Implementation shadow introduction

Plane shadow

According to the direction of light , Project each vertex of the object onto the plane ground . On .、

Principles of Mathematics : Similar triangles

shortcoming ： It can only be projected to the plane , The projected object must be between the light and the plane

Shadow casting

Set one for each shadow projector A camera , Render shadow shadows into shadow textures , Render shadow receiver , And blend with shadow texture

stay Unity To realize ：

Projector Components : Its parameters can be set to generate a visual vertebral body ,Projector Will use the material provided to it , Draw all the objects in its visual vertebra , Use Render Texture Generate shadow maps , Paint the shadow shadow volume to the shadow texture , to Project Sets the material for blending objects and shadow maps

Shadow mapping

No depth map on the right , Record the depth value of each pixel , The darker the color, the closer it is to the camera , The white area represents the sky box .

The core idea of shadow mapping ： Pixels in the shadow are not visible , That is to say, the depth value of the pixel in the shadow is greater than the depth value of the pixel in the shadow map .

technological process ： First, generate shadow mapping from the perspective of care , Then generate the whole scene object from the camera's perspective , After converting the camera coordinate system to the shadow mapping coordinate system , Guessing the depth of two textures is , If the pixel depth value in the camera perspective is greater than the pixel depth value in the shadow map , Then the pixel belongs to shadow .

Unity Screen space shadow mapping in

1. Render the depth map of screen space ;
2. Render shadow mapping from the direction of the light source ;
3. Do a shadow collection calculation in screen space (Shadows Collector), This calculation will get a - Screen space shadow texture ;
4. When drawing objects , Use the screen coordinates of the object uv, sampling 3 Screen space shadow map generated in .

What shadow mapping does in one frame

Optimization of shadow mapping

Self shadowing problem

Due to the limited resolution of shadow mapping , Discrete sampling points and values . The deviation on may cause incorrect self shadowing , Also known as Z- Fighting Or Shadow Pink (Surface Acne)

When comparing depth , To avoid surface self shadowing , The fault tolerance threshold needs to be set
● Depth migration (Depth Bias)
● Normal offset (Normal Bias)
But when the depth offset is set too large , Will cause light leakage . That is, there is a disconnection between the shadow and the projector , It's also called Peter Panning problem .

From shadow

Light leakage phenomenon ​​​​​

 

Self shadow and shadow artifacts ​​​

Depth migration : Increasing the depth offset brings the pixel closer to the light source ;
Normal offset : Offset outward in the direction of the surface normal ;
The offset unit is the texture element of shadow mapping ;
stay Shadow Receive The calculation phase , Pixel by pixel ;
Only use in shadow depth test , Will not affect the real scene .

Unity Offset optimization in

Do offset in lift Wenli stage , Let the shelter reverse offset , That is, subtract the offset , That's better performance

 

Aliasing problem

Look at shadow mapping in the process of signal reconstruction :
● Initial sampling : Render shadow mapping ;
● Resampling : Sample the signal from the perspective of the camera ( Shadow mapping ) Resampling .

Perspective aliasing

● Shadow mapping is evenly distributed in world space .
● After the object in the visual cone passes through the perspective projection , Near and far away , The pixels in the near plane and the far plane are the same ;
● There are significantly fewer sampling points for elements close to the observer .

  Solution ： Cascade shadow mapping

Divide the visual cone into several sub visual cones ; Calculate independent shadow maps of equal size for each sub view cone ;

  Resampling error

Shadow mapping is a dynamically generated texture ; wave filtering —— Solution of texture sampling error

wave filtering
● Image processing , Filter to emphasize some features or remove some unnecessary parts of the image ;
● Filtering is a neighborhood operator , The final output value of a given pixel is determined by the value of the pixel around the pixel .
Shadow filtering
● Use a portion of the shadow map sample points to calculate a specified value View The method of sampling the final shadow result of the point .

 PCF wave filtering

1. Generate shadow mapping from light direction ;
2. from view Perspective rendering scene , Use shadow mapping for depth testing ;
3. When the coloring point passes the depth test , Take the specified size around the pixel ( Filter core ) The scope of the . Calculate their visible average as output .

PCF Different implementations of filtering

Number of samples K
● Regular filtering ,3*3 or 5*5;
● use Poisson Disk To distribute a certain number of sampling points .
Filter kernel function
● Gaussian function as filter function .

Unity Shadow mapping optimization summary

● Use depth offset and normal offset to solve the problem of self shadow ;
● Use cascaded shadow mapping to solve the problem of perspective projection ;
● Use PCF Solve the problem of resampling , And produce the effect of soft shadow .

Tutorial address