# A Way of Rendering Crescent-shaped Shadows under Solar Eclipse

Posted by Daqi's Blog on January 1, 2018

Happy new year! I haven’t posted for a long time. However, I have done many exciting projects in the last half year and I’ll upload some of them soon. The first project I want to share with you is the render I created in in the Utah Teapot Rendering Competition. We were all awed by the great solar eclipse on August 21th. Do you remember the crescent-shaped shadow casted by tree leaves? It was so beautiful that the first time I saw it, I want to render it with ray tracing. Before working on this competition, I thought that there must be some complex math to figure out to simulate this rare phenomenon. However, the problem turned out to be embarrassingly simple. We can just model the actual geometry of sun and moon and trace rays. You might think that it is a crazy idea. In fact, instead of creating a sun with a diameter of 1.4 million kilometers and putting it 150 million kilometers away, we can simply put a 1-unit wide sun 100 units away from our scene, where 1 unit is approximately how big our scene is. It is supposed to have almost the same result. Then we use the same kind of trick to put a moon with a slightly smaller diameter and slightly ahead of the sun to make sure that the sun is eclipsed by it with a crescent shape. By treating the sun as an isotropic spherical emitter, the moon as a diffuse occluding sphere, and use a tree model with detailed alpha-masked leaf texture, I got results that are surprisingly good and also fast to compute. In such a simple way, I created a nice image with a glass Utah teapot sitting under a tree on a lawn behind the Warnock Engineering Building with crescent-shaped leaf shadows at the background.

The render. Click to enlarge.

See the "sun" and "moon"? This is really how it works!

a close-up showing the crescent shape by eclipse

Hope you like this project. Sometimes complex effects are really that simple!

Remark:

The tree model and the grass model (include their texture) are downloaded from TurboSquird.com
Grass: https://www.turbosquid.com/FullPreview/Index.cfm/ID/868103
Tree: https://www.turbosquid.com/FullPreview/Index.cfm/ID/884484

http://texturelib.com/texture/?path=/Textures/brick/pavement/brick_pavement_0099