Tweak float dither calculates positive DC bias for luma

[msg7086]

Analysis attribution: This issue analysis was created by Gemini 3.7 Flash.
The GitHub user who submits this issue does not claim authorship of the analysis.

Finding

• Finding ID: F33-tweak-float-dither-luma-bias
• Status: Reproduced defect
• Severity: Medium
• Affected component: Tweak
• Source location: avs_core/filters/levels.cpp:1642-1666

Summary

In Tweak::Tweak(), bias_dither_luma subtracts 1 unconditionally when dither == true. For 32-bit float clips (pixelsize == 4), where dither_strength has been scaled down by 65536, the unscaled -1 term produces a large positive DC offset (+0.498046875), causing all luma pixels to be brightened by ~ +0.001945 in float dithering.

Affected Code

  if (pixelsize == 4)
    dither_strength /= 65536.0f; // same dither range as for a 16 bit clip

  if (dither) {
    // lut scale settings
    scale_dither_luma = 256; // lower 256 is dither value
    divisor_dither_luma *= 256;
    bias_dither_luma = -(256.0f * dither_strength - 1) / 2;
...
    scale_dither_chroma = 16; // lower 16 is dither value
    divisor_dither_chroma *= 16;
    bias_dither_chroma = -(16.0f * dither_strength - (pixelsize==4 ? 1/256.0f : 1)) / 2; // -7.5
  }

void Tweak::tweak_calc_luma(BYTE *srcp, int src_pitch, float minY, float maxY, int width, int height)
{
  float ditherval = 0.0f;
  for (int y = 0; y < height; ++y) {
    const int _y = (y << 4) & 0xf0;
    for (int x = 0; x < width; ++x) {
      if (dither)
        ditherval = (ditherMap[(x & 0x0f) | _y] * dither_strength + bias_dither_luma) / (float)scale_dither_luma;

Correct Behavior

Adjust bias_dither_luma for 32-bit float similarly to chroma ((pixelsize == 4 ? 1/256.0f : 1) or appropriate float scaling), centering ditherval around zero.

Reproduction

Call Tweak(clip, hue=0.0, sat=1.0, bright=0.0, cont=1.0, dither=true) on a 32-bit float clip (CS_YUV444PS).

Observed Result

observed: output luma is shifted upwards by ~ +0.00195 due to unbalanced positive dither bias
expected: output luma dither is zero-centered and preserves overall average brightness

Impact

Applying Tweak(dither=true) on float video introduces a visible whitening/brightening artifact across the luma plane.

Validation Criteria

Float luma dithering values are symmetric and zero-centered without introducing a DC bias offset.

Version and Environment

• AviSynthPlus revision: 80da03a5ee437c2b103dbebc9fab43480a9d97c6
• Platform and compiler: Linux x86_64 / Windows x64 GCC/MSVC
• CPU features used: host-default build.