81c71fbbb9
The previous code used the max of the texel's width or height footprint in world space to compute the offset; this could both overestimate or underestimate the bias causing some peter panning or acne. The new code replaces a sqrt with a dot, but is otherwise similar.
66 lines
3.3 KiB
GLSL
66 lines
3.3 KiB
GLSL
//------------------------------------------------------------------------------
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// Shadowing
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//------------------------------------------------------------------------------
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#if defined(VARIANT_HAS_SHADOWING)
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/**
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* Computes the light space position of the specified world space point.
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* The returned point may contain a bias to attempt to eliminate common
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* shadowing artifacts such as "acne". To achieve this, the world space
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* normal at the point must also be passed to this function.
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* Normal bias is not used for VSM.
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*/
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highp vec4 computeLightSpacePosition(highp vec3 p, const highp vec3 n,
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const highp vec3 dir, const highp vec2 b, highp_mat4 lightFromWorldMatrix) {
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#if !defined(VARIANT_HAS_VSM)
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// --------------------------------------------------------------------------------------
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// Anisotropic Normal Bias for Shadow Mapping
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// --------------------------------------------------------------------------------------
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// To prevent shadow acne, we must push the geometry along its normal to clear the
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// quantization steps of the shadow map's discrete depth grid. The exact physical depth
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// error we must clear is proportional to the shadow texel's world-space dimensions.
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//
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// This implementation computes the exact geometric projection of the rectangular
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// shadow map texel onto the surface normal.
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//
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// 1. Coordinate Space Transition:
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// We project the world-space normal onto the light's X and Y basis vectors (L_right,
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// L_up). This gives us the lateral components of the normal in Light Space (n_Lx, n_Ly).
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//
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// 2. The Implicit sin(theta) Slope Scale:
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// Because the normal is a unit vector, the magnitude of its lateral components in
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// light space inherently equals sin(theta), where theta is the angle of incidence.
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// This perfectly and automatically scales the bias from 0.0 (top-down, flat surface)
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// to maximum (grazing angle).
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//
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// 3. Exact Anisotropic Footprint (The L1 Norm):
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// Shadow texels are rarely perfectly square due to Cascaded Shadow Maps (CSM) or
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// Light Space Perspective Shadow Maps (LiSPSM). Jx and Jy are the physical world-space
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// dimensions of the texel.
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// By evaluating `abs(n_Lx * Jx) + abs(n_Ly * Jy)`, we compute the exact scalar
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// projection of the rectangular texel footprint.
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// - It is superior to `max(Jx, Jy)` which assumes a massive square and causes Peter Panning.
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// - It is superior to `length()` which assumes an ellipse and under-biases the corners.
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// --------------------------------------------------------------------------------------
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// Extract the first row (Light's Right vector in World Space)
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highp vec3 L_right = vec3(lightFromWorldMatrix[0][0], lightFromWorldMatrix[1][0], lightFromWorldMatrix[2][0]);
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// Extract the second row (Light's Up vector in World Space)
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highp vec3 L_up = vec3(lightFromWorldMatrix[0][1], lightFromWorldMatrix[1][1], lightFromWorldMatrix[2][1]);
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// Project the world normal onto the shadow map's 2D grid
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highp float n_Lx = dot(n, L_right);
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highp float n_Ly = dot(n, L_up);
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// Apply the anisotropic normal bias
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p += n * (abs(n_Lx * b.x) + abs(n_Ly * b.y));
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#endif
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return mulMat4x4Float3(lightFromWorldMatrix, p);
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}
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#endif // VARIANT_HAS_SHADOWING
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