Another example of light absorption (also known as "Beer's law"). This is similar to "transparency colour" in Terragen v0.9:
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New! Volumetric shading of the participating medium. In v0.9 you could control the subsurface diffuse colour, but the lighting was only calculated at the surface of the water which would give an unnatural hard shadow and exaggerated shading on rough water. Now there is an option to sample the lighting at multiple points along the ray to give volumetric shadows under the water, although they are quite subtle in this example render:
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Matt