Center-Aligned Pixel PWM With Trailing Dither for Dark Banding
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Solution Overview
Problem
Existing liquid crystal displays (LCDs) face issues with dark banding artifacts and fringe-field effects due to non-linear pixel activation times, particularly in head-mounted displays like AR and VR headsets, when using leading edge aligned PWM modulation, which conventional dithering methods fail to address effectively.
Innovation Solution
A center aligned PWM signal is modulated with trailing edge dithering, where the dithering bit is encoded separately to adjust the trailing edge of the pulse, aligning leading edges of adjacent pixels and reducing dark banding artifacts through spatial and temporal dithering techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If leading edge aligned PWM modulation is used, then pixel activation is simplified, but dark banding artifacts and fringe-field effects occur due to non-linear pixel activation times
Solution Approach 1:
The patent applies asymmetry by using center-aligned PWM modulation instead of symmetric leading-edge alignment, where the pulse is centered within the modulation period. This asymmetric positioning of the pulse relative to the modulation cycle eliminates the dark banding artifacts caused by non-linear pixel activation times while maintaining simplified control operation.
Solution Approach 2:
The patent changes the temporal parameters of the PWM signal by introducing trailing edge dithering, which dynamically adjusts the pulse width and position. This parameter modification allows the system to maintain center alignment while adding temporal variation to eliminate banding artifacts, resolving the contradiction between operational simplicity and artifact reduction.
2Measurement precision
If conventional dithering methods are applied, then grayscale precision is improved, but dark banding artifacts persist due to non-linear pixel activation
Solution Approach 1:
The patent applies preliminary action by pre-aligning the leading edges of PWM pulses across all pixels through center alignment before dithering is applied. This preliminary temporal synchronization prevents the non-linear activation that causes banding, allowing conventional dithering to improve grayscale precision without reintroducing artifacts.
Solution Approach 2:
The patent uses periodic center-aligned PWM modulation with trailing edge dithering that operates in a periodic manner. The dithering is applied rhythmically to the trailing edge of each pulse, maintaining the center alignment benefit while progressively eliminating banding artifacts through repeated temporal variation, thereby improving grayscale precision without persistent banding.
3Stability of the object's composition
If center aligned PWM is used, then temporal motion stability is improved, but trailing edge variation is needed to eliminate banding artifacts
Solution Approach 1:
The patent applies local quality by applying dithering only to the trailing edge of the PWM pulse while keeping the leading edge and center alignment fixed. This localized modification maintains temporal motion stability from center alignment while introducing just enough variation at the trailing edge to eliminate banding artifacts, avoiding the need for complex overall modulation changes.
Solution Approach 2:
The patent uses partial action by applying trailing edge dithering only to the necessary extent to eliminate banding artifacts, rather than completely redesigning the modulation scheme. This partial modification to the trailing edge provides sufficient temporal variation to remove artifacts while preserving the stability benefits of center alignment, avoiding excessive complexity.
Data Source
AI summary
A method and system for controlling a pixel of a display using pulse width modulation (PWM). A desired grayscale value for the pixel is processed to generate a binary encoding having a plurality of select bits encoding a select value, and a dither bit encoding a dither value, encoded by a scheme in which a first select value and a first dither value indicate a center-aligned pulse of a first width having a leading edge at a first temporal position, a second select value and the first dither value indicates a center-aligned pulse of a second width having a leading edge at a second temporal position, and the first select value and a second dither value indicate a center-aligned pulse of a second width having a leading edge at the first temporal position. The binary encoding is provided to the display for pulse width modulation of the pixel.