The native raster implementation is adapted from Vega 6.4.0's vega-geo
contours, density2D and quantize algorithms, with the following notices.
Vega's marching-squares and box-blur implementations are adapted from
d3-contour. No JavaScript implementation is shipped or executed by Raven.

Vega / University of Washington Interactive Data Lab
https://github.com/vega/vega/tree/v6.4.0/packages/vega-geo

Copyright (c) 2015-2023, University of Washington Interactive Data Lab
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this
   list of conditions and the following disclaimer.

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   this list of conditions and the following disclaimer in the documentation
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3. Neither the name of the copyright holder nor the names of its contributors
   may be used to endorse or promote products derived from this software
   without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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d3-contour / Mike Bostock
https://github.com/d3/d3-contour

Copyright 2012-2023 Mike Bostock

d3-array / Mike Bostock (threshold tick steps and bandwidth statistics)
https://github.com/d3/d3-array

Copyright 2010-2023 Mike Bostock

Permission to use, copy, modify, and/or distribute this software for any purpose
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