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Common CG Terminology

By RoboticZombie1

I will try to update this guide periodically and use it as a space to breakdown common CG terms into their origins. It may get pretty technical at times. It often helps me to better understand things in the long term if I know how they're built and where they came from. If you notice that I got something wrong, or would like me to go into depth about a different subject, please feel free to send me a message and I'll be happy to address it.

Award

Created by

RoboticZombie1
In-Game
METAL GEAR & METAL GEAR 2: Solid Snake

Category: Gameplay Basics, Secrets, Story or Lore, Walkthroughs

Posted

Updated

6 Jun, 2021 @ 6:37am

27 Jul @ 3:48pm

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Guide Index

Overview

Breaking Down the Exposure Value

Shading Techniques

Comments

Breaking Down the Exposure Value

Originally posted by RoboticZombie1:
Note: the following stops are not to be confused with f-stops.
The former refers to a relative amount of light, while the latter represents aperture size.

Considering that incremental stops represent the doubling or halving the amount of light, we can use simple logarithms to reverse the exponentiation and identify the number of stops captured at a given ISO:

//let x represent the number of stops. //let ISO represent the sensitivity of the current film back as it relates to a standard of 100. 2^x * 100 = ISO 2^x = ISO / 100 x = log2(ISO / 100) x = log10(ISO / 100) / log10(2) //using this formula, an iso of 800 is sensitive to 3 additional stops of light compared to an iso of 100.

Originally posted by RoboticZombie1:
Note: if using a calculator, you may find it difficult to compute base two logarithm. A work around would be to instead use the change of base formula; which involves the base ten logarithm of the exponent, divided by the base ten logarithm of the new base.

Exposure Value (EV) represents how much, and for how long, a film back is exposed to light. This implies the necessity of two variables: the size of the aperture allowing light to pass through, and the timing of the shutter attached to it.

The aperture size is commonly notated as an f-stop, or simplified as an f-number, which describes the diameter of a lenses aperture as it relates to it's focal length.
Originally posted by RoboticZombie1:
Note: Most modern lenses simplify the f-number as an item in a sequence of incremental powers of the square root of two. Each subsequent value representing an aperture that is either double or half the size of the previous.
The amount of time that light is allotted to pass through the aperture is determined by the shutter speed; commonly measured in fractions of a second.

//let EV represent Exposure Value under an ISO 100 sensitivity. //let ap represent the f-number. //let ss represent the shutter speed. EV = log2(ap^2 / ss) EV = log10(ap^2 / ss) / log10(2) //using this formula, an aperture with an f-number of 2.8 and a shutter speed of 1/30 will yield an Exposure Value of 8.

Once an Exposure Value has been determined under the default exposure of an ISO 100, it can be offset by the number of stops available within additional ISO sensitivities:

//let EV represent Exposure Value under an ISO 100 sensitivity. //let x represent the number of stops. EV = EV - x

Shading Techniques

Originally posted by Phong, 1975:
In computer graphics, a shading function is defined as a function that yields the intensity value of each point on an object's body, based on the characteristics of the light source, the object, and the position of the observer.

The position of any point on a surface to be shaded.

The orientation of a surface at a given point, P.

The incident light direction, as determined by the vector drawn between the position of a light source and a given point, P.

ꞷᵢ

The angle of incidence, as determined by the angle between the light direction, L, and the surface normal, N.

The reflection direction, or the outgoing light direction.

ꞷₒ

The angle of reflection, as determined by the angle between the reflection direction, R, and the surface normal, N.

The position of the viewer, or camera.

The viewing direction, as determined by the vector drawn between the viewer's position, E, and a given point, P.

//The Law of Reflection states that when light is reflected off a perfectly mirrored surface, the angle of incidence is equal to the angle of reflection. ꞷᵢ = ꞷₒ

Facing Ratio

The Facing Ratio is a simple shading technique that highlights the degree to which the geometric faces of a mesh are angled towards the viewer. It involves calculating the dot product the surface normal, N, and the viewing direction, V.

Originally posted by RoboticZombie1:
The dot product, or scalar product, of two normalized vectors is equal to the cosine of the angle between the same two vectors; and will return a float value that ranges between -1.0 and 1.0.

When the surface normal, N, and the viewing direction, V, are parallel to one another, the angle between them is 0°, and the dot product will return a value of 1.0.
When the surface normal, N, and the viewing direction, V, are perpendicular to one another, the angle between them is 90°, and the dot product will return a value of 0.0.
If the angle between the surface normal, N, and the viewing direction, V, is greater than 90°, then the dot product will return a negative value between 0.0 and -1.0.

//The viewing direction vector can be found using the surface point and the viewer's position. //Normalize the viewing direction. //Computing the facing ratio is as simple as calculating dot product of the surface normal and the viewing direction //Clamp the lower values in order to prevent negative numbers from backfaces. V = E - P V = ‖V‖ facing_ratio = N·V clamped_facing_ratio = max(0.0, N·V)

In the screenshot above, an example of the Facing Ratio is implemented using Blender. Below, there is another example of the Facing Ratio, but instead using the geometry node's True Normal output in place of the surface normal.

Originally posted by RoboticZombie1:
Note the visible faceting when using the geometry node's True Normal rather than the Normal output. In Blender, the Normal output takes the shading method of the object into consideration. This means that if the object is smooth shaded, then interpolated normals are created in order to simulate a continuous surface. Using the True Normal output is synonymous with a flat shading method, or utilizing the face normals directly.