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Question about L2 spherical harmonics calculation in UnityCG.cginc

Discussion in 'Shaders' started by Darkisu, Feb 11, 2019 at 10:38 AM.

  1. Darkisu

    Darkisu

    Joined:
    Oct 24, 2018
    Posts:
    1
    I noticed a spherical harmonics calculation in UnityCG.cginc where the L2 spherical harmonics lighting decompression was calculated as below:

    half4 vB = normal.xyzz * normal.yzzx;
    x1.r = dot(unity_SHBr,vB);
    x1.g = dot(unity_SHBg,vB);
    x1.b = dot(unity_SHBb,vB);


    My question is that according to this Wikipedia page, the calculation of Y(2, 0) is :
    SH Y20.JPG
    Then why is the SHEvalLinearL2(half4 normal) function calculate Y(2, 0) simply multiplying Z^2 with a value?
    Won't it cause the result contribution among Z axis a little bit stronger than other two axis?
     
  2. jvo3dc

    jvo3dc

    Joined:
    Oct 11, 2013
    Posts:
    1,291
    There is the mathematical formula of spherical harmonics and then there is the practical one. In the conversion, a few things commonly happen:
    - The constant multiplier is removed or simplified, which doesn't actually change the result. The main importance is that the values fall in the target range if needed. (For example 0 to 1 if needed by the texture/rendertexture.)
    - The harmonics are reordered, which also doesn't change the result. This to maximize vector operations. (Like normal.xyzz * normal.yzzx)

    Without the constant multipliers and assuming r=1 (normalized normal vector), the L2 SH coefficients are:
    • 1
    • y
    • z
    • x
    • xy
    • yz
    • 2zz-xx-yy
    • xz
    • xx-yy
    Besides reordering, the only one that is different in Unity is indeed 2zz-xx-yy, which is just implemented as zz. If I look at the contour of the original formula, I can understand the optimization:

    (Third row, middle image)

    The zz approximation loses the (negative) yellow torus, but maintains the double peak at positive and negative z. In essence, it just adds Y0,0 to Y2,0:
    • 1 = xx+yy+zz
    • 2zz-xx-yy + 1 =
    • 2zz-xx-yy+xx+yy+zz =
    • 3zz
    If you then rescale again, you end up with just z^2, which written differently is:
    • z^2 = A*Y2,0 + B*Y0,0 (for specific values of A and B.)
     
    Darkisu and Invertex like this.