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+/*
+ * Copyright (c) 2014 Vittorio Giovara <vittorio.giovara@gmail.com>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * @ingroup lavu_video_display
+ * Display matrix
+ */
+
+#ifndef AVUTIL_DISPLAY_H
+#define AVUTIL_DISPLAY_H
+
+#include <stdint.h>
+
+/**
+ * @defgroup lavu_video_display Display transformation matrix functions
+ * @ingroup lavu_video
+ *
+ * The display transformation matrix specifies an affine transformation that
+ * should be applied to video frames for correct presentation. It is compatible
+ * with the matrices stored in the ISO/IEC 14496-12 container format.
+ *
+ * The data is a 3x3 matrix represented as a 9-element array:
+ *
+ * @code{.unparsed}
+ *                                  | a b u |
+ *   (a, b, u, c, d, v, x, y, w) -> | c d v |
+ *                                  | x y w |
+ * @endcode
+ *
+ * All numbers are stored in native endianness, as 16.16 fixed-point values,
+ * except for u, v and w, which are stored as 2.30 fixed-point values.
+ *
+ * The transformation maps a point (p, q) in the source (pre-transformation)
+ * frame to the point (p', q') in the destination (post-transformation) frame as
+ * follows:
+ *
+ * @code{.unparsed}
+ *               | a b u |
+ *   (p, q, 1) . | c d v | = z * (p', q', 1)
+ *               | x y w |
+ * @endcode
+ *
+ * The transformation can also be more explicitly written in components as
+ * follows:
+ *
+ * @code{.unparsed}
+ *   p' = (a * p + c * q + x) / z;
+ *   q' = (b * p + d * q + y) / z;
+ *   z  =  u * p + v * q + w
+ * @endcode
+ *
+ * @{
+ */
+
+/**
+ * Extract the rotation component of the transformation matrix.
+ *
+ * @param matrix the transformation matrix
+ * @return the angle (in degrees) by which the transformation rotates the frame
+ *         counterclockwise. The angle will be in range [-180.0, 180.0],
+ *         or NaN if the matrix is singular.
+ *
+ * @note floating point numbers are inherently inexact, so callers are
+ *       recommended to round the return value to nearest integer before use.
+ */
+double av_display_rotation_get(const int32_t matrix[9]);
+
+/**
+ * Initialize a transformation matrix describing a pure clockwise
+ * rotation by the specified angle (in degrees).
+ *
+ * @param[out] matrix a transformation matrix (will be fully overwritten
+ *                    by this function)
+ * @param angle rotation angle in degrees.
+ */
+void av_display_rotation_set(int32_t matrix[9], double angle);
+
+/**
+ * Flip the input matrix horizontally and/or vertically.
+ *
+ * @param[in,out] matrix a transformation matrix
+ * @param hflip whether the matrix should be flipped horizontally
+ * @param vflip whether the matrix should be flipped vertically
+ */
+void av_display_matrix_flip(int32_t matrix[9], int hflip, int vflip);
+
+/**
+ * @}
+ */
+
+#endif /* AVUTIL_DISPLAY_H */