COM_algorithm_van_vliet_gaussian_blur.hh

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/* SPDX-FileCopyrightText: 2024 Blender Authors
 *
 * SPDX-License-Identifier: GPL-2.0-or-later */
 
#pragma once
 
#include "BLI_math_vector_types.hh"
 
#include "COM_context.hh"
#include "COM_result.hh"
 
namespace blender::compositor {
 
/* Blur the input using a fourth order IIR filter approximating a Gaussian filter of the given
 * sigma computed using Van Vliet's design method. This is based on the following paper:
 *
 *   Van Vliet, Lucas J., Ian T. Young, and Piet W. Verbeek. "Recursive Gaussian derivative
 *   filters." Proceedings. Fourteenth International Conference on Pattern Recognition (Cat. No.
 *   98EX170). Vol. 1. IEEE, 1998.
 *
 * However, we internally split the fourth order IIR filter into two second order sections in order
 * to improve its numerical stability and improve parallelism. See the implementation for more
 * information.
 *
 * This differs from the standard symmetric separable blur algorithm in that it is faster for high
 * sigma values, the downside is that it consumes more memory and is only an approximation that
 * might suffer from fringing and artifacts, though those are typically unnoticeable. This filter
 * is not accurate for sigma values less than 32, in those cases, use the Deriche filter instead.
 * Further, for sigma values less than 3, use direct convolution instead, since it is faster and
 * more accurate. Neumann boundary is assumed.
 *
 * The output is written to the given output result, which will be allocated internally and is thus
 * expected not to be previously allocated. */
void van_vliet_gaussian_blur(Context &context,
                             const Result &input,
                             Result &output,
                             const float2 &sigma);
 
}  // namespace blender::compositor