Thursday, October 5, 2017

Red Shadowline

A Twitter user asks: "Hey James. Can't find the answer to this anywhere else, but do you know what you would call this red line on the skin?"

Answer: That looks like subsurface scattering. The sunlight penetrates the skin and scatters a short distance beneath the surface.

It comes up to the surface across the shadow line with the same reddish color you see when you hold your fingers.

The warm color in the nasolabial fold is a combination of subsurface scattering and reflected light from the illuminated alar planes of the nose.
Previously on GurneyJourney
Subsurface Scattering I
Subsurface Scattering II


alexdantas said...

Hi James,

Do you have any examples of this effect on darker skins?

Greetings from a brazilian in Spain.

Peter Drubetskoy said...
This comment has been removed by the author.
Peter Drubetskoy said...

James, how does one separate subsurface scattering from simple transparency? In your linked post on this as well as in your book, you illustrate the phenomenon with photographs where the light source is shining through the object (hand/orange), which to me seems just to be the transmitted light due to transparency. Are there photographs that make the effect as strikingly obvious as in the Rubens painting? Or am I still missing the point?
Also, are we sure the red line on the face in the photograph in this post is not just some sort of chromatic aberration?

A Colonel of Truth said...

I recall reading something, long ago, about Sargent, on some portraits, painting a similar red line and vision “experts” concluding he probably saw it due to astigmatism. Cannot put my hands on the source at the moment. An interesting idea.

marina said...

actually, in the case of the portrait,that have a very potent source of white light and dark areas tha area in the intesection that look like a purple border, is called fringing, and it could be of various colors. It`s an artifact that its caused by the distance of the subject and the diference of light , bad hadled by the sensor of the digital camera.

Styles Osunde said...

Something i´ve noticed that a similar saturated line appears around my environments, on rocks and the like and i have no way to explain it, it cant be subsurface scattering unless rocks have blood, and my geology teacher lied to us all.

Luca said...

Styles, the subsurface scattering happens with many translucent materials, not just skin (think to candles, for example). Perhaps with rocks it's less common, but if the minerals they are made of are translucent, it happens with them too, or there may be some surface covering them that creates the effect.

About the picture, i thought the same of Marina, some kind of artifact of the camera, i mean. Perhaps it's because in that point (in the halftone between highlight and terminator, using James terminology) colors are usually more saturated and the camera sensor have more problem (my guessing, i'm not an expert of photography).

Peter Drubetskoy said...

Just a couple of pointers:
Chromatic aberration
Purple Fringing

Judy P. said...

This post, as well as the diverse astute comments, make me proud to study representational art! I hope to be as well-versed as the writers here. I notice this effect especially between the fingers of my small grandchildren, proof that direct observation from life is the best art education.

Luca said...

Since there are elements for both the subsurface (the color above all, artifacts looks more purple, as you said) and the camera artifact (subsurface looks a bit more diffuse, it's a bit strange that follows exactly the line), i think the only way to solve the problem is...trying to replicate the very same effect and observe it from reality and then with a camera. Scientific method, i mean. If we see it just on camera, it's an artifact. If we can see it in reality, it's subsurface scattering (enhanced by the contrast with the projected shadow of the cap,that makes it look brighter and with higher chroma, i suppose). :)

Luca said...

Uhm...with the help of Photoshop, i think i solved the enigma. In the image i'm linking to, the upper part is desaturad, in the lower one value is taken away and just saturation and hue change (not exactly, actually, but quite close to that). So, subsurface scattering don't affect value (upper image: the effect is almost invisible), but it does affect saturation. The bearded man has the same hue+saturation everywhere we could expect a scattering, like with James hand. The same is the for misterious red line, which at this point looks a scattering too. It seems that , from that particular angle, the slope of light was exactly the one needed to enter the skin and bounce back.

mrhappy199 said...

I must say that I never thought I would see a picture of Action Bronson on your blog haha!