Sunday, April 25, 2010

Painting Highlights: The Size of Specular Reflections

When we look at a portrait, figure or any object we seek to paint we are faced with a variety of highlights (specular reflections*) big and small, hard edged and soft edged. The shape of the form that reflects the highlight determines the orientation of its patterns (I hope to cover this aspect in an upcoming post) while the size is determined by the size of the convexity (or concavity--although I will only be presenting diagrams of convexities) and size of the light source.In figure 1 you can see how the size of the light source determines the degrees of a convexity that reflect the highlight at the angle of incidence = angle of reflection. If you imagine a point on one end of the light source and its geometry to the viewer, then the opposite point and its geometry to the viewer and the gradation of points between with their respective angles to the viewer you end up with the highlight's size according to the size of the light source. This establishes a constant we can rely on in any given scene: a viewer from a point in space will see a set of specular reflection in a set parameter of degrees.
So, why do we have highlights of different size throughout the figure when clearly the size of the light source remains constant?

This is answered in figure 2. Here we can see three convex forms (as depicted by the three circles inside each other) that are reflecting a highlight in the same range of degrees. The size of the highlight zone grows in proportion to the scale of the convex form.
One of the most obvious examples of this in everyday life would be the type of reflections you encounter on a vinyl record (figure 3). Each little groove has a tiny side that makes up convexities of progressively smaller sizes. The highlight pattern that emerges looks much like the range of degrees illustrated in figure 2.

UPDATE: Some of you may recognize that I've removed a portion of this post (some of the above diagrams will now seem a bit out of context, but may still have worth). This is because I need to rethink aspects of what I posted concerning the intensity of highlights. I don't want to put misleading information out there so I will revisit the topic in the near future with everything sorted out. In the way that teaching has been helpful in forming my concepts over the years it seems that blogging may be useful in the same way. I write something as I've discussed it and in rereading it I become more particular about the language I use and its implications. I hope to remain this vigilant about the information I offer and perhaps this demonstrates my desire to find the truth behind our visual experience.

I would appreciate any and all feedback.

*note* I will interchangeably use "highlight" and "specular reflection" to refer to the same phenomenon. Highlight is more convenient when talking about the net visual effect and is therefore good artist language. Specular reflection refers more to the parts of the highlight that produce the properties associated with highlights. Pure specular reflection would be found in a perfect mirror where there would be no absorption or diffusion to dilute it's effect.


  1. wow. i had no idea you released an ep. I hope you don't mind but i am cutting and pasting your blog entries and diagrams into a word document but i am backspacing over your name and inserting mine. please stop putting the copyright tags on your images--they are hard to erase in photoshop.
    seriously--this is a really illuminating post (all puns intended).

  2. Thanks Carol, you know I only post this stuff so that I can get your witty responses! I will be releasing that record shortly but I'll send you a free autographed copy now.

  3. Wow Scott, another illuminating post!
    In this case, it is a topic in which I never thought nor read about it!

    Keeping in mind the form of the surface that reflect the highlight could be one of the most useful things to quickly determinate the size of the HL.

    Thanks Scott for posting this!
    Really useful.

  4. Thanks for the comment Ariel, I'm glad you found this useful.

  5. Brilliant! Thank U.

  6. Scott, hi!
    I'm revisiting this post because lately I have found difficulties painting highlights... is like I found myself painting them as if they were part of the form light, when I actually know that they aren't.

    I know that highlights could be recognized because they move with the viewer, and because the local color of the object began to be replaced by the color of the light source... but is hard to manage for me to things:
    a. To discriminate IN the painting the highest form light from the highlight
    b. To avoid graduate the form light IN the direction of the highlight, instead of doing it in the direction of the lightest light (the plane most facing the light) of the form light.

    Also, the edge quality...

    Well, I think painting is hard! :)

    See you
    And again, I love this blog!


  7. Spectacular or specular posts and thanks for your highlights! ;-)