[ta- frost wolf \u may- hundred people plan] 2.2 model and material space

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Rendering pipeline and model foundation

Realization principle of the model

Connect the dots into a line , Line enclosing surface , And then form a polygon , So far, a model in the model space has been formed .

UV

UV Like a mineral water bottle , Cut it apart . Tile in a two-dimensional coordinate system .
Every position of the bottle （ Every vertex of the model ）, In three-dimensional space and two-dimensional space can be one-to-one correspondence . The corresponding position of the vertex in the two-dimensional coordinate system is the texture coordinate of the vertex .

therefore , Each vertex can use the texture coordinates to obtain the information stored in the map .

Model UV

In modeling software Conduct UV an ,UV Will be placed in a horizontal direction for U Vertical is V, Range （0-1） In a two-dimensional coordinate system
Expanded UV stay SP Draw a map in （ Diffuse map 、 Normal map 、 height map 、 Metallicity mapping 、AO Mapping ）

Information contained in a model （ With OBJ File as an example ）

V Vertex coordinate data （ Of a single vertex in model space XYZ coordinate ）
VT UVW Map （ The horizontal direction is U, The vertical direction is V, The scope is 0-1 Between ）
VN Vertex normals （ The current peak Z The orientation of the axis , This Z The axis is the orientation of the vertex itself in model space . And the vertex normals ultimately determine the orientation of the face ）
Vertex color （ Of a single vertex RGBA Channel color information ）

OBJ And FBX Format comparison

fbx The format supports vertex Color, Multi quadrant UV as well as LOD（ Level of detail model ）

Material foundation

texture of material

Some objects don't scatter much when they reflect light （Scatter）, This produces a small high spot ;
And some objects will scatter a lot of light , Produce a highlight with a larger radius

Diffuse reflection

Lambertian The method simply considers that light is reflected evenly ;

Specular reflection

Specular reflection is the reflection of incident light according to the surface normal , And there is energy only in the direction of reflection , The energy in other directions is 0;（ Add ：Games101 Mentioned in , Specular reflection , The reflection angle is not necessarily equal to the incident angle , The highlight area can be seen only within a certain range of the specular reflection angle , So the energy in other directions is 0 It's a general statement ）

refraction

For the dielectric of glass , In addition to reflection, it also refracts part of the light into the object according to the refractive index of the object , The amount of reflected and refracted energy is determined by Fresnel's law .

Rough specular reflection （ Add the specular reflection above ）

Frosted metal surface

Rough specular reflection

All kinds of ground glass

Multi layer material

Besides the material of the object itself , The outer layer is also attached to other materials such as ： Transparent paint, etc

Subsurface scattering

Subsurface scattering objects are generally translucent objects , for example ： Jade 、 Candles, etc ;

Multilayer skin model

Think of the skin as three layers , Respectively ： Grease layer , Epidermis , Dermis .
The grease layer provides direct reflection , Produce a highlight effect on the skin surface ;
Another part of the light enters the surface layer through refraction , Light is partially absorbed after entering these layers （ To produce color ） And scattering , And then from the exit point near the incident point in the skin . This process produces the effect of subsurface scattering .

Change the material surface

reason ：

1. There is no perfectly smooth plane in the real world ;
2. A model vertex carries a finite number of normals .
   Therefore, normal mapping can be used to supplement
   And diffuse reflection , highlights , The normal is involved in the calculation of refraction , So make adjustments to the normals , Can affect the result of its illumination calculation .

Model data analysis

1. Vertex animation ： In the vertex shader , Modify the vertex position of the model . So as to achieve the effect of model movement ;
2. Texture animation ： In the fragment shader , Modify magical UV Information , When sampling maps , Produce movement effect due to displacement ;
3. Vertex color ： At the time of rendering , Affect output results . Control the color range

Texture animation

principle 1

By modifying the normal map , Change the direction of reflection and refraction . Normals by 1 change 2, Reflected light is also caused by 1 change 2, As shown in the figure below ：

Simply modify the normal , Keep other data unchanged , Can change the texture and appearance of an object .

principle 2

change UV Sample points to produce animation

Vertex animation

Operate on the vertices of the model in the vertex shader to produce the effect of animation .
Vertex shaders are applied to every vertex , The data of each vertex is different , So the same calculation formula is on different vertices , The calculated results are also different
In some 3D modeling software , The animation K Frame can also be understood as a kind of vertex animation ;

Vertex color

Control vertex color in vertex shader , application ： shadow , Line thickness, etc

Barycentric coordinate color interpolation

Use vertex coordinates for interpolation （ Is not very good , Let's see later Games101 Did you say ）

Vertex normals and face normals

A good understanding , Vertex normals are awesome .

Expand

In cartoon rendering , Generally, the back of the model is expanded outward along the normal direction , Then output a color , So as to achieve the effect of tracing , In objects that do not use smooth shading , The size of the face remains unchanged , Recolor will cause the fracture of the outer contour , This is caused by the discontinuity of normals .

summary

What other uses do vertex colors have ？

1. Can be used to pre specify lighting 、 Light blocking and other visual effects .
2. Render the model separately as a mask , Including but not limited to lighting calculation separately , Deal with animation, etc. separately ;
3. Coloring with vertex colors ;
   The influence of model smoothing group on normal ？
   After using smoothing , Vertices re interpolate normals according to the smoothed model , Faces under the same smoothing group will have a smoothing effect ; As shown in the lower part of the figure below （ source ）：