my_text_data/cheatsheets/computer graphics cheatsheet.txt

WORK IN PROGRESS

This is not a complete cheatsheet of CG, just things I personally want to have a cheatsheet of.

- 3D
  - perspective correction (texturing):
    - We have a triangle in 3D and texture coordinates u and v.
    - Principle: values 1/z, u/z and v/z are linear in screen space! We linearly interpolate these and
      from them compute the correct u and v at each pixel.
    - Algorithm:
      1. Compute 1/z, u/z and v/z at each triangle vertex.
      2. Linearly interpolate the three values on both side.
      3. Linearly interpolate the three values between the both sides.
      4. Compute the correct values from the interpolated ones as:
         z_correct = 1 / (1/z)
         u_correct = (u/z) * z
         v_correct = (v/z) * z
- radiometry
  - Measurement of electromegnetic radiation including light.
  - measured quantities:
    - basic quantities:
      - radiant energy (Q,[J])
      - radiant flux/power (P,[W = J/s])
      - radiant flux density ([W/m^2]): for directional lights
        - irradiance: flux flowing INTO a surface
        - radiosity/exitance: flux flowing OUT of the surface
      - radiant intensity (I,[W/sr]): for point light sources
      - radiance (L,[W/(sr * m^2)]: Illumination in a single ray of light on a surface area, this is the quantity
                                    measured when evaluating the rendering equation - IMPORTANT. (The reason for
                                    sr in the unit: iny physics we can't get a value in a single point, we have
                                    to integrate over the solid angle)
    - spectral quantities: The same, but additionally parametrized by the wavelength.
  - functions:
    - BRDF (bidirectional reflectance distribution function)
      - most often used
      - input: incoming direction, outgoing direction (both with respect to the surface normal)
      - output: ratio of outcoming radiance to incoming radiance
    - BTDF (bidirectional transmittance distribution function)
      - same as BRDF, but the outgoing direction in on the other side of the surface, i.e. says
        how the incoming light gets into the surface instead of how it's reflected.
    - BSDF (bidirectional scattering distribution function)
      - BRDF + BTDF, i.e. for incoming light direction and outgoing light direction (which can be
        above or below the surface) returns the ration of the incoming to outcoming radiance.
    - SVBRDF (spatially varying bidirectional reflectance distribution function)
      - Like BRDF but also takes an extra parameter: the position on the surface, i.e. models
        surfaces whose BRDF differs across the surface.
    - BSSRDF (bidirectional surface scattering reflectance distribution function)
      - Takes into account also subsurface scattering etc., defines the relationship between any
        incoming and outcoming light ray (which may also be at a DIFFERENT POINT of the surface).
    - BTF (bidirectional texture function)
      - For a given texture coordinate, viewing and illumination angle tells the outcoming light.
- photometry
  - Same as radiometry, but with respect to human eye, i.e. everything is multiplies by a photometric curve.

- screen-space effects
  - ambient occlusion
    - SSAO (screen space ambient occlusion)
      - simple, sometimes inaccurate
      - For each pixel samples a normal-oriented hemisphere for occlusions and decides how much occlusion appears.
    - SSDO (screen space directional occlusion)
      - generalization of SSAO
      - Takes into account directions and so allows for "colored" occlusion shadow in scenes with multiple lights.
        Also computes one bounce of indirect ilumination and so approximates global ilumination.

    - HBAO (horizon based ambient occlusion)
      - 
    - HBAO+
    - GTSO
    - HemiAO
    - GTAO (ground truth ambient occlison)
      - most accurate

- PBR (physically based rendering)
  - A set of ideas and philosophies, using physics for close-to-reality rendering.
  - core ideas
    - using "measured" textures of physical surface properties
      - albedo map: What wavelengths are absorbed by the surface, defines its color. In
                    non-PBR diffuse texture was used instead. The difference is: albedo
                    has no ligting (AO, shadows, ...) in it!
      - one of the following
        - metallness "workflow"
          - two maps:
            - metallness map: How metal (more reflective, little diffuse, ...) or insulant (uncolored specular
                              reflection, ...) the material is.
            - roughness map: How rough the surface is due to microsurfaces. The inverse is glossiness map.
          - more "PBR"
          - more difficult to mess up the material, but also more difficult to create unrealistic materials
          - uses less memory (doesn't specify the color of specular reflections)
          - can have artifacts
        - specular "workflow": less "PBR", easier to mess up the material
          - specularity map: Explicit color (and intensity) of the specular reflections (black = no specular
                             reflections).
          - glossiness map: How glossy the surface is (inverse is the roughness map).
      - normal/bump maps as usual
    - energy conservation: When increasing reflectivity of material, the specular reflections get smaller
                           and brighter, but other areas (diffuse reflections) get accordingly darker
                           (e.g. if a cloth gets wet, it look darker).
    - Fresnel: Surfaces reflect more light when viewed at grazing angles (Fresnel's law says that more
               light is reflected when hitting the surface at low angle). E.g. a rendered ball will be
               brighter at edges. 

- pixel art upscaling methods
  - general upscaling methods (nearest-neighbour, bilinear, bicubic, vector tracing, ...)
  - specialized methods: because pixel art offers some presumptions (small palleter, small-scale features, common patterns etc.)
    - EPX: oldest, simple (4 neighbour pixels, 5 conditions), OK for fonts
    -