# What are the mechanics of "bounce" light? How does it work?

I'm a learning 2D artist, trying to learn more about light and I am having trouble with diffuse "bounce" light. I would think that reflected/bounce light would illuminate certain areas evenly. In the picture below, if the cylinder is being lit by a sun, then why do points A and B have different values? Does this have anything to do with light falloff? And why is the bounce light most intense on the bottom of the side instead of the front?

Most of the stuff I have read about bounce light doesn't talk about the mechanics of it, other than a sort of general explanation. When actually rendering, I want my decisions to be informed when I make them. On real life examples of objects, the reflected light forms certain shapes and patterns, and I just don't understand how they are made. So if anyone can help, and could give an in depth look into how bounce light works, it would be greatly appreciated. :)

(The questions inside the image)

A. Shouldn't a and b on the right be the same value since they are being hit at the same angle? yet, both in Blender and in real life they are not the same value.

B. What am I missing here?

C. Is the angle at which the light hits the object the only thing that determines its value?

D. Does this mean there are more rays hitting B and there are A?

E. How do I determine how many rays are hitting an area?

F. And how does the cast shadow get lit back up, where are those bounces corning from?

• The strength of the bounce is quite pronounced, its not that big in all casses but that really depends on the lightning conditions. Commented Aug 6, 2017 at 18:45
• The reflectance(?) and luminosity of the green plane will have an influence on the intensity of light that reaches the bottom of the cylinder. For e.g. imagine if the green plain were a mirror... Commented Aug 6, 2017 at 21:38
• @johnp if its highly reflective then it wouldnt do any diffuse reflection. Only direct in which case it would look like 2 directional light. Commented Aug 7, 2017 at 4:35

The first and most basic concept of light you should know is:

Direct light vs Diffuse or soft light. You probably already know it on source lights, I will describe it because is the same concept.

1. You have a spot light, where the beam source is a tiny spot (a) and cast a hard shadow (b).

2. When you have a bigger sized source light it becomes a diffused one. The beams from the point (c) go in a diferent direction and cast a shadow in another point (d)

3. Of course, there are not just 2 points of light, but an infinite array of little light sources, which, combined gives a soft shadow (e), where the darker zone is defided by (b) and the lightest zone by (d) in the previous diagrams.

1. In real life, this light either it is absorbed or bounces. The angle on a reflective surface (mirror) of this reflected ray (g) is the same opposite angle as the incident one (f).

Some rays simply go away from your object and do not come back (h).

1. But in a diffuse material, a matt material, these rays do not bounce at that angle alone, they explode, they scatter (i) and those that on a reflective surface would not hit the object anymore, come back and lit a bit the object (j).

1. One important thing to understand, is that normal light scatter, this is, are not parallel rays, they come from a point to different directions, If you are close, you get more concentrated rays (k), so the further away you are from this little point, the fewer (l) rays hit the zone.

1. One additional concept is the angle of difusion. This is the concept that light will less likely to bounce at some angles. More rays will bounce at some angles similar to the incident angle (m) than an angle too far from this "ideal" angle (n).

1. On a very, very rough material, the little bumps on it, bounce more light back (o) than further away, they are like little walls. An example of this material could be a carpet.

1. The main light law here to understand how distance afects the ilumination is the inverse square law, that states that the further away you are from the light source the less light you get in a square inverse proportion.

A. Shouldn't a and b on the right be the same value since they are being hit at the same angle? yet, both in Blender and in real life they are not the same value.

No, they are not hit at the same angle because the material you are using is not a mirror, it is a matt material, exactly like a pool table. See point 6.

B. What am I missing here?

What you are missing is that the further away from the source light you are the less light you get, this is the inverse square law.

C. Is the angle at which the light hits the object the only thing that determines its value?

How the light hit an object is determined by

• Angle of the incident light on the reflective material

• Material, color, and roughness (glossy vs matt and all the in between values).

• Distance from the source light. This is specially true on a matt material, this matt material (the floor) is a new light source now.

D. Does this mean there are more rays hitting B and there are A?

Yes.

E. How do I determine how many rays are hitting an area?

By the inverse square law. The closer you are to this (new) source light (the matt material) more light you have, the further away less light you get.

But you need to determine two light sources, the incident light and the bounced light. In this case the inverse square law applies more to the bounced light, as this is the main problem you are describing.

F. And how does the cast shadow get lit back up, where are those bounces corning from?

You are mixing concepts.

The cast shadow in your diagram is gray, this is implying that there is an ambient light that you are not seeing, the roof of a room, the sky of the planet.

But on your diagram what you consider cast shadow is self shadow, that, it is also gray because the ambiet ligh, but have some bounced green light from all over the floor.

In the image you are posting, the "only the bounced light" render, there is a chance you are "forcing" the Blender material too much making it not natural.

I am simulating roughly this bounced light.

See the angles depending on the type of the floor's material. Your example has more light on the right, but it looks more like if this reflective surface is in an angle.

• Explanation of your point E is incorrect as it now exists. Also F is not correct and should be illustrated looking down (in a plan view.)
– Stan
Commented Aug 7, 2017 at 14:24
• There are a few issues in each of the above 5 examples—some trivial, some conceptual, some with the terminology. For example, soft shadow for soft edge. No. 2 is bogus and should be No. 3. Stay tuned.
– Stan
Commented Aug 7, 2017 at 14:34
• Point source vs diffuse source, direct vs. indirect, it can be bent, bounced, blocked (bounced), and absorbed. I believe dispersion is the concept illustrated by No. 7 which is greater at "Brewster's angle." It would appear more specular due to angular differences. Fresnel involves different concepts (index of refraction.)
– Stan
Commented Aug 7, 2017 at 14:50
• Inverse square law affects luminance with distance not the number of rays which are infinite. What confuses the issue is the attempt to "ray-trace" for the area of the intercepted angle. The attempt to illustrate the concept is a challenge for technical artists.
– Stan
Commented Aug 7, 2017 at 14:55
• 7 is diffusion. Dispersion involves the "spread" of a collimated (parallel) beam of light as it travels from the source.
– Stan
Commented Aug 7, 2017 at 15:06

Light falloff is irrelevant since the surface is somewhat uniformly light (light falloff, only really meaningfully applies to small surfaces sending light). The explanation of how it actually works is pretty meaningless if you paint by hand eye coordination. It is a completely another thing when your actually working with 3d images like the one here.

But if you want to understand it at a model level then here you go. The amount of the bounce light is proportional to how much the surface is visible to each point. In this case its the table below is less visible the higher you go. The color diminishing follows the formula (π-atan(a))/π Where a is the normalized height (a = A/B).

Off course reality inst a slice so you end up winging it.

All this said: For a 2d artists its better to look at references and learn form that.

• I think the math confuses what you can actually observe. I would think that light falloff is exactly what is being illustrated by the question.
– Stan
Commented Aug 6, 2017 at 21:20
• @stan yes the image is somewhat, correct, didnt measure, just very pronounced. bit it isnt light fallof as such. Commented Aug 7, 2017 at 4:11

Here's my explanation which involves a concept in physics that I learned in photography.

Light diminishes in intensity the further away it is. For your example, the brightest area is closest (right next) to the lit surface it sits upon. The top of the cylinder is further away from the surface so the light hitting it is less intense as it travels (bounces) further from the base.

Your illustrations show this beautifully. The bright green allows you to see and distinguish that light has bounced from the green surface onto the neutral grey cylinder.

Perceptually, you notice the effect more in the shadow of the cylinder as a greater proportion of the bounced light shows where the main light has been masked by the cylinder itself. (That concept is known as Weber's Law.)

For those physicists among you, the physics concept is the inverse-square law: light intensity varies with the square of the distance.

Your answer: The bounced light is a different diffuse light source for argument's sake.

A simple approach:

Have any source of light which distributes the light into different directions. The physical fact: "As the distance increases, it makes the objects less illuminated". This is because the same amount of light spreads to larger area when the distance grows(see NOTE1 ).

In sunlight your head and foot can seem equivalently illuminated, because the distaces to the sun are virtually the same. The about 5 feet difference is in percents insignificant. The differences should be compared merely in percents than in feets or meters.

Any illuminated surface must be considered as a light source. Your green plate is not an exception. The brightest bounced green is near the green platform and the green illumination diminishes as the distance increases.

The highest green illumination does not mean the most visible green illumination, because the direct sunlight easily makes so much stronger illumination that our very limited eyes nor our 1000x more limited cameras simply cannot detect the slight green under the strong white light. The strong light easily saturates the ability to see the colors. (The same is true with our displays which really get saturated and show only white, if the general brightness is too high)

Thus in the shadow the bounced green can appear to be much more visible than the much stronger green near the platform but under a strong direct sunlight.

NOTE1: Not true for lasers or other nearly zero beam angle sources at short distances. But well obsevable when using ordinary light sources. For example go to a dark room with a flashlight. Direct the light to the wall or door. See, how the illumination gets dimmer as you walk further. At the same time the illuminated area grows.

• Actually in the case of area lights the illumination does not decrease with distance per see since each point cancels the next which is the reason you can see a yellow wall as same yellow as far as you can see it. Just the angle of how much of it is visible. Now dont get me wrong the effect is similar on sides of a cylinder since what they see reduces as on a edge but if the plane is Huge like a football field and you have a face pointing down it is as if the light would not fall off at all untill the fistance is considerable. Commented Aug 7, 2017 at 4:32