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2021_06_22_technical_explanation_of_deep_down.md

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Technical Explanation of "Deep Down" Graphics

Agenda

  • Material
  • Light
  • Hair

Material

Panta Rhei's objective

  • High quality texture
    • Make it look richer than ever
  • Unified quality
    • Have similar quality no matter who makes it
    • Use it anywhere = separate lighting
  • "Physically Based Rendering" to achieve

Material

  • Normal shading
    • OrenNayer + GGX
      • At the time of TGS exhibition, it was Oren Nayer + Blinn Phong, but it will be changed later
  • Skin shading
    • Pre-integrated Skin Shading + GGX

GGX

  • Beautiful highlights of details
    • Leather, metal and plastic textures are more flexible than Blinn-Phong

Skin sample

Texture adjustment value

Value to adjust Meaning
Albedo Diffuse reflectance Non-metals have color, metals are black
Reflectance Gloss reflectance Non-metals are almost black, metals have colors
Glossiness Surface smoothness
Normal Normal
Emissive Optional self-luminous component

Panta Rhei uses 3D-Coat and MAYA shaders to make adjustments using these values

Shader editor

  • Create shaders with graphical editing
    • Graph method for editing by linking nodes

  • Instant preview available

Detail layer

Water monster

Screen space effect

Shader editor pros/cons

  • Pros
    • Artists can create while checking the appearance for themselves
  • Cons
    • The number of nodes increases and tends to be complicated
    • Performance is not considered

Comparison with the conventional

  • Conventional (MT FRAMEWORK)
    • Material (setting based on empirical rules)
    • A combination of shader nodes
  • Currently (Panta Rhei)
    • Material (Physically based. Created with a small number of parameters)
    • Artist-based shaders

Light

Panta Rhei's objective

  • Dynamic light source processing as much as possible
    • Automatically generated dungeons
      • No lightmap in the dungeon
    • handle indirect lighting dynamically
  • Improved rendering quality

Lighting

  • HDR lighting & linear space
    • Rendered in FP16
  • In-game lighting
    • Direct lighting
      • Tile-based deferred + forward
    • Indirect lighting
      • Irradiance volume + parallax correction environment map

Rendering Pipeline

GBuffer

  • Albedo and Reflectance compressed with YCbCr
    • Cb and Cr are stored alternately in pixel units in RT3
  • Options: Save decal information, BRDF type

Problems with YCbC

  • Albedo and reflectance with YCbCr
    • Due to the nature of encoding, color difference information
    • Colors close to black cannot be reproduced accurately
      • Purple etc. occur when exposed to high brightness
      • 8-bit accuracy
  • It is better to put RGB normally
    • Consider introducing Metallic's idea for the same capacity

Rendering Pipeline

Tile Based Deferred Shading

  • Two-stage configuration
    • Create light list
      • Creating light information that is also used for translucency, etc.
  • Direct lighting processing

Light list

  • Create a light list by dividing the image into 16x16 units
    • Create a frustum from tile depth information
      • Divide the frastam into 32 pieces in the line-of-sight direction
        • The 32nd is a frustum in the range of maximum and minimum depth values
        • The rest is a frustum divided evenly from the minimum value
        • Judgment of intersection with the light source for each frustum

Creating a light list

  • Calculate and save the number of lights after culling
    • Get memory blocks from the writelist pool
      • Keep head offset and number of tiles
  • The pool is uint2's Structured Buffer
    • 32-bit write index
    • 32-bit culling mask

Light culling

Direct lighting

  • Processed as a light source with size
    • Spherical light source only
      • Light intensity, magnitude, distance squared attenuation
    • Points, spots, directional
    • Supports shadow and projected textures

Example of direct lighting

Direct lighting shader

  • Process images in 16x16 units with CS
    • Get from light list
      • Get only lights with the 32nd bit enabled
      • Stored in Shared Memory for each light type
    • Expand loops by light type
      • Use the best calculation within each loop
      • VGPR bloated when shadow filter is involved

Rendering Pipeline

Forward

  • Used for skin and translucent
    • Lighting using the results of the light list
  • Search for the mask of the light list from the Z value at the time of drawing
    • Supports shadow and texture projection

Rendering Pipeline

Indirect lighting

Processed at once with pixel shader

  • Irradiance Volume
  • Screen space reflection
  • Parallax Corrected Cubemap

Irradiance volume

Direct lighting

Direct lighting + irradiance volume

Irradiance volume

  • Diffuse reflection of indirect lighting
  • The light that hits the surface is reflected
    • You can also place fake and invisible luminescent polygons

  • Created while the game is loading
    • Use voxel cone tracing
  • Create voxels for your scene
    • Output voxel information from PS to list for each model
      • Illuminated color information and occlusion information
    • Enter the list in voxels (256x256x256)
    • Create voxel mipmaps
  • Creating the final irradiance volume
    • 128x128x128 structure that covers the entire scene
    • Cone tracing by dividing the entire spherical surface into 12 directions
      • Each cone trace result is added as 4 base colors
        • Implemented based on Farcry 3
      • Stored in 3 128x128x128 3D textures

Screen space reflection

  • Image-based ray tracing

  • Linear exploration and dichotomy
    • Glossy reflection is not supported

Parallax correction environment map

  • also called "Parallax Corrected Cubemaps"

Direct lighting

Direct lighting + Parallax correction environment map

Parallax correction environment map

  • Used for diffuse and specular reflections
  • Essential for metal texture

  • Up to 96 in one scene with "deep down"
    • Surface roughness changes the mip level of the environment map
      • Resolution up to 128x128, mipmap up to 2x2
      • Color information is retained in RGBE format on R8G8B8A8

  • Environmental map capture and filtering on load
    • Taken with a tetrahedron(四面体)
    • Create all mip levels at once with CS
  • Parallax correction environment map exploration with stackless tree
    • AABB only BVH
    • Works well as it has almost the same branching direction

Indirect lighting

  • 效果对比

Direct lighting

Indirect lighting

Shadow

  • Supports points, spots, and cascades
    • Shadows are output on a single texture

Accelerate shadow map creation

  • Most light sources do not move or rotate
    • Variable light intensity and range
  • Cache depth in a dedicated temporary buffer
    • Static light source caches static objects
      • Export only moving characters and objects
      • Omit most depth map updates
      • Memory consumption, processing time reduced

Others

SSAO

  • Use ScreenAmbientObscuarance
    • Change loop unrolling to processing in 4 units
      • Good parallelism and execution speed
// before
for (int i = 0; i < NUM_SAMPLES; i++)
    sum += sampleAO(ssC, C, n_C, ssDiskRadius, i, randomPatternRotationAngle);

// after
[loop]
for (int i = 0; i < NUM_SAMPLES/4; i+=4) {
    sum += sampleAO(ssC, C, n_C, ssDiskRadius, i, randomPatternRotationAngle);
    sum += sampleAO(ssC, C, n_C, ssDiskRadius, i+1, randomPatternRotationAngle);
    sum += sampleAO(ssC, C, n_C, ssDiskRadius, i+2, randomPatternRotationAngle);
    sum += sampleAO(ssC, C, n_C, ssDiskRadius, i+3, randomPatternRotationAngle);
}

Texture

  • New texture compression BC7 and BC6H
    • HDR image BC6H
      • Used in environment maps, etc.
    • Color and normal BC7
      • High quality where the alpha channel is not used
  • BC1 where there is not enough memory
  • Roughness uses BC4

Hair

Deep down's objective

  • Express flowing hair
    • Shake it according to the motion
  • Add variations
  • Light processing

Shell method

  • Stacking layers in the normal direction
  • Good appearance if normal and line of sight are parallel
  • Stacking is visible near the contour
    • A significant number of layers are needed to make the stacking inconspicuous at high resolution
  • It is difficult to express flowing hair

Implanting polygons

  • Implanting polygons with hair texture
    • Common methods for expressing hair
  • Can express hair flow and sway
  • The wider the area where hair grows, the more polygon planting work will be done.
    • It's even harder with variations

Deep down hair expression

  • Use a tessellator to grow polylines
    • No need to pre-embed in model mesh
    • The artist just adjusts the parameters
      • Easy to grow without hassle
      • Easy to add variations
    • Consider hair sway with tessellator

Process flow - at the time of loading

  • Generate a point Pn(n=1,...N) that is the source of the polyline from the vertex Pm of the standard pose of the mesh model.
    • Use Compute Shader
    • Uniform variability
  • RWBuffer
    • Triangle index I(i, j, k)
    • Local coordinates p(u, v) in the triangle

Process flow - every frame

  • Mesh model skinning Pm -> P`m
  • Coordinates of polyline elements after skinning Pn -> P`n

  • Back-to-front sorting for translucency priority

  • Polyline generation with tessellator
    • Consider orientation and length

Complete

Speed up writing

  • Lighting calculations in pixel shaders are heavy
    • A large number of pixel shaders run for one pixel
  • Perform lighting calculations in domain shaders
    • Pros: Writing calculation can be done in the number of times of TessFactor
    • Cons: Highlights are less likely to appear

Hair sway

  • Simulation is heavy
    • No need for accurate simulation in the first place => Use speed
  • Read/Write Buffer
    • The result of the previous frame remains in the buffer
    • Velocity = Result of current frame-Result of previous frame

Summary

  • Change to physically based rendering
    • New texture expression
      • Skin expression
      • Use artist-made shaders
    • New lighting
      • Direct lighting - a light source with an area
      • Indirect lighting - irradiance volume, parallax correction environment map
  • Use compute shaders
    • Writing
    • Hair