Discrete Color Matching in Document Scanners
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Solution Overview
Problem
Large-format document scanners with multiple cameras face challenges in uniform color reproduction due to variations in camera sensors and light sources, leading to visible artifacts under certain lighting conditions, known as metamerism, which are difficult to correct analytically.
Innovation Solution
A method is introduced to correct image values by computing and storing color correction values from overlapping scan areas, using a color difference between cameras to adapt color correction to specific ranges of colors, ensuring uniform color reproduction across the scan line.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If multiple cameras are used to cover large document formats, then the scanning area is increased, but color uniformity deteriorates due to camera-to-camera variations
Solution Approach 1:
The patent introduces an intermediary color correction process that mediates between multiple cameras with different spectral responses. A reference camera captures ground truth color values, and these serve as the intermediary standard against which other cameras are compared and corrected. The color correction values act as intermediaries to translate readings from different cameras into a unified color space, resolving the color uniformity issue while maintaining multi-camera coverage capability
Solution Approach 2:
The patent changes the parameter space by introducing color correction values that are specific to different color ranges (represented by color index values). Instead of applying a uniform correction across all colors, the system computes and applies different correction values for different color regions. This parameter-based correction approach allows the system to compensate for spectral variations in different parts of the color spectrum, thereby improving color uniformity across multiple cameras while preserving the ability to scan large areas
2Device complexity
If conventional offset correction is applied to reduce camera response differences, then the complexity of color correction is reduced, but visible color artifacts appear under certain lighting conditions
Solution Approach 1:
The patent performs preliminary color correction value computation during the scanning process itself, using overlap areas where both reference and test cameras capture the same region. This preliminary action allows the system to compute accurate color correction values before final image output, enabling the system to handle metamerism and lighting condition variations without adding complexity to the main scanning operation. The correction is prepared in advance based on actual captured data rather than theoretical offsets
Solution Approach 2:
The patent implements a feedback mechanism where color correction values are computed based on actual color differences observed in overlap areas between cameras. The system continuously monitors color discrepancies and uses this feedback to adjust correction values. This feedback loop ensures that color artifacts are minimized by adapting corrections to actual observed conditions rather than relying on fixed offset values, thereby eliminating visible artifacts while maintaining manageable system complexity
3Measurement precision
If color correction values are computed for all possible color ranges, then color accuracy is improved, but memory requirements and processing time increase
Solution Approach 1:
The patent segments the color space into discrete color ranges represented by color index values. Instead of computing corrections for every possible color value continuously, the system divides the color spectrum into manageable segments and computes correction values for representative colors in each segment. This segmentation approach maintains color accuracy across the full spectrum while significantly reducing the computational burden and memory requirements compared to handling all possible color values
Solution Approach 2:
The patent computes color correction values selectively for colors that are actually present in the document being scanned, rather than pre-computing corrections for all possible colors. The system analyzes the color content of the document and computes corrections only for the relevant color ranges detected. This partial action approach improves color accuracy for the actual document content while avoiding the time and memory costs of computing corrections for colors that may not be present
Data Source
AI summary
A method, and document scanner system, for correcting image values recorded by an optical document scanner with multiple color cameras arranged in a staggered configuration with overlap scan areas includes storing color correction values mapped to respective color index values, where a color correction value is computed from a color difference between a first color value and a second color value of the respective overlap sets, and correcting a third color value which is a color value from the second set of color image values by a color correction value which is looked up via a color index value determined from the third color value. Thereby color variations among staggered cameras are corrected.