51 lines
1.1 KiB
C++
51 lines
1.1 KiB
C++
// Copyright (c) the JPEG XL Project Authors. All rights reserved.
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//
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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#include "lib/jxl/enc_linalg.h"
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#include <cmath>
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#include "lib/jxl/base/status.h"
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namespace jxl {
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void ConvertToDiagonal(const Matrix2x2& A, Vector2& diag, Matrix2x2& U) {
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#if JXL_ENABLE_ASSERT
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// Check A is symmetric.
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JXL_ASSERT(std::abs(A[0][1] - A[1][0]) < 1e-15);
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#endif
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double b = -(A[0][0] + A[1][1]);
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double c = A[0][0] * A[1][1] - A[0][1] * A[0][1];
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double d = b * b - 4.0 * c;
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if (std::abs(A[0][1]) < 1e-10 || d < 0) {
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// Already diagonal.
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diag[0] = A[0][0];
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diag[1] = A[1][1];
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U[0][0] = U[1][1] = 1.0;
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U[0][1] = U[1][0] = 0.0;
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return;
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}
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double sqd = std::sqrt(d);
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double l1 = (-b - sqd) * 0.5;
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double l2 = (-b + sqd) * 0.5;
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Vector2 v1 = {A[0][0] - l1, A[1][0]};
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double v1n = 1.0 / std::hypot(v1[0], v1[1]);
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v1[0] = v1[0] * v1n;
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v1[1] = v1[1] * v1n;
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diag[0] = l1;
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diag[1] = l2;
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U[0][0] = v1[1];
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U[0][1] = -v1[0];
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U[1][0] = v1[0];
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U[1][1] = v1[1];
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}
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} // namespace jxl
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