Cycles Units & Colors

Units

All object, mesh and camera distance units are in meters.

The values written out into the render float buffer are measured in radiance.

radiance = {Watts \over sr \cdot m ^ 2}

Background closures output radiance as well.

Emission closures may output different things depending on where they are used. Surfaces by default output radiant exitance, but have an option to emit radiant power over the entire surface area.

exitance = {Watts \over m^2}

power = {Watts \over 1}

Point lights emit radiant power, and distant lights emit radiance.

The output of BSDF nodes is unitless, it is a probability of light being reflected or transmitted.

Color Spaces

All colors in the render kernel are referred to as scene linear. This also means that we do not make a distinction between different color spaces for e.g. bump maps and diffuse maps at that level. We intend to keep color space conversions out of the render kernel.

Also images textures are assumed to be scene linear, or scene linear + sRGB transfer function for compression into 8 bit. When we add support for more color spaces later, we should do this on file read, before mipmapping and interpolation to ensure proper energy preservation.

The float buffer resulting from the render is also scene linear. For display there is a conversion from this float buffer to an 8bit srgb buffer suitable for computer screens.

Spectral Rendering

We currently do not support spectral rendering, and only use RGB colors. Part of the reason is that Open Shading Language also uses RGB. Even if the shading system does not, integrators could still be made to use spectral colors, which would at least make light interaction spectrally correct.