[Aqsis-commits] SF.net SVN: aqsis: [1865] branches/texfiltering/aqsis/texturing/texlib

[<c4...@us...>]

Revision: 1865
          http://aqsis.svn.sourceforge.net/aqsis/?rev=1865&view=rev
Author:   c42f
Date:     2008-01-27 05:37:11 -0800 (Sun, 27 Jan 2008)

Log Message:
-----------
* Change the scale of texture blurring to bring it more inline with that
  implied by the RI spec.  (Previously, we were getting excessive blurring.)

Modified Paths:
--------------
    branches/texfiltering/aqsis/texturing/texlib/ewafilter.cpp
    branches/texfiltering/aqsis/texturing/texlib/ewafilter.h

Modified: branches/texfiltering/aqsis/texturing/texlib/ewafilter.cpp
===================================================================
--- branches/texfiltering/aqsis/texturing/texlib/ewafilter.cpp	2008-01-23 02:19:38 UTC (rev 1864)
+++ branches/texfiltering/aqsis/texturing/texlib/ewafilter.cpp	2008-01-27 13:37:11 UTC (rev 1865)
@@ -103,10 +103,11 @@
 	// recommend using 1 for these variances, but this results in excess
 	// blurring.
 	//
-	// Default reconstruction filter variance - this is the variance of the
-	// filter used to reconstruct a continuous image from the underlying
-	// discrete samples.
-	const TqFloat reconsVar = 1.3/(2*M_PI);
+	// Default standard deviation for reconstruction filter - this is the
+	// stddev (=sqrt(variance)) of the filter used to reconstruct a continuous
+	// image from the underlying discrete samples.
+	const TqFloat reconsStdDev = 0.454864184147; // std::sqrt(1.3/(2*M_PI))
+	const TqFloat reconsVar = reconsStdDev*reconsStdDev;
 	// "Prefilter" variance - this is the variance of the antialiasing filter
 	// which is used immediately before resampling onto the discrete grid.
 	//
@@ -121,13 +122,28 @@
 	if(sBlur > 0 || tBlur > 0)
 	{
 		// The reconstruction variance matrix provides a very nice way of
-		// incorporating extra filter blurring if desired.  Here we do this by
-		// adding the extra blur to the reconstruction variance matrix.
-		TqFloat sVariance = sBlur*baseResS;
-		sVariance = sVariance*sVariance + reconsVar;
-		TqFloat tVariance = tBlur*baseResT;
-		tVariance = tVariance*tVariance + reconsVar;
-		coVar += SqMatrix2D(sVariance, tVariance);
+		// incorporating extra filter blurring if desired, without distorting
+		// the filter region in a weird manner.  We do this by adding the
+		// standard deviations of the base reconstruction filter with a scaled
+		// blur in the s and t directions.
+		//
+		// The RI spec (version 3.2, page 149) says that blur is "an additional
+		// area to be added to the texture area filtered in both the s and t
+		// directions, expressed in units of texture coordinates".  From their
+		// example it seems like they meant length rather than area...
+		//
+		// Since we're working in raster units, we scale the blur by the base
+		// texture resolution.  This gives the blur in units of number of
+		// pixel widths to add to the filter in the two directions.  To turn
+		// this into a standard devation for a gaussian filter, we scale by
+		// 1/sqrt(12) which is the standard deviation of a box filter of width
+		// 1.  (If we don't do this, the blur is much larger than desired,
+		// since the support of a gaussian of standard deviation 1 occupies
+		// much more than a pixel.)
+		const TqFloat oneOnSqrt12 = 0.288675134595;
+		TqFloat sStdDev = sBlur*baseResS*oneOnSqrt12 + reconsStdDev;
+		TqFloat tStdDev = tBlur*baseResT*oneOnSqrt12 + reconsStdDev;
+		coVar += SqMatrix2D(sStdDev*sStdDev, tStdDev*tStdDev);
 	}
 	else
 	{

Modified: branches/texfiltering/aqsis/texturing/texlib/ewafilter.h
===================================================================
--- branches/texfiltering/aqsis/texturing/texlib/ewafilter.h	2008-01-23 02:19:38 UTC (rev 1864)
+++ branches/texfiltering/aqsis/texturing/texlib/ewafilter.h	2008-01-27 13:37:11 UTC (rev 1865)
@@ -124,7 +124,7 @@
 		 */
 		inline CqEwaFilterWeights(const SqSampleQuad& sQuad, 
 				TqFloat baseResS, TqFloat baseResT,
-				TqFloat sBlur = -1, TqFloat tBlur = -1,
+				TqFloat sBlur = 0, TqFloat tBlur = 0,
 				TqFloat logEdgeWeight = 4, 
 				TqFloat maxAspectRatio = 20);
 


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