fix(ot128): introduce anti-flicker downsample dithering pattern

[mojomex]

PR Type

• Improvement

Related Links

• feat(hesai): implement mask-based pruning filter for Hesai #251 -- introduced mask based pruning filter

Description

⚠️ The below use cases are not affected by the flickering issue (already working fine):

• masks converted from feat(hesai): add a ring section filter to Hesai sensors #182
• black-and-white masks
• OT128 in standard/non-high-resolution mode
• other sensors

While the mask based pruning filter introduced in #251 works well for all supported Hesai sensors, including OT128 in standard mode, OT128's high resolution mode employs a dithering pattern that interferes with the pruning filter's one.

This results in flicker and could degrade perception performance etc.
This PR extends the DownsampleMaskFilter by allowing sensors to specify a transform function for azimuth/channel coordinates passed to the dithering algorithm. This allows the developer to include knowledge about the sensor's dithering/scan pattern into the mask dithering step.
All sensors except OT128 are using the default transform, as before, with only OT128 using a specialized one.

For details on how OT128 dithers, see the chapter B.4. Laser firing time of each channel of the OT128 User Manual.

Basically, the pattern is repeating every 2 blocks x 4 channels, so the specialized transform ensures that the points that change within one tile are mapped to the same index for dithering.

🟢 Evaluation

Filter disabled (unaffected by this PR)

The dithering pattern causes some oscillating points but the image is overall stable.

downsample_flicker_nomask-2025-03-07_18.00.20.mp4

Filter enabled, before fix

The interference between sensor dithering and downsample filter causes strong flickering.

downsample_flicker_before-2025-03-07_17.56.40.mp4

Filter enabled, after fix

The specialized dithering pattern plays well with the sensor dithering.
Osillations are more noticeable than in the disabled case due to there being larger gaps between points.
The actual oscillation amplitude is the same.

downsample_flicker_after-2025-03-07_17.58.22.mp4

Review Procedure

• Code review
• Compare implementation with datasheet
• Test with downsample mask (different downsample factors or gradients)

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• Commits are properly organized and messages are according to the guideline
• (Optional) Unit tests have been written for new behavior
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Post-Review Checklist for the PR Author

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• All open points are addressed and tracked via issues or tickets

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[codecov]

Codecov Report

Attention: Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.

Project coverage is 28.47%. Comparing base (39fbbe7) to head (b3e6d12).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
- Coverage   30.23%   28.47%   -1.76%     
==========================================
  Files         112      112              
  Lines        9695     9607      -88     
  Branches     3652     2949     -703     
==========================================
- Hits         2931     2736     -195     
- Misses       6224     6398     +174     
+ Partials      540      473      -67     

Flag	Coverage Δ
differential	28.47% <35.71%> (?)
total	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Common	27.30% <100.00%> (-22.94%)	⬇️
Hesai	30.79% <0.00%> (+0.38%)	⬆️
Velodyne	37.83% <ø> (+0.17%)	⬆️
Continental	28.15% <ø> (+0.17%)	⬆️
Robosense	0.00% <ø> (ø)

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