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JoinIdenticalVertices: Performance optimizations by Krishty („Fuck the System”). Yields a 9x speedup in first benchmarks with meshes > 2k triangles.

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git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@780 67173fc5-114c-0410-ac8e-9d2fd5bffc1f
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aramis_acg
2010-07-11 23:07:11 +00:00
parent 9e8a9586b3
commit a9fd02c14e
4 changed files with 160 additions and 12 deletions
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139
code/SpatialSort.cpp
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@@ -46,6 +46,11 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
using namespace Assimp;
// CHAR_BIT seems to be defined under MVSC, but not under GCC. Pray that the correct value is 8.
#ifndef CHAR_BIT
# define CHAR_BIT 8
#endif
// ------------------------------------------------------------------------------------------------
// Constructs a spatially sorted representation from the given position array.
SpatialSort::SpatialSort( const aiVector3D* pPositions, unsigned int pNumPositions,
@@ -168,6 +173,140 @@ void SpatialSort::FindPositions( const aiVector3D& pPosition,
// that's it
}
namespace {
// Binary, signed-integer representation of a single-precision floating-point value.
// IEEE 754 says: "If two floating-point numbers in the same format are ordered then they are
// ordered the same way when their bits are reinterpreted as sign-magnitude integers."
// This allows us to convert all floating-point numbers to signed integers of arbitrary size
// and then use them to work with ULPs (Units in the Last Place, for high-precision
// computations) or to compare them (integer comparisons are faster than floating-point
// comparisons on many platforms).
typedef signed int BinFloat;
// --------------------------------------------------------------------------------------------
// Converts the bit pattern of a floating-point number to its signed integer representation.
BinFloat ToBinary( const float & pValue) {
// If this assertion fails, signed int is not big enough to store a float on your platform.
// Please correct the declaration of BinFloat a few lines above - but do it in a portable,
// #ifdef'd manner!
BOOST_STATIC_ASSERT( sizeof(BinFloat) >= sizeof(float));
#if defined( _MSC_VER)
// If this assertion fails, Visual C++ has finally moved to ILP64. This means that this
// code has just become legacy code! Find out the current value of _MSC_VER and modify
// the #if above so it evaluates false on the current and all upcoming VC versions (or
// on the current platform, if LP64 or LLP64 are still used on other platforms).
BOOST_STATIC_ASSERT( sizeof(BinFloat) == sizeof(float));
// This works best on Visual C++, but other compilers have their problems with it.
const BinFloat binValue = reinterpret_cast<BinFloat const &>(pValue);
#else
// On many compilers, reinterpreting a float address as an integer causes aliasing
// problems. This is an ugly but more or less safe way of doing it.
union {
float asFloat;
BinFloat asBin;
} conversion;
conversion.asBin = 0; // zero empty space in case sizeof(BinFloat) > sizeof(float)
conversion.asFloat = pValue;
const BinFloat binValue = conversion.asBin;
#endif
// floating-point numbers are of sign-magnitude format, so find out what signed number
// representation we must convert negative values to.
// See http://en.wikipedia.org/wiki/Signed_number_representations.
// Two's complement?
if( (-42 == (~42 + 1)) && (binValue & 0x80000000))
return BinFloat(1 << (CHAR_BIT * sizeof(BinFloat) - 1)) - binValue;
// One's complement?
else if( (-42 == ~42) && (binValue & 0x80000000))
return BinFloat(-0) - binValue;
// Sign-magnitude?
else if( (-42 == (42 | (-0))) && (binValue & 0x80000000)) // -0 = 1000... binary
return binValue;
else
return binValue;
}
} // namespace
// ------------------------------------------------------------------------------------------------
// Fills an array with indices of all positions indentical to the given position. In opposite to
// FindPositions(), not an epsilon is used but a (very low) tolerance of four floating-point units.
void SpatialSort::FindIdenticalPositions( const aiVector3D& pPosition,
std::vector<unsigned int>& poResults) const
{
// Epsilons have a huge disadvantage: they are of constant precision, while floating-point
// values are of log2 precision. If you apply e=0.01 to 100, the epsilon is rather small, but
// if you apply it to 0.001, it is enormous.
// The best way to overcome this is the unit in the last place (ULP). A precision of 2 ULPs
// tells us that a float does not differ more than 2 bits from the "real" value. ULPs are of
// logarithmic precision - around 1, they are 1÷(2^24) and around 10000, they are 0.00125.
// For standard C math, we can assume a precision of 0.5 ULPs according to IEEE 754. The
// incoming vertex positions might have already been transformed, probably using rather
// inaccurate SSE instructions, so we assume a tolerance of 4 ULPs to safely identify
// identical vertex positions.
static const int toleranceInULPs = 4;
// An interesting point is that the inaccuracy grows linear with the number of operations:
// multiplying to numbers, each inaccurate to four ULPs, results in an inaccuracy of four ULPs
// plus 0.5 ULPs for the multiplication.
// To compute the distance to the plane, a dot product is needed - that is a multiplication and
// an addition on each number.
static const int distanceToleranceInULPs = toleranceInULPs + 1;
// The squared distance between two 3D vectors is computed the same way, but with an additional
// subtraction.
static const int distance3DToleranceInULPs = distanceToleranceInULPs + 1;
// Convert the plane distance to its signed integer representation so the ULPs tolerance can be
// applied. For some reason, VC won't optimize two calls of the bit pattern conversion.
const BinFloat minDistBinary = ToBinary( pPosition * mPlaneNormal) - distanceToleranceInULPs;
const BinFloat maxDistBinary = minDistBinary + 2 * distanceToleranceInULPs;
// clear the array in this strange fashion because a simple clear() would also deallocate
// the array which we want to avoid
poResults.erase( poResults.begin(), poResults.end());
// do a binary search for the minimal distance to start the iteration there
unsigned int index = (unsigned int)mPositions.size() / 2;
unsigned int binaryStepSize = (unsigned int)mPositions.size() / 4;
while( binaryStepSize > 1)
{
// Ugly, but conditional jumps are faster with integers than with floats
if( minDistBinary > ToBinary(mPositions[index].mDistance))
index += binaryStepSize;
else
index -= binaryStepSize;
binaryStepSize /= 2;
}
// depending on the direction of the last step we need to single step a bit back or forth
// to find the actual beginning element of the range
while( index > 0 && minDistBinary < ToBinary(mPositions[index].mDistance) )
index--;
while( index < (mPositions.size() - 1) && minDistBinary > ToBinary(mPositions[index].mDistance))
index++;
// Now start iterating from there until the first position lays outside of the distance range.
// Add all positions inside the distance range within the tolerance to the result aray
std::vector<Entry>::const_iterator it = mPositions.begin() + index;
while( ToBinary(it->mDistance) < maxDistBinary)
{
if( distance3DToleranceInULPs >= ToBinary((it->mPosition - pPosition).SquareLength()))
poResults.push_back(it->mIndex);
++it;
if( it == mPositions.end())
break;
}
// that's it
}
// ------------------------------------------------------------------------------------------------
unsigned int SpatialSort::GenerateMappingTable(std::vector<unsigned int>& fill,float pRadius) const
{