[TA frost wolf \u may hundred people plan] Figure 3.5 early-z and z-prepass

3.5.1 Depth testing

Used to solve the occlusion problem of visible objects
You can customize the conditions of depth test

Problems caused by in-depth testing

Blocked objects will not appear on the screen, resulting in performance waste .

3.5.2 Early-Z

The depth test is introduced before the calculation of the slice shader , Eliminate unnecessary pieces .

Add

Early-Z You can also add template tests in the process of

Failure conditions

1. Turn on Alpha Test or clip/discard Wait for the discard fragment operation ;
2. Manually modify GPU The depth obtained by interpolation ： The situation is the same 1
3. Turn on Alpha Blend： Generally, objects with transparency blending turned on will turn off depth writing ,ZWrite Off
4. Turn off depth test Depth Test

Efficient use of Early-Z

If the rendering order in the following figure is from far to near （3-2-1）, There is no optimization effect

Ideal situation ： Sort objects from near to far, and then render

3.5.3 Z-prepass

Use two Pass：

• At the first Pass namely Z-Prepass Only the depth is written in , No color is calculated and output ;
• In the second Pass Turn off depth writing , And set the depth comparison function to equal ;
• If the test fails, no subsequent slice calculation will be carried out , Save resources ;

thus , Each object will render these two pass, Of all objects Z-Prepass As a result, a minimum depth buffer is automatically formed Z-Buffer.

The difference lies in ： If it is manual sorting, it costs a lot cpu Calculation , If it is Z-Prepass Words , still GPU Calculation .

Pseudo code implementation ：

The resulting problems —— Dynamic batch processing

An object has multiple pass Of shader It is impossible to perform dynamic batch processing , That is to make drawcall Double , as follows ：

The way 2： Separated in advance PrePass

• The original first Pass（ namely Z-Prepass） Separate out as a single Shader, And use this first Shader Put the whole scene Opaque Object rendering side .
• At this time, only the second material is left Pass, Still turn off deep writing , And set the depth comparison function to equal .
  

3.5.4 Z-Prepass Other questions

Z-Prepass The calculated consumption of ： Through experiments, we can see that efficiency may not be improved in small scenes ;
Whether to use... Should be selected according to the actual situation Z-Prepass

By combining the two methods , Achieve the correct rendering effect .（ Can render the back of hair ）

Homework

preZ Test the effect of

Not written in advance Zbuffer The effect of

By adding the following Pass, achieve preZ Purpose

// Extra pass that renders to depth buffer only
		Pass {
    
			ZWrite On
			ColorMask 0
		}

preZ Result display

summary earlyZ The limitation of

• Early-Z It is a function supported by hardware , Unable to get API To make the call
• If it does Alpha Clip Or deep write , Can cause EarlyZ invalid
• Early-Z Usually by preZ To achieve , But in DrawCall Used in particularly high scenes preZ Will cause negative optimization （ A lot of sorting takes up performance ）
• If it's on Alpha Test（Discard）, It can lead to Early-Z invalid , Even occlusion will occur again Pixel Calculation ;
• Alpha Blend You can talk to Early-Z At the same time , Can carry out the correct Early Calculation （ Because deep writing is turned off ）