Replacement reference DCT implementation.

patch by Dylan Yudaken, dyudaken gmail com

Originally committed as revision 18273 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/dctref.c

+/*
+ * reference discrete cosine transform (double precision)
+ * Copyright (C) 2009 Dylan Yudaken
+ *
+ * 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 libavcodec/dctref.c
+ * reference discrete cosine transform (double precision)
+ *
+ * @author Dylan Yudaken (dyudaken at gmail)
+ *
+ * @note This file could be optimized a lot, but is for
+ * reference and so readability is better.
+ */
+
+#include "libavutil/mathematics.h"
+static double coefficients[8 * 8];
+
+/**
+ * Initialize the double precision discrete cosine transform
+ * functions fdct & idct.
+ */
+av_cold void ff_ref_dct_init(void)
+{
+    unsigned int i, j;
+
+    for (j = 0; j < 8; ++j) {
+        coefficients[j] = sqrt(0.125);
+        for (i = 8; i < 64; i += 8) {
+            coefficients[i + j] = 0.5 * cos(i * (j + 0.5) * M_PI / 64.0);
+        }
+    }
+}
+
+/**
+ * Transform 8x8 block of data with a double precision forward DCT <br>
+ * This is a reference implementation.
+ *
+ * @param block pointer to 8x8 block of data to transform
+ */
+void ff_ref_fdct(short *block)
+{
+    /* implement the equation: block = coefficients * block * coefficients' */
+
+    unsigned int i, j, k;
+    double out[8 * 8];
+
+    /* out = coefficients * block */
+    for (i = 0; i < 64; i += 8) {
+        for (j = 0; j < 8; ++j) {
+            double tmp = 0;
+            for (k = 0; k < 8; ++k) {
+                tmp += coefficients[i + k] * block[k * 8 + j];
+            }
+            out[i + j] = tmp * 8;
+        }
+    }
+
+    /* block = out * (coefficients') */
+    for (j = 0; j < 8; ++j) {
+        for (i = 0; i < 64; i += 8) {
+            double tmp = 0;
+            for (k = 0; k < 8; ++k) {
+                tmp += out[i + k] * coefficients[j * 8 + k];
+            }
+            block[i + j] = floor(tmp + 0.499999999999);
+        }
+    }
+}
+
+/**
+ * Transform 8x8 block of data with a double precision inverse DCT <br>
+ * This is a reference implementation.
+ *
+ * @param block pointer to 8x8 block of data to transform
+ */
+void ff_ref_idct(short *block)
+{
+    /* implement the equation: block = (coefficients') * block * coefficients */
+
+    unsigned int i, j, k;
+    double out[8 * 8];
+
+    /* out = block * coefficients */
+    for (i = 0; i < 64; i += 8) {
+        for (j = 0; j < 8; ++j) {
+            double tmp = 0;
+            for (k = 0; k < 8; ++k) {
+                tmp += block[i + k] * coefficients[k * 8 + j];
+            }
+            out[i + j] = tmp;
+        }
+    }
+
+    /* block = (coefficients') * out */
+    for (i = 0; i < 8; ++i) {
+        for (j = 0; j < 8; ++j) {
+            double tmp = 0;
+            for (k = 0; k < 64; k += 8) {
+                tmp += coefficients[k + i] * out[k + j];
+            }
+            block[i * 8 + j] = floor(tmp + 0.5);
+        }
+    }
+}