ref: a66c945c5973246561b3ff710886018f14e6679c
parent: 152ce6b2b901279d1154ef45e8b387eb23cad3a0
author: Yaowu Xu <yaowu@google.com>
date: Fri Oct 14 05:58:35 EDT 2011
added a last stage rounding for 8x8 inverse dct Prior to the added rounding, tests on randomly generated data showed that forward-inverse transform round trip errors are about 3.02/block for input range [-10,10] and 2.68/block for input range [-256, 255]. The added rounding reduced the errors to 0.031/block for input range [-10,10] and 0.037/block for input range [-256, 255]. Maximum round trip error on for any pixel position is 1. The average errors are calculated based on 100,000 blocks of randomly with the specified ranges. Paul mentioned in discussion that the change was not clear on why we need change the rounding, so Patch 2 intends to make the rationale obvious in code, it merged the two separate shifts into one, and the two separate rounding factors into one. Patch 1 and 2 have same numerical test results. Change-Id: Ic5e2f5463de17253084d8b2398c4a210194b20de
--- a/vp8/common/idctllm.c
+++ b/vp8/common/idctllm.c
@@ -369,7 +369,7 @@
((blk[8 * 0] + 32) >> 6);
return;
}
- x0 = (blk[8 * 0] << 8) + 8192;
+ x0 = (blk[8 * 0] << 8) + 16384;
/* first stage */
x8 = W7 * (x4 + x5) + 4;
@@ -399,14 +399,14 @@
x4 = (181 * (x4 - x5) + 128) >> 8;
/* fourth stage */
- blk[8 * 0] = (x7 + x1) >> 14;
- blk[8 * 1] = (x3 + x2) >> 14;
- blk[8 * 2] = (x0 + x4) >> 14;
- blk[8 * 3] = (x8 + x6) >> 14;
- blk[8 * 4] = (x8 - x6) >> 14;
- blk[8 * 5] = (x0 - x4) >> 14;
- blk[8 * 6] = (x3 - x2) >> 14;
- blk[8 * 7] = (x7 - x1) >> 14;
+ blk[8 * 0] = (x7 + x1 ) >> 15;
+ blk[8 * 1] = (x3 + x2 ) >> 15;
+ blk[8 * 2] = (x0 + x4 ) >> 15;
+ blk[8 * 3] = (x8 + x6 ) >> 15;
+ blk[8 * 4] = (x8 - x6 ) >> 15;
+ blk[8 * 5] = (x0 - x4 ) >> 15;
+ blk[8 * 6] = (x3 - x2 ) >> 15;
+ blk[8 * 7] = (x7 - x1 ) >> 15;
}
#define TX_DIM 8
@@ -434,7 +434,7 @@
{for (j = 0; j < TX_DIM; j++)
{- block[i*shortpitch+j] = X[i * TX_DIM + j]>>1;
+ block[i*shortpitch+j] = X[i * TX_DIM + j];
}
}
}