shading - Fabio Pellacini13 projects in digital art • shading © 2007 fabio pellacini • 25 mapping • takes an image and apply on a surface • mapping function – for each point

1

projects in digital art • shading © 2007 fabio pellacini • 1

shading


overview

• what is shading? • surface characteristics

– color, light response, texture, roughness • light response

– brdfs, non-physical styles • texturing and painting

– projections, parameterization, procedurals – bumps and displacements

• shader creation – shading trees, shading languages

2


what is shading?

• defining the material characteristics – color – light response – textures – roughness


examples

3


examples - color


examples - light response

4


examples - texture


examples – roughness

5


color

• the main color of an object

• what is color? – human perception of light, i.e. power spectrum of

electromagnetic radiation – very complicated subjects!

• involves a lot of physiology and psychology • not fully understood yet

– we will only care about color in digital images


color

• visible colors can be describe using 3 numbers – which ones? – choose the easier representation

• red, green, blue (RGB) – color used by monitors to create images

1,0,0 0,1,0 0,0,1 1,1,1

0,0,0 0.5,0,0 0,0.5,0 0,0,0.5

1,1,0 1,1,0 1,1,0

6


color

• RGB is hard to use – changes to lightness and saturation

• hue, saturation, value (HSV) – easier to specify colors – based on human perception

0,1,1 0.4,1,1 0.7,1,1 0,0,1

0,0,0 0,1,0.5 0.4,1,0.5 0.7,1,0.5

0.2,1,1 0.9,1,1 0.5,1,1


light response

• specifies how a surface reflects light – will be covered in detail in upcoming lecture – often called Light reflection model

7


light response – phong model

N

R V L



8





9





10


non-photorealistic styles (NPR)

• realistic materials or cartoon-like ones? – choice of animation style


non-photorealistic styles (NPR)

Phong NPR

© G

ooch

et

al.

11


cartoon look: real or NPR?

• avoid 2d cell animation look – today’s 2D animation often rendered in 3D

• Pixar/PDI look: a mixture of the real and NPR – realistic, physically-based building boxes – combined in non-realistic ways


texturing

• real world: surface properties vary across surfaces

12


texturing

• CG: change shading parameters – for each point on the surface


texturing

• often textures are images

13


mapping

• takes an image and apply on a surface

• mapping function – for each point on the surface specifies a point in the

image

Mapping Function

Surface Image


projections

• defines mapping by projection – projection follows simple shapes

• plane, box, cylinder, sphere

• useful for man made object and simple shapes

14


projections - planar


projections - cubical

15


projections - cylindrical


projections – spherical

16


tiling

• repeat texture when mapping is out of it

Untiled Tiled


distortions

• textures get minified and magnified on surfaces – always try to minimize this

Distorted Undistorted

17


seems

• seems are created by tiling or joining texture edges


fundamental problems

• seems and distortions are unavoidable – inherent in any form of mapping

• mathematically speaking – seems originates from discontinuities in the mapping

function – distortions originates from the behavior of the second

derivative of the mapping function

18


parameterization

• projections do not work if geometry if different

• store the mapping function onto the geometry – store texture coordinates for each control point – interpolate along the surface – often called UV mapping

• parameterization: process of assigning intrinsic texture coordinates onto objects


subdivision surfaces and mapping

Level 0 Level 1 Level 2

19


subdivision surfaces and mapping

Parameterization Projection


visualizing and editing UV maps

20


procedural texturing

• texture variations are generated algorithmically – user can tweak parameters that control texture look – often specified in 3d for solid materials

[Wol

fe /

SG

97 S

lide

set]


painting

• in industry almost all textures are from paintings – better match with the rest of the look – only required details

• paint directly on the surface – avoid seems and distortions – but cannot be reused on other meshes

21


painting


painting

• painted textures appear distorted when viewed

• this “pre-distortion” avoid distortion issues when applied to the given surface

• if not using a 3d paint tool, the artist needs to apply this distortion as he works – in practice this is never done

23


painting - stretching


types of maps

• can modify every surface property – diffuse color – specular color – transparency – surface geometry

24


diffuse maps

• variations to diffuse color


specular maps

• variation of the highlights

25


transparency maps

• variation of object transparency


bump maps

• variations of surface normals

26


displacement maps

• variations of surface position and normals


bump vs. displacement maps

Bump map Displacement map

27


bump vs. displacement maps

Bump map Displacement map


type of maps

• almost always combine various types

28


shader creation

• shading trees – shaders are collections of nodes in a graph – visual representation – used in all the modeling software

• shading languages – shaders are programs – programmer representation

• much more flexible – used in productions by the more technically inclined

• but disappearing since too hard to control


shading trees - examples

• phong shading with a texture

29


shading languages - examples

• phong shading with a texture

surface phong(uniform color C; uniform float Kd, Ks, roughness; string txt) { Cs = C * texture(txt, s, t) * diffuse(N) + Ks * specular(N,E,roughness); }

Documents

shading - Fabio Pellacini13 projects in digital art • shading © 2007 fabio pellacini • 25 mapping • takes an image and apply on a surface • mapping function – for each point