27 February 2015 Efficient illuminant correction in the local, linear, learned (L3) method
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To speed the development of novel camera architectures we proposed a method, L3 (Local, Linear and Learned),that automatically creates an optimized image processing pipeline. The L3 method assigns each sensor pixel into one of 400 classes, and applies class-dependent local linear transforms that map the sensor data from a pixel and its neighbors into the target output (e.g., CIE XYZ rendered under a D65 illuminant). The transforms are precomputed from training data and stored in a table used for image rendering. The training data are generated by camera simulation, consisting of sensor responses and rendered CIE XYZ outputs. The sensor and rendering illuminant can be equal (same-illuminant table) or different (cross-illuminant table). In the original implementation, illuminant correction is achieved with cross-illuminant tables, and one table is required for each illuminant. We find, however, that a single same-illuminant table (D65) effectively converts sensor data for many different same-illuminant conditions. Hence, we propose to render the data by applying the same-illuminant D65 table to the sensor data, followed by a linear illuminant correction transform. The mean color reproduction error using the same-illuminant table is on the order of 4▵E units, which is only slightly larger than the cross-illuminant table error. This approach reduces table storage requirements significantly without substantially degrading color reproduction accuracy.
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Francois G. Germain, Francois G. Germain, Iretiayo A. Akinola, Iretiayo A. Akinola, Qiyuan Tian, Qiyuan Tian, Steven P. Lansel, Steven P. Lansel, Brian A. Wandell, Brian A. Wandell, "Efficient illuminant correction in the local, linear, learned (L3) method", Proc. SPIE 9404, Digital Photography XI, 940404 (27 February 2015); doi: 10.1117/12.2083277; https://doi.org/10.1117/12.2083277

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