Studying the Color of Skin and its representation

The color of flesh that one perceives is caused by multiple processes of light reflection.  Diffuse reflection, specular reflection and subsurface scattering contribute to the perceived color of skin.  Through studying how light's interaction affects the color of skin I was able to use that information as a basis to paint skin.

Diffuse reflection occurs when light rays projected from a light source reflect at various angles from its meeting with the surface of a form.

The example above shows only a fraction of the portion of light rays emitted from a light source.  In diffuse reflection, millions of light rays, composed of photons, are projected from the light source to meet the surface of a form and then reflect at various angles.  The diffusely reflected light rays enter the eye at varying levels of brightness to create an image of a form under a light source(s).  This causes the form to appear darker as its surface turns away from the light.

Specular reflection is the mirror-like reflection of light on a smooth glossy surface where the angle of incoming light projected in a single direction from the light source, the angle of incidence, is reflected at the same angle into a viewer's eye, the angle of reflection.  Therefore, the angle of incidence equals the angle of reflection. Specular reflection is only visible on a form at the halfway point between the angle of incidence and the angle of reflection into a viewer's eye.  Many specular reflections create a highlight area.

Subsurface scattering is a process of light reflection where light penetrates the surface of a translucent object and is scattered by interacting with the material before it exits the object from a different point than it entered from.

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. 

The light penetrates through the outer layer of skin to interact with the reddish color of the sub-dermis before the light exits the surface, in this case on the other side of the light. (4)  Because the epidermis (outer layer of skin) is translucent it can be thought of as a veil through which we can see some of the layer underneath it.

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)

 When light interacts with skin some light rays are absorbed or reflected from the color of the epidermis while some light rays penetrate through the epidermis and interact with the color of the sub-dermis to be absorbed or involved in subsurface scattering before exiting the surface.  Some light rays penetrate even further to reach the bone layer.(1)

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)

With these properties of light's interaction with paint in mind, I experimented with representing the color of skin through its light effects in my painting.

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.

Then I more opaquely painted the color of the form as I perceived it.  Due to the opacity of the painted layer, it was able to reflect light more diffusely (representing diffuse reflection in the epidermis of skin), while still allowing for light to pass through it to also reflect the more reddish hue of the under-layer (representing subsurface scattering in the sub-dermis of skin).  And the color of the canvas in combination with the amount of light passing through its surface contributed to the luminosity of the color that is ultimately reflected from the painting as light rays exit the surface with properties of the layers that it has interacted with.  This painting seemed to have more of a glow than my other pieces where I used a different process.

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.

References

- (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

-(2) https://commons.wikimedia.org/wiki/File:Humanrbc1000x.jpg

- (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

-(5) http://www.gamedev.net/topic/585052-real-time-skin-shader/

Many of these concepts of light are explained in further detail on Doug Flynt's blog and in Scott Wadell's video webisode here.