************
Introduction
************

.. figure:: /images/render_cycles_introduction_overview.jpg

Cycles is Blender's physically-based path tracer for production rendering.
It is designed to provide physically based results out-of-the-box,
with artistic control and flexible shading nodes for production needs.

To use Cycles, select it as the *Render Engine* in the Render properties.
For :doc:`GPU accelerated rendering </render/cycles/gpu_rendering>`,
enable compatible devices in :menuselection:`Preferences --> System --> Cycles Render Devices`.

.. seealso::

   The `Cycles website <https://www.cycles-renderer.org/>`__ with more information and a gallery.


.. todo: expand to describe how ray tracing works.

Light Rays
==========

Ray types can be divided into four categories:

#. Camera: the ray comes straight from the camera.
#. Reflection: the ray is generated by a reflection off a surface.
#. Transmission: the ray is generated by a transmission through a surface.
#. Shadow: the ray is used for (transparent) shadows.

Reflection and transmission rays can further have these properties:

- Diffuse: the ray is generated by a diffuse reflection or transmission (translucency).
- Glossy: the ray is generated by a glossy specular reflection or transmission.
- Singular: the ray is generated by a perfectly sharp reflection or transmission.

The Light Path node can be used to find out the type of ray the shading is being computed for.

.. figure:: /images/render_cycles_render-settings_light-paths_rays.svg
   :align: center

.. seealso::

   The object :ref:`ray visibility <cycles-ray-visibility>` settings.


.. _render-cycles-light-paths-transparency:

Transparency Rays
-----------------

The transparent :abbr:`BSDF (Bidirectional scattering distribution function)` shader is given
special treatment. When a ray passes through it, light passes straight on,
as if there was no geometry there.
The ray type does not change when passing through a transparent BSDF.

Alpha pass output is also different for the transparent BSDF.
Other transmission BSDFs are considered opaque,
because they change the light direction. As such they cannot be used for
alpha-over compositing, while this is possible with the transparent BSDF.

Note that, while semantically the ray passes through as if no geometry was hit,
rendering performance is affected as each transparency step requires executing the shader and tracing a ray.