TDU Photoreal Lighting Course Week 2

TDU Photoreal Lighting Course Week 2

Considering Ambient Occlusion.

What is ambient occlusion (AO)? -  AO is a 3D rendering technique which is trying to cheaply estimate the effect of objects blocking available light as seen in real life - for example the creases between sofa cushions, the corners and edges of a ceiling, small gaps and cracks in tarmac. 

It is technically a Global Illumination (GI) technique in 3D, however it is only an approximation of true GI, as an AO pass assumes a constant and consistent amount of ambient light throughout the scene and isn't calculated using any of the scene lighting.

In the photo above the shadowing between the cushions isn't caused by a shadow from a direct light, and is instead a result of the other cushion against it limiting the amount of existing light can reach the surface.

That the AO calculation assumes constant and even light throughout the scene results in it being less realistic in scenes with a mix of bright direct and soft ambient lighting as an artist only has control of the AO on a global level. In soft and ambient lighting AO has a softer and larger spread where-as in strong sunlight or strong direct light AO has a much tighter spread.

The above is an example of a scene with a clear mix of both strong direct light and soft ambient light, which would result in an AO pass being less accurate and realistic.

Soft AO as you'd expect to see
in softer lighting conditions
(such as interior artificial lighting).

Tight AO as you'd expect to see
in strong direct lighting conditions
(such as exterior on a sunny day)

Another problem with AO's realism is it's lack of colour information. In reality shadows we see are not simply black and greys but are darker shades of colour. The standard practice most people start off using for AO passes is to multiple them over the top of a render - however this can result in an image losing colour information and becoming more washed out as shades of greys and blacks are being layered over the top of the image.

Comparisons of the standard darkening blend modes.

To try and get around this lack of colour information I personally tend to use an AO pass firstly using multiply  with a low-mid opacity to set the overall tone and range of the AO, and over the top I add a duplicate with a linear colour burn and very low - low opacity as this darkens the colour information without sacrificing saturation, but avoids the over-contrast introduced with standard colour burn (which can be seen clearly above). AO should be a subtle effect generally, so I try and keep it subtle and in keeping with the lighting around it.

This is simply my own approach to using AO so I'll be interested to see how everyone else utilises it in composting.