godot/thirdparty/meshoptimizer/stripifier.cpp

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// This file is part of meshoptimizer library; see meshoptimizer.h for version/license details
#include "meshoptimizer.h"
#include <assert.h>
#include <limits.h>
#include <string.h>
// This work is based on:
// Francine Evans, Steven Skiena and Amitabh Varshney. Optimizing Triangle Strips for Fast Rendering. 1996
namespace meshopt
{
static unsigned int findStripFirst(const unsigned int buffer[][3], unsigned int buffer_size, const unsigned int* valence)
{
unsigned int index = 0;
unsigned int iv = ~0u;
for (size_t i = 0; i < buffer_size; ++i)
{
unsigned int va = valence[buffer[i][0]], vb = valence[buffer[i][1]], vc = valence[buffer[i][2]];
unsigned int v = (va < vb && va < vc) ? va : (vb < vc) ? vb : vc;
if (v < iv)
{
index = unsigned(i);
iv = v;
}
}
return index;
}
static int findStripNext(const unsigned int buffer[][3], unsigned int buffer_size, unsigned int e0, unsigned int e1)
{
for (size_t i = 0; i < buffer_size; ++i)
{
unsigned int a = buffer[i][0], b = buffer[i][1], c = buffer[i][2];
if (e0 == a && e1 == b)
return (int(i) << 2) | 2;
else if (e0 == b && e1 == c)
return (int(i) << 2) | 0;
else if (e0 == c && e1 == a)
return (int(i) << 2) | 1;
}
return -1;
}
} // namespace meshopt
size_t meshopt_stripify(unsigned int* destination, const unsigned int* indices, size_t index_count, size_t vertex_count, unsigned int restart_index)
{
assert(destination != indices);
assert(index_count % 3 == 0);
using namespace meshopt;
meshopt_Allocator allocator;
const size_t buffer_capacity = 8;
unsigned int buffer[buffer_capacity][3] = {};
unsigned int buffer_size = 0;
size_t index_offset = 0;
unsigned int strip[2] = {};
unsigned int parity = 0;
size_t strip_size = 0;
// compute vertex valence; this is used to prioritize starting triangle for strips
unsigned int* valence = allocator.allocate<unsigned int>(vertex_count);
memset(valence, 0, vertex_count * sizeof(unsigned int));
for (size_t i = 0; i < index_count; ++i)
{
unsigned int index = indices[i];
assert(index < vertex_count);
valence[index]++;
}
int next = -1;
while (buffer_size > 0 || index_offset < index_count)
{
assert(next < 0 || (size_t(next >> 2) < buffer_size && (next & 3) < 3));
// fill triangle buffer
while (buffer_size < buffer_capacity && index_offset < index_count)
{
buffer[buffer_size][0] = indices[index_offset + 0];
buffer[buffer_size][1] = indices[index_offset + 1];
buffer[buffer_size][2] = indices[index_offset + 2];
buffer_size++;
index_offset += 3;
}
assert(buffer_size > 0);
if (next >= 0)
{
unsigned int i = next >> 2;
unsigned int a = buffer[i][0], b = buffer[i][1], c = buffer[i][2];
unsigned int v = buffer[i][next & 3];
// ordered removal from the buffer
memmove(buffer[i], buffer[i + 1], (buffer_size - i - 1) * sizeof(buffer[0]));
buffer_size--;
// update vertex valences for strip start heuristic
valence[a]--;
valence[b]--;
valence[c]--;
// find next triangle (note that edge order flips on every iteration)
// in some cases we need to perform a swap to pick a different outgoing triangle edge
// for [a b c], the default strip edge is [b c], but we might want to use [a c]
int cont = findStripNext(buffer, buffer_size, parity ? strip[1] : v, parity ? v : strip[1]);
int swap = cont < 0 ? findStripNext(buffer, buffer_size, parity ? v : strip[0], parity ? strip[0] : v) : -1;
if (cont < 0 && swap >= 0)
{
// [a b c] => [a b a c]
destination[strip_size++] = strip[0];
destination[strip_size++] = v;
// next strip has same winding
// ? a b => b a v
strip[1] = v;
next = swap;
}
else
{
// emit the next vertex in the strip
destination[strip_size++] = v;
// next strip has flipped winding
strip[0] = strip[1];
strip[1] = v;
parity ^= 1;
next = cont;
}
}
else
{
// if we didn't find anything, we need to find the next new triangle
// we use a heuristic to maximize the strip length
unsigned int i = findStripFirst(buffer, buffer_size, &valence[0]);
unsigned int a = buffer[i][0], b = buffer[i][1], c = buffer[i][2];
// ordered removal from the buffer
memmove(buffer[i], buffer[i + 1], (buffer_size - i - 1) * sizeof(buffer[0]));
buffer_size--;
// update vertex valences for strip start heuristic
valence[a]--;
valence[b]--;
valence[c]--;
// we need to pre-rotate the triangle so that we will find a match in the existing buffer on the next iteration
int ea = findStripNext(buffer, buffer_size, c, b);
int eb = findStripNext(buffer, buffer_size, a, c);
int ec = findStripNext(buffer, buffer_size, b, a);
// in some cases we can have several matching edges; since we can pick any edge, we pick the one with the smallest
// triangle index in the buffer. this reduces the effect of stripification on ACMR and additionally - for unclear
// reasons - slightly improves the stripification efficiency
int mine = INT_MAX;
mine = (ea >= 0 && mine > ea) ? ea : mine;
mine = (eb >= 0 && mine > eb) ? eb : mine;
mine = (ec >= 0 && mine > ec) ? ec : mine;
if (ea == mine)
{
// keep abc
next = ea;
}
else if (eb == mine)
{
// abc -> bca
unsigned int t = a;
a = b, b = c, c = t;
next = eb;
}
else if (ec == mine)
{
// abc -> cab
unsigned int t = c;
c = b, b = a, a = t;
next = ec;
}
if (restart_index)
{
if (strip_size)
destination[strip_size++] = restart_index;
destination[strip_size++] = a;
destination[strip_size++] = b;
destination[strip_size++] = c;
// new strip always starts with the same edge winding
strip[0] = b;
strip[1] = c;
parity = 1;
}
else
{
if (strip_size)
{
// connect last strip using degenerate triangles
destination[strip_size++] = strip[1];
destination[strip_size++] = a;
}
// note that we may need to flip the emitted triangle based on parity
// we always end up with outgoing edge "cb" in the end
unsigned int e0 = parity ? c : b;
unsigned int e1 = parity ? b : c;
destination[strip_size++] = a;
destination[strip_size++] = e0;
destination[strip_size++] = e1;
strip[0] = e0;
strip[1] = e1;
parity ^= 1;
}
}
}
return strip_size;
}
size_t meshopt_stripifyBound(size_t index_count)
{
assert(index_count % 3 == 0);
// worst case without restarts is 2 degenerate indices and 3 indices per triangle
// worst case with restarts is 1 restart index and 3 indices per triangle
return (index_count / 3) * 5;
}
size_t meshopt_unstripify(unsigned int* destination, const unsigned int* indices, size_t index_count, unsigned int restart_index)
{
assert(destination != indices);
size_t offset = 0;
size_t start = 0;
for (size_t i = 0; i < index_count; ++i)
{
if (restart_index && indices[i] == restart_index)
{
start = i + 1;
}
else if (i - start >= 2)
{
unsigned int a = indices[i - 2], b = indices[i - 1], c = indices[i];
// flip winding for odd triangles
if ((i - start) & 1)
{
unsigned int t = a;
a = b, b = t;
}
// although we use restart indices, strip swaps still produce degenerate triangles, so skip them
if (a != b && a != c && b != c)
{
destination[offset + 0] = a;
destination[offset + 1] = b;
destination[offset + 2] = c;
offset += 3;
}
}
}
return offset;
}
size_t meshopt_unstripifyBound(size_t index_count)
{
assert(index_count == 0 || index_count >= 3);
return (index_count == 0) ? 0 : (index_count - 2) * 3;
}