Optical Flow Estimation Using Varied Scan Orders
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Solution Overview
Problem
Conventional optical flow estimation methods using hierarchical motion search face challenges due to less accurate initial motion vectors, which impact subsequent resolution levels, leading to poor optical flow estimates, especially at the top half of the image.
Innovation Solution
The approach involves varying the scan order for different levels of image analysis, leveraging high-quality optical flow estimates from previous scans to reduce the reliance on lower quality estimates, such as rotating the image representation by 180 degrees between scans, and alternating scan orders between passes to improve hint quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a fixed scan order (e.g., top to bottom, left to right) is used at all resolution levels, then the processing is simple and consistent, but the initial motion vectors at the start of the scan (e.g., top half of the image) are less accurate due to lack of spatial hints
Solution Approach 1:
The scan order is made dynamic by varying it across different resolution levels. Specifically, even-resolution levels use one scan order (e.g., top-to-bottom) while odd-resolution levels use the opposite scan order (e.g., bottom-to-top). This dynamic adjustment ensures that spatial hints from neighboring locations are more readily available throughout the scan, improving motion vector accuracy without requiring complex per-location adjustments.
Solution Approach 2:
The scan order alternates periodically between resolution levels. Each resolution level uses a scan order that is opposite to the previous level, creating a periodic pattern. This periodic alternation ensures that by the time the scan reaches any given location, spatial hints from previously scanned locations at the same resolution level are more likely to be available, improving estimation accuracy.
2Ease of operation
If the scan starts from the top left corner and proceeds row by row, then the processing order is straightforward, but the top half of the image suffers from poor optical flow estimates due to insufficient spatial hints
Solution Approach 1:
The scan order is inverted for alternating resolution levels. While even-resolution levels scan from top to bottom, odd-resolution levels scan from bottom to top. This inversion ensures that when processing any given region, the spatial hints from previously processed neighboring regions are more likely to be available, thereby improving optical flow estimation accuracy while maintaining implementation simplicity through a systematic alternation pattern.
3Adaptability or versatility
If the same scan order is used across all resolution levels, then the algorithm is consistent and easy to implement, but inaccurate initial motion vectors at early scan locations propagate errors to subsequent resolution levels
Solution Approach 1:
The algorithm introduces dynamic variation in scan order across resolution levels while maintaining overall structural consistency. Even-resolution levels use one scan direction and odd-resolution levels use the opposite direction. This dynamic adaptation allows the algorithm to improve motion vector accuracy by ensuring spatial hints are available, while the systematic alternation pattern preserves algorithmic consistency and ease of implementation.
Solution Approach 2:
The scan order parameter is changed based on the resolution level. Specifically, the scan direction parameter alternates between levels: levels with even indices use one direction (e.g., top-to-bottom) while levels with odd indices use the opposite direction (e.g., bottom-to-top). This parameter change improves reliability by ensuring better spatial hint availability without fundamentally altering the algorithm's structure.
Data Source
AI summary
In various examples, optical flow estimate (OFE) quality is improved when employing a hint-based algorithm in multi-level hierarchical motion estimation by using different scan orders at different resolution levels. A scan of an image performed with a scan order may initially leverage OFEs from a previous scan of the image, where the previous scan was performed using a different scan order. The OFEs leveraged from the previous scan are more likely to be of high accuracy until sufficient spatial hints are available to the hint-based algorithm for the scan to reduce the impact of potentially lower quality OFEs resulting from the different scan order of the previous scan.