gpu_prog_for_video_game_27.md

• 27: Projective Textures
  • What is projective texturing ?
  • Texture matrix
  • ProjectTexture -- setup
  • ProjectTexture -- uniforms
  • ProjectTexture -- structures
  • ProjectTexture -- vertex program
  • ProjectTexture -- fragment program
  • C# script to set up projector matrix
  • Watch out for the reverse projection!

27: Projective Textures

What is projective texturing ?

• An intuition for projective texturing

  • The slide projector analogy
  • 
  • project onto various objects in the scene

• Another exmaple: different locations

  • 

Texture matrix

from "The Cg Tutorial", p.252

• 
• The way we formulate this is basically to setup the same set of matrix transformation we would, when taking 3D object in the scene and projecting them onto a camera plane, except here we're using the light position and orientation as if the light itself is the camera.
• so we still have the same modeling matrix which translates model coordiantes into world space.
• and then transform into a view space, but this view space is from the point of the projecting light, not the camera.
• similarly we will go through a projection matrix, again, from the point of view of the light, not the camera.
• the last matrix is used to shift everything from [-1,1] to [0,1] because that's what texture lookups expect.

ProjectTexture -- setup

• 
  • "ForwardAdd" lightmode

ProjectTexture -- uniforms

sampler2D _BaseTex;
float4 _BaseTex_ST;
sampler2D _SpotPower;

float _SpotPower;

float4x4 _myProjectorMatrixVP;
float3 _spotlightDir;

• _myProjectorMatrixVP view projection matrix for the light

ProjectTexture -- structures

struct a2v { // application to vertex
    float4 positionOS: POSITION;
    float3 normalOS: NORMAL;
    float2 uv : TEXCOORD0;
}; 

struct v2f { // vertex 2 fragment
    float4 sv: SV_POSITION;
    float2 uv: TEXCOORD0;
    float3 positionWS: TEXCOORD1;
    float3 normalWS: TEXCOORD2;
    float4 positionProjected: TEXCOORD3;
};

ProjectTexture -- vertex program

• 

ProjectTexture -- fragment program

• 
  • the lightDir vector is pointing from vertex to the light, the _spotlightDir is pointing out of the light, so I put the minus sign on lightDir in spotlightEffect computation, and take dot product on that to get something falls off as the angle increases to get nice spot effect.
    • and then we take it to some pow, and by changing the power we can change the size of the spot.

  diffAlmost *= tex2Dproj( _ProjTex, input.positionProjected );

    - new version of `tex2D` : `tex2Dproj`
    - instead of giving a 2D `uv` coordinate, you give it a 4D vector that represents s,t,r,q coordinates space related to the light source.  and this `tex2Dproj` handles that divide by q.
  

C# script to set up projector matrix

• 
• this is a c# script to define component that will attach to a camera object. but this camera object is attached to is not the main camera, it's sort of a fake camera we're using for the spot light effect.
• I set this [ExecuteInEditMode] directive that tells unity to run this in editor, not just in game.
• once we have myCamera component, we can go ask it what's the projection matrix that you're using and what the world to camera matrix is.
  • myCamera.projectionMatrix , myCamera.worldToCameraMatrix
• And now I'm goging to engage in some bad programming practice.

  // Quick & dirty; it would generally be better to set properties in specific materials.
  // :  use component.material.Setxxxx ?
  Shader.SetGlobalMatrix( "_myProjectorMatrixVP", projectionMatrix );
  Shader.SetGlobalVector( "_spotlightDir", transform.forward );

  • the Shader class has some static methods, those static methods will go through EVERY single material . And each time they find a shader that has the variable with the specific name they set the values. But it does that for EVERY single material. So this wouldn't work if you want to have multiple spot light. And looking string in unity is extreamly time comsuming.

Watch out for the reverse projection!

• 

• There is one interesting effect that sometimes you have to watch out for , the underlying projection matrix math doesn't care which direction you're going.

  • so you can have this weird mirrored projections what you might have to be careful about.

• The most common use actually is as the core of implementing shadows maps, which is method of handling shadow effects.

• I should let you know that unity does have spot light builtin to it, along with the ability for those spot light to project textures.

  • Unity implements this in the way that is different that what I describes here.