of the surface. The effects of Fresnel 2 on a surface can vary widely from subtle changes and color shifts as a surface bends away from the camera, to cartoony and bizarre effects with different techniques.
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The basic underlying principal of the Fresnel shader is to control how a surface appears both from a head-on view and as it curves around away from the camera. This means that any single surface can have different appearances all depending on how the camera is viewing it and from what angle. A perfect example would be a person's face. Looking at a face straight on, the polygons facing you could appear pale and unsaturated and of unremarkable appearance, but as the surface bends around the cheeks and into the eye sockets, you can alter the surface such that the coloring (and other parameters) becomes deeper and more intense.
That's just one example. You could also tell Fresnel2 to make that face be totally white and unshaded (low diffuse by high luminence) as seen straight on, but the sides and edges to be black. This would end up giving a cartoon-like or cell-shaded appearance, and as the face turned, the effect would constantly update itself in such a way that the edges would always be hard and darkened, the facing part always pale and washed out.
The trees have single polygons leaves that are facing all different directions. Some are flat, facing towards the camera, some bend this way and that way and some are turned completely away from the camera. Without any sort of fresnel shading, the leaves are basically a single color, shaded simply by the way a light hits them.
Using a fractal noise texture can work, but it does not provide the desired results. The noise texture bleeds over from one leave to the next and gives everything a very fuzzy appearance because the colors have blended together.
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But a simple application of the Fresnel 2 shader makes each leaf take on its own color, and the colors are sharp and clear.
Adding Luminance makes the leaves facing the camera brighter. To give the leaves that are not directly facing the camera a hard and opaque look requires the addition of the Transparency channel.
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This also makes the leaves that are facing the camera more transparent. The leaves look much more natural and soft.
With a few clicks of the sliders to adjust the range colors, results like these below can be achieved within seconds:
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