Diffuse reflection occurs when light rays projected from a light source reflect at various angles from its meeting with the surface of a form.
|Diffuse Reflection (light ray does not penetrate the surface), Subsurface Scattering (light ray penetrates the surface)|
On human skin this interaction of subsurface scattering allows for the reflection of the sub-dermis hue (the color of flesh and blood) in its combination with the epidermis hue. While subsurface scattering occurs at various levels of light intensity, this effect is heightened in the presences of a great deal of light. A hand placed over a flash light emphasizes the effect of subsurface scattering.
Human skin is made up of many layers and each has its own properties. The epidermis is the outer layer of skin and primarily exhibits diffuse and specular reflection. Lights interaction with the sub-dermis of skin (flesh and blood) primarily results in subsurface scattering. Blood is partially made up of red blood cells, composed of hemoglobin, and its properties contribute to the reddish hue perceived in the color of skin.
|Human red blood cells magnified 1000 times (2)|
All of these interactions that light has with skin creates a specific effect on the perceived color of skin. Due to the multi-layered process that creates the color of skin, I attempted using a layered painting process to try to represent the color of skin and the effects of light that make it visible. I experimented with this process on a painting of an ear and will use that painting to illustrate the thought process that I have been following.
Applying Light Concepts to Painting
The interaction of light with a painted surface involves diffuse reflection and subsurface scattering as well. Some light rays interact with the opaque layer of paint on the surface while some light rays continue to penetrate the surface to the color under-layer and then to the color of the canvas before light is reflected from the surface of a painting, producing a combined effect of each layers color.(3)
I began by making my under-layer reddish in hue to represent the sub-dermis layer of skin. This layer was painted thinly to allow for light to pass through it before reflecting from the canvas layer.
3-D Modeling Layers
This painting process was largely inspired by investigations into 3-D modeling. Many 3-D modeling programs separate the effects of subsurface scattering, diffuse reflection and specular reflection into different layers before representing their combined effect. The consideration of each specific effect of light contributes to stunning representations of human skin.
|Image from David Moratilla|
|3-D modeled image from David Moratilla|
|Image with no subsurface scattering on the left, compared to being shown with subsurface scattering on the right||(5)|
While the painting process described in this post is just an experiment, I am not sure how effective it has been at representing the color of skin, especially in the photograph shown, but these investigations are something for me to continue experimenting with or at least thinking about in the future. There are many other factors that could be considered to specify the effect of the process, including the specific amount of blood in a given area, the properties of melanin in a given area, the density of hair on skin etc., and each factor could alter the painting process to produce more specific results. So I look forward to continuing investigations with painting skin.
- (1) Blevins, N. (2006, June 12). Translucency and Sub-Surface Scattering. Retrieved July 15, 2015, from http://www.neilblevins.com/cg_education/translucency/translucency.htm
- (3) Gamblin, R. (2003, October 1). Why Classical and Contemporary Paintings Look So Different? Retrieved July 15, 2015, from http://www.gamblincolors.com/newsletters/studionotes12.html
- (4) https://en.wikipedia.org/wiki/Subsurface_scattering#/media/File:Skin_Subsurface_Scattering.jpg
Many of these concepts of light are explained in further detail on Doug Flynt's blog and in Scott Wadell's video webisode here.