Reflectivity and viewing angle

Smooth surfaces are more reflective when you look at them at shallow angles.


You can see this subtle phenomenon best by looking at cars in an open parking lot. Note how the foreshortened plane on the left is highly specular, or mirrorlike, reflecting the scene beyond with a wide range of values.

The back window and tailgate do not reflect the sky or the surrounding cars as brightly.

In previous posts, we’ve seen a related phenomenon with smooth water surfaces, which also become more reflective at grazing angles of incidence. In the case of a transparent fluid surface like water, light that is not reflected is refracted beneath the surface.

A smooth solid surface like a painted car or a pool ball or an apple is generally a blend of specular and diffuse reflection.

But even surfaces that we think of as diffuse reflectors, such as ordinary paper, can be remarkably specular at shallow angles. Hold up a piece of paper at an extremely shallow angle to your eye, so that you’re sighting right along the surface of the paper. If you have bold light and dark lines above that surface (such as the mullions on a window), the paper will behave almost like a mirror, reflecting a fairly clear image of the scene beyond. (Good party trick.)
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Thanks, Kevin Bjorke, for telling me about this.  

More on such things in "Color and Light: A Guide for the Realist Painter"

Related previous posts on GJ:
Escher’s Three Worlds
Color in Mountain Streams
Transparency of Water
Specular and Diffuse Reflection

Caustics inside eyes

Caustic effects are those little spots or arcs of light that happen when sunlight shines through wavy water or a water-filled glass. The curving surface acts like a lens to focus the light into a small area.



Caustics also appear within eyes when the direct sunlight travels diagonally through the cornea. Blog reader P.A. Farris sent me these photos she took of a black-crowned night heron. The caustic is an arc-like shape on the opposite side of the pupil from the highlight.


If the light comes from far enough to the side, the caustic becomes a focused point.

The same effects happen with human eyes. The light is coming from the right, and the caustic appears brightest as a curved shape at the edge of the iris. In the raking light on the iris, you can also see how the pupil bunches up.

Thanks, P. A. Farris
More on caustics in Color and Light: A Guide for the Realist Painter
Previously on GJ:
Caustics
Nephroid Caustics
Caustic Reflections
Wikipedia on caustic optics

Rocket Plume

This photograph shows the vapor plume from a space shuttle rising up into the evening sky. It illustrates several important points about the interaction of light and atmosphere.


The sun has just set behind us, and it’s six degrees below the horizon. Most of the plume is in the “earth shadow,” not touched by direct sunlight. As you follow the plume upward into the higher altitudes, it catches the light of the setting sun, first red, then orange, yellow, and finally white as you go miles above the earth.

The red colors come from sunlight that has traveled through the longest stretch of the earth’s atmosphere. The color of the sun itself goes through a similar color progression as it sets. Sunlight traveling on a near tangent to the surface of the Earth has had the most blue light scattered or removed from each ray, leaving the red color.

The moon is a few hours from full, and is therefore almost 180 degrees opposite the sun at the antisolar point. The plume is casting a shadowbeam  through the illuminated atmosphere. That shadow points directly toward the antisolar point. You can think of the dark line as a slice of unilluminated vapor seen edge-on. If that slice were completely unilluminated, we'd see stars through it.

It’s a good reminder that the light blue color of the sky is merely “the blue dome of the sky itself is really a semitransparent film of air interposed over the blackness of space.”
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(These points are further explained and illustrated in Color and Light, page 176, 180, and 190).
Image from Atmospheric Optics.
Previously on GJ: Shadowbeams

Light Pillar and Sub Sun

A light pillar is an optical phenomenon that occurs when the light of the sun reflects off tiny ice crystals floating in the air, forming a vertical column above the sun.

It usually happens when the sun is low in the sky, and sometimes even after it has set.


Light pillars can also occur near the light of the moon or streetlights, as long as ice crystals are floating in the air near you.

Ice crystals are often shaped like tiny flat hexagonal plates. Like falling leaves, they tend to float downward with the flat surface parallel to the ground.


Sometimes you can see a reflection of the sun off the top surface of these floating crystals. Looking down from an airplane window, a “sub sun” reflection will occasionally appear in a region below the horizon underneath the sun.

In the case of the magnificent photo above, the secondary light effect is a sub-parhelion, more rarely observed. It's caused by light that reflects off of the hexagonal internal surfaces of the ice crystals.  
Wikipedia on light pillar
Sub sun image from EPOD.
Nice explanation with diagrams of the ice crystals on Weather Doctor

Escher’s three worlds

M.C. Escher’s lithograph "Three Worlds" is a good illustration of the behavior of light on a smooth water surface.


At the far end of the pond, the water approaches the reflectivity of a perfect mirror because we’re looking straight across it at a very shallow angle. At shallow angles, most of the light bounces off the water (reflection) rather than angling down into it (refraction.)

At steep angles of view, the opposite happens: we see less reflection and more refraction. Therefore the water is dark and we see the fish more than the sky or the trees.

Wikipedia about "Three Worlds"
Color and Light, page 200.
Previously on GurneyJourney: Transparency of Water

Ice on the Bus

Note to concept artists: If you’re designing a frozen world, consider the fact that ice can form on the inside of things as well as on the outside.


Here’s a bus in Siberia. The wipers scrape the outside of the windshield. On the inside, the dashboard is covered with frozen precipitation. The bus driver sees the lights of the dashboard through a layer of ice.



Moisture condenses on everything, just like the frost in your freezer.

If someone leaves a window cracked open, the snow blows in and forms drifts.
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Pictures from “Siberian Ghost Cities” on Dark Roasted Blend

Clouds: Growth and Dispersion

Clouds have a life cycle. They grow and they decay. We don’t really perceive the process because it happens so slowly.

If you speed up the action, you can see the growth side of the equation as the clouds emerge in tight, convex cells.



Watch the top of the frame, and you can also see the dispersion. Clouds break into fragments, called “fractus” clouds, and then melt into the sky. Because the water vapor is less dense, fractus clouds are never as bright white as the growth cells.

Artist James Perry Wilson included both phases of a cloud’s life in this detail of a painted diorama backdrop at the Museum of Natural History in New York.
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Fractus clouds on Wikipedia
Earlier GJ Posts: “Capturing a Cumulus”
"James Perry Wilson's Dioramas"

Nephroid Caustics

Oh, I forgot to mention…there’s another kind of caustic reflection that’s as near to you as your coffee mug: a nephroid caustic. That’s the name for the little shape that forms when sunlight slants into an empty cup or bowl.

These mathematical figures are called “nephroid curves” because they sometimes have a kidney-like shape, and they’re called “caustics” because they’re focused rays, so they could potentially burn something (but dang, the won’t keep my coffee hot).

Note that in the photo there’s also a caustic halo bouncing off the outside of the mug, too. Any time you’ve got glass or metal objects in direct sunlight, there will be lots of caustic effects all over the place.

The giveaway that it’s a caustic effect (as opposed to plain old reflected light or highlights) is that the light is focused into a definite shape with a bright line or edge around it.

And now you have a topic that you can try out at the coffee machine to find out who are your kindred visual geeks.
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Previously on GurneyJourney: Caustics, Caustic Reflections
Wikipedia on optical caustic effects and nephroid geometry.
CG Caustic rendering tutorial

Caustic Reflections

In a previous post we looked at the fun trick of optics is called caustics, where undulating waves act like lenses to cause a network of dancing lines on the sea floor. That’s called a caustic projection.

The same kind of thing happens when light bounces up from the water surface. It’s called a caustic reflection. It’s a common sight on the sides of docked sailboats or on the undersides of bridges in Venice.

In this case the caustic lines are fairly parallel to each other because the waves are, too. The light is hitting the water at a low angle (allowing a greater amount of light to be reflected upward), and the caustic reflections spread out and go a little out focus on the near side of the bridge.

A caustic pattern appears on the inside surface of the vaulted arch in this painting from Dinotopia: Journey to Chandara.

This figure shows how the two kinds of caustics work. The waves act like curving mirrors and lenses at the same time.
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Photo by imappi2 on Flickr
Previously on GJ: Caustic Projections

Superior Mirage

Ordinary ground mirages look like puddles on hot roads in the desert. They’re called inferior mirages and they’re fairly common.

Far more rare and magical are superior mirages, also known as fata morgana. They occur when a layer of very cold air is overtopped by a warm air layer. The change of densities bends the light and inverts faraway images.

Phantom icebergs, ships, mountains, or even entire cities appear upside down, floating in the air. The observed objects are often so far away that they would normally be invisible, hidden behind the curvature of the earth,

Superior mirages usually occur in polar regions over ice or cold water. Sometimes they look like spiky mountains. In Iceland these are called halgerndingar. Floating cities or ships are called hillingar in Icelandic.

In her book Half-Broke Horses, Jeannette Walls describes her grandmother’s eyewitness account of a floating town in the high desert of Arizona: “There, floating in the air above the horizon, was an upside-down town. You could see the low, flat stores, the adobe church, the horses tied to the hitching posts, and the people walking in the streets.”

Images from Astronomy Cafe
More examples at the Mirage Gallery

Reflections of Masts in Rippled Water

Here are two photographs from a 1903 text for artists about reflections on water.

Left: “Knotted reflections of masts.” Right: “Broken reflection of sail. The mast, being taller, is reflected as a continuous winding line.”

The book is called Light and Water: A Study of Reflexion and Colour in River, Lake, and Sea, by Sir Montagu Pollock. It gives a thorough analysis of reflections on smooth, rippled, and wavy water, with perspective diagrams and explanations. You can download it for free as a PDF at the Internet Archive.

Shadowbeams

The painting below shows two shadowbeams, which are slightly darker than the background sky, slanting down to the left, where they intersect a cast shadow on the floor of the Hudson Valley.

The shadowbeams above were cast by natural clouds, but they most often occur when a jet contrail aligns with the line of sight. The dark beam below is cast by a contrail that's not visible in the photo; it's to the left offscreen.

The shadow is a bar of unilluminated vapor seen edge-on. The adjoining illuminated air is a notch lighter in value. The darker beam is usually only visible when there is a light hazy sky behind it.
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Previously: Sunbeams
Contrail shadow photo courtesy Atmoptical.

Gryposaurus, Part 2

With the maquette described in yesterday's post, I went outside and set it up on a piece of filmmaker's grip equipment called a C-stand. Now I could experiment both with different angles and different light directions and see exactly what was happening with the shadows.


I found some very tiny leaves and put them in the dinosaur's mouth. I wanted to see real leaves to study the transmitted light. I also went to a botanical garden to photograph magnolia leaves, and used those leaf shapes for reference.

Here's the final painting in oil on illustration board. I concentrated detail and dark accents around the eye and the tongue, which in the maquette is curling back to grab the leaves.

The far forest goes way out of focus to subconsciously suggest that this is a wildlife photo. Technically I handled this with white nylon flat brushes after a bristle block-in.

Shallow depth of field is common in wildlife photographs because they are usually shot with telephoto lenses, which have a very narrow focal plane.

I also used an effect called "bokeh," which I haven't really defined yet on the blog. Bokeh (Wikipedia explanation here) is that cool photographic effect where far away bright highlights or sky holes become circles that increase in size with distance.

Even though I'm a traditional painter, I'm using a lot of photographic effects here quite deliberately to create a photographic impression and to blend the images naturally in Ranger Rick. Although I used those effects, I didn't trace the reference photos because there were a million ways I wanted to improve on them.

A lot of what I've learned about light and color and vision has come from my conversations with professional photographers, who think about imagemaking a little differently than artists usually do. Hope all this stuff isn't too dry and technical.
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I'll be heading into New York City today for the Spectrum opening and Art Out Loud, so probably won't post tomorrow.

Smokestreams

When you’re drawing or painting a stream of smoke rising in still air, here are a couple of things to keep in mind.

The column of smoke begins as a fairly uninterrupted column of less dense gas. As it rises, the column begins to interact with the surrounding air by forming a curling vortex.

Each vortex usually spirals outward from the central line and breaks up as it gets higher. The most unexpected shapes happen higher up.

It helps to think of the smoke column as a three dimensional semi-transparent plastic bag, with some parts seen edge-on and therefore more opaque.
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Two of these images are by Irene Muller. They all come from the image collection “Dark Roasted Blend.”