Projective texturing and falloff

Question

I quite understand how projective texturing works. I implemented successfully a shader for that following nvidia doc.

The major problem I'm facing is that with that implementation the projector frustum is used only to determine the texture coordinate in projective space, but it doesn't clip anything outside the projective frustum volume.

In other words, if I have a projector pointing toward a object, the texture will be projected on it even if the object is outside its frustum. In addition when projector is almost parallell to the projected object, the texture stretching is too evident and I would like to fade it out.

Now, I'm trying to understand how Unity handles their built-in projectors. I found this example. Here's vertex and fragment relevant code:

v2f vert (float4 vertex : POSITION)
{
  v2f o;
  o.pos = mul (UNITY_MATRIX_MVP, vertex);
  o.uvShadow = mul (_Projector, vertex);
  o.uvFalloff = mul (_ProjectorClip, vertex);
  return o;
}

fixed4 frag (v2f i) : SV_Target
{
  fixed4 texS = tex2Dproj (_ShadowTex, UNITY_PROJ_COORD(i.uvShadow));
  texS.rgb *= _Color.rgb;
  texS.a = 1.0-texS.a;

  fixed4 texF = tex2Dproj (_FalloffTex, UNITY_PROJ_COORD(i.uvFalloff));
  fixed4 res = texS * texF.a;
  return res;
}

I think that _Projector matrix is a classical matrix to transform vertices coordinates into projector space. (In fact the fragment just use the transformed coordinates to sample the projected texture).

What I'm really missing is how Unity construct _ProjectionClip matrix. The transformed vertex coordinates are used to sample the falloff texture (that I think it is what I really need).

Does anyone know how the _ProjectionClip is constructed? Or how to achieve a similar effect?

Note: I did something similar for calculating spotlights falloff, but there I used a single matrix multiplication in the vertex shader to transform vertices into lightspace, and the squared distance in the fragment shader to calculate texture coords for a lookup into the attenuation texture.

Answer 1

For clipping pixels outside the frustum, you can use a texture with the border mode enabled and a black border. (Or a black 1-pixel border authored directly in the texture). Alternatively, in an additive blending pass, you can add code to the shader to discard the pixel if the UVs are outside the unit square.

For fading the projection out when the projected object is close to parallel, the ordinary lighting N·L factor will mostly take care of this. You can fade things out sooner by biasing the N·L factor a little bit.

As far as the Unity shader goes, my guess is it simply lets you have different scales for the main texture and falloff texture. For instance, the main texture could be some image and the falloff texture might be a white circle fading into a black background, which multiplies with the main texture and restricts it to only appear in that circle. The user could adjust the frusta for the two textures independently.

In other words, the _ProjectionClip matrix is simply another classical projection matrix, potentially built with different parameters. It doesn't magically clip anything; it's just that some combination of factors reduces the light color to zero outside the desired frustum.