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Shadow Map "Receiver" Depth Bias

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I've been trying to implement "receiver plane" depth bias based on these two references:
As I understand it the basic principle is as follows: convert the screen-space depth derivatives into shadow map space to work out how much the slope changes as you move around in the shadow map, then use this information to compute a per-tap depth bias. Here's my GLSL code, more or less as it appears in Tuft's presentation (the Microsoft reference above, slide 52 onwards):
//  get shadow map texture coords (viewSpacePosition is extracted from the 
//  z buffer and view ray)
  vec4 shadowCoords = uShadowMatrix * vec4(viewSpacePosition, 1.0);
  shadowCoords.xyz /= shadowCoords.w;
//  light space -> screen space matrix
  mat2 jacobian = mat2(
    dFdx(shadowCoords.x),		dFdy(shadowCoords.x),
    dFdx(shadowCoords.y),		dFdy(shadowCoords.y)
  jacobian = inverse(jacobian);
//  get depth ratios (how depth varies as we move in shadow map)
  vec2 texelSize = 1.0 / vec2(textureSize(uShadowTex, 0));
  vec2 right = jacobian * vec2(texelSize.x, 0.0);
  vec2 up = jacobian * vec2(0.0, texelSize.y);
  vec2 dfDepth = vec2(dFdx(linearDepth), dFdy(linearDepth)); // linearDepth = linearized depth from z buffer
//  get depth deltas (per-tap depth offsets for filtering)
  float rightDelta = right.x * dfDepth.x + right.y * dfDepth.y;
  float upDelta = up.x * dfDepth.x + up.y * dfDepth.y;
//  perform shadow map filtering (25 tap box filter)
  float shadowResult = 0.0;
  for (int i = -2; i < 2; ++i) {
    for (int j = -2; j < 2; ++j) {
      vec2 offset = vec2(i, j);
      float sdepth = texture(uShadowTex, shadowCoords.xy + offset * texelSize).r;
    //  get bias for this tap
      float sbias = offset.x * rightDelta + offset.y * upDelta;
    //  apply bias
      sdepth += sbias * uSlopeScale + uConstantBias;
      shadowResult += step(shadowCoords.z, sdepth);
  shadowResult /= 25.0;
The shader above executes in the following context: I'm rendering front faces into the shadow map. The shader reads the depth from the hardware z buffer and gets the view space position by linearizing this depth and multiplying by a view ray (typical deferred rendering stuff). The matrix "uShadowMatrix" (which converts from view space to shadow map space) is constructed as follows:
shadowMatrix = mat44(
shadowMatrix *= shadowCamra.getProjectionMatrix();
shadowMatrix *= shadowCamra.getViewMatrix();
shadowMatrix *= currentCamera.getWorldMatrix(); // inverse view matrix
I'm not sure that either of these things makes any difference. Also, I adjust uSlopeScale in the range [-2,2] and get no difference whether it is positive or negative. See the screenshots below.
I'm completely stumped and suspect that I'm missing something crucial, but I can't seemt to figure out what - all insights welcome!
Here's what I'm currently getting, note also that the large banding artefacts are view-dependant (they slide around as the camera moves):


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Sorry to bump this, but I'm still stumped wacko.png


I'd be happy just to see some working code, if anyone has a working implementation?

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