Sketchdump #238: Color and Light (Gurney Studies)

I started doing studies with James Gurney’s book “Color and Light” (which I highly recommend!). Taking notes on every chapter and doing master/photo/life studies and/or application sketches as I find useful.

Moonlight
– light from the moon is slightly redder than direct sunlight
– color receptive cones barely function because it’s so dim
– Purkinje Shift: rods (value seeing) are slightly more sensitive for greenish wavelengths —> greens slightly brighter, red (roses) dark/black
– Khan/Pattanaik Hypothesis: neural activity of rods spills over to (blue) cones they touch, tricking the brain into thinking it sees blue (no physiological evidence for that though)

Color constancy
= automatic habit of interpreting local colors as stable and unchanging
–  can be confusing to choose/mix the correct color without context
– isolate color in question by covering up the surroundings
– knowledge of this effect has to be applied at imaginative painting

Adaptation and Contrast
– colors and perception of those influence each other (afterimages, colors looking different depending on surrounding colors…)
– hue, saturation, brightness of background induces opposite qualities in objects
– looking at color influences color seen next
– visual system balanced color temperatures like a camera
– color constancy (see above)
– smaller object = less distinct color
—> enhance effects in painting, always look for those things in RL, always compare

Transmitted light
= sun travelling through thin, semitransparent material, i.e. leaves or stained glass
–> colorful
-Four types of light on surfaces like leaves:
1. Transmitted light (colored)
2. Shadow plane facing downward (darkest green, picks up reflected light from surroundings)
3. Shadow plane facing upward (slightly bluer because of the sky)
4. Directly lit area (high value and texture, low chroma)
– Same applies to (backlit) trees

Subsurface Scattering
– glowy, pretty, makes things look alive
– most visible: translucent flesh, small forms, backlit
– also present in other conditions (lit from the front), but less visible

Color zones of the face
– forehead: yellow/golden (bone)
– forehead to bottom of nose: red (blood)
– nose to chin: blue/green/grey (hair, deoxygenated blood)
– more pronounced in men
– accentuate this blue/green hue to accentuate red lip color

Hair
– big brushes
– simplify masses
– soften edges
– control highlights
– think of a ribbon: hightlight goes across, not along the curve

Caustics
= bands of light resulting from refraction or reflection from glass/waves of water
– “imperfect lenses”
– curvature determines shapes
– can be cast upwards from waves, reflecting on architecture

Reflections
– Specular reflection: Light rays bounce off surface at same relative angle as they approached it
– Diffuse reflection: Light rays bounce in different directions (=matte surface)
– Most surfaces are a combination
– More reflective = broader value range (metal!)
– Convex reflective surface reflects miniature view of scene around object
– specular pattern is to be considered when rendering

Highlights
= Specular reflection of light on wet or shiny surfaces
– How to place them: Imagine a mirrow placed in a way that the light source gets reflected to the eye –> highlights are exactly on planes parallel to that mirror
– help describe form
– annular (=ringförmige) highlights: highlights are not only found in the middle of objects, but also in a circular pattern made up from scratches etc (scratched metal on pot lid, icy tree branches)

Color Corona
– bright light source or highlight surrounded by colorful light (-> lensflare)
– sun, streetlight, car headlight, solar highlight
– glow color = source color

Motion Blur
1. motion blur: object moves (blurs), camera static
– any (fast) moving object has a blurred edge (think of animation)
– faster moving = more blurred
2. speed blur: camera moves with object, background blurs
– BG blurs radially from vanishing point, following the movement
– closer to camera = more blur

Photo vs. observation
– look out for these things:
Clipping = information loss in dark and light areas, which become more black/white than they appear to the eye
– Color shifts and chroma get lost, as well as weak sources (like reflected light)

Sky blue
– sky = two gradients overlapping each other (changing in hue, value, chroma)
1. solar glare
2. horizon glow
– horizon = lighter than zenith (more atmosphere to look through)
– lighter, less saturated warmer towards the sun (away from sun: more violet)
– clouds: dark center, light edge (near the sun) vs. light at top/center, dark at side and base (sun behind the viewer)

Atmospheric perspective
– dark areas are affected first (lighter, bluer)
– white objects become warmer and stay visible longest
– only happens when air between viewer and object is illuminated!
– dust, moisture, haze, smog enhance effect

hands10 gurney2The day I shot the subsurface scattering photographs I was also able to catch a lovely prism:

subsurfaceprism2

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