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Updated meshoptimizer.

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This commit is contained in:
Бранимир Караџић 2024-06-25 22:36:33 -07:00
parent 14750e1392
commit 85b33a0f63
7 changed files with 145 additions and 50 deletions
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2
3rdparty/meshoptimizer/src/allocator.cpp vendored
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@ -1,7 +1,7 @@
// This file is part of meshoptimizer library; see meshoptimizer.h for version/license details
#include "meshoptimizer.h"
void meshopt_setAllocator(void* (MESHOPTIMIZER_ALLOC_CALLCONV *allocate)(size_t), void (MESHOPTIMIZER_ALLOC_CALLCONV *deallocate)(void*))
void meshopt_setAllocator(void*(MESHOPTIMIZER_ALLOC_CALLCONV* allocate)(size_t), void(MESHOPTIMIZER_ALLOC_CALLCONV* deallocate)(void*))
{
meshopt_Allocator::Storage::allocate = allocate;
meshopt_Allocator::Storage::deallocate = deallocate;

2
3rdparty/meshoptimizer/src/clusterizer.cpp vendored
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@ -441,7 +441,7 @@ static size_t kdtreeBuild(size_t offset, KDNode* nodes, size_t node_count, const
}
// split axis is one where the variance is largest
unsigned int axis = vars[0] >= vars[1] && vars[0] >= vars[2] ? 0 : vars[1] >= vars[2] ? 1 : 2;
unsigned int axis = (vars[0] >= vars[1] && vars[0] >= vars[2]) ? 0 : (vars[1] >= vars[2] ? 1 : 2);
float split = mean[axis];
size_t middle = kdtreePartition(indices, count, points, stride, axis, split);

8
3rdparty/meshoptimizer/src/indexcodec.cpp vendored
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@ -33,7 +33,7 @@ static int rotateTriangle(unsigned int a, unsigned int b, unsigned int c, unsign
{
(void)a;
return (b == next) ? 1 : (c == next) ? 2 : 0;
return (b == next) ? 1 : (c == next ? 2 : 0);
}
static int getEdgeFifo(EdgeFifo fifo, unsigned int a, unsigned int b, unsigned int c, size_t offset)
@ -217,7 +217,7 @@ size_t meshopt_encodeIndexBuffer(unsigned char* buffer, size_t buffer_size, cons
int fe = fer >> 2;
int fc = getVertexFifo(vertexfifo, c, vertexfifooffset);
int fec = (fc >= 1 && fc < fecmax) ? fc : (c == next) ? (next++, 0) : 15;
int fec = (fc >= 1 && fc < fecmax) ? fc : (c == next ? (next++, 0) : 15);
if (fec == 15 && version >= 1)
{
@ -267,8 +267,8 @@ size_t meshopt_encodeIndexBuffer(unsigned char* buffer, size_t buffer_size, cons
// after rotation, a is almost always equal to next, so we don't waste bits on FIFO encoding for a
int fea = (a == next) ? (next++, 0) : 15;
int feb = (fb >= 0 && fb < 14) ? (fb + 1) : (b == next) ? (next++, 0) : 15;
int fec = (fc >= 0 && fc < 14) ? (fc + 1) : (c == next) ? (next++, 0) : 15;
int feb = (fb >= 0 && fb < 14) ? fb + 1 : (b == next ? (next++, 0) : 15);
int fec = (fc >= 0 && fc < 14) ? fc + 1 : (c == next ? (next++, 0) : 15);
// we encode feb & fec in 4 bits using a table if possible, and as a full byte otherwise
unsigned char codeaux = (unsigned char)((feb << 4) | fec);

30
3rdparty/meshoptimizer/src/meshoptimizer.h vendored
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@ -1,5 +1,5 @@
/**
* meshoptimizer - version 0.20
* meshoptimizer - version 0.21
*
* Copyright (C) 2016-2024, by Arseny Kapoulkine (arseny.kapoulkine@gmail.com)
* Report bugs and download new versions at https://github.com/zeux/meshoptimizer
@ -12,7 +12,7 @@
#include <stddef.h>
/* Version macro; major * 1000 + minor * 10 + patch */
#define MESHOPTIMIZER_VERSION 200 /* 0.20 */
#define MESHOPTIMIZER_VERSION 210 /* 0.21 */
/* If no API is defined, assume default */
#ifndef MESHOPTIMIZER_API
@ -311,12 +311,12 @@ MESHOPTIMIZER_EXPERIMENTAL void meshopt_decodeFilterExp(void* buffer, size_t cou
*/
enum meshopt_EncodeExpMode
{
/* When encoding exponents, use separate values for each component (maximum quality) */
meshopt_EncodeExpSeparate,
/* When encoding exponents, use shared value for all components of each vector (better compression) */
meshopt_EncodeExpSharedVector,
/* When encoding exponents, use shared value for each component of all vectors (best compression) */
meshopt_EncodeExpSharedComponent,
/* When encoding exponents, use separate values for each component (maximum quality) */
meshopt_EncodeExpSeparate,
/* When encoding exponents, use shared value for all components of each vector (better compression) */
meshopt_EncodeExpSharedVector,
/* When encoding exponents, use shared value for each component of all vectors (best compression) */
meshopt_EncodeExpSharedComponent,
};
MESHOPTIMIZER_EXPERIMENTAL void meshopt_encodeFilterOct(void* destination, size_t count, size_t stride, int bits, const float* data);
@ -328,8 +328,12 @@ MESHOPTIMIZER_EXPERIMENTAL void meshopt_encodeFilterExp(void* destination, size_
*/
enum
{
/* Do not move vertices that are located on the topological border (vertices on triangle edges that don't have a paired triangle). Useful for simplifying portions of the larger mesh. */
meshopt_SimplifyLockBorder = 1 << 0,
/* Do not move vertices that are located on the topological border (vertices on triangle edges that don't have a paired triangle). Useful for simplifying portions of the larger mesh. */
meshopt_SimplifyLockBorder = 1 << 0,
/* Improve simplification performance assuming input indices are a sparse subset of the mesh. Note that error becomes relative to subset extents. */
meshopt_SimplifySparse = 1 << 1,
/* Treat error limit and resulting error as absolute instead of relative to mesh extents. */
meshopt_SimplifyErrorAbsolute = 1 << 2,
};
/**
@ -969,10 +973,10 @@ inline size_t meshopt_simplify(T* destination, const T* indices, size_t index_co
template <typename T>
inline size_t meshopt_simplifyWithAttributes(T* destination, const T* indices, size_t index_count, const float* vertex_positions, size_t vertex_count, size_t vertex_positions_stride, const float* vertex_attributes, size_t vertex_attributes_stride, const float* attribute_weights, size_t attribute_count, const unsigned char* vertex_lock, size_t target_index_count, float target_error, unsigned int options, float* result_error)
{
meshopt_IndexAdapter<T> in(NULL, indices, index_count);
meshopt_IndexAdapter<T> out(destination, NULL, index_count);
meshopt_IndexAdapter<T> in(NULL, indices, index_count);
meshopt_IndexAdapter<T> out(destination, NULL, index_count);
return meshopt_simplifyWithAttributes(out.data, in.data, index_count, vertex_positions, vertex_count, vertex_positions_stride, vertex_attributes, vertex_attributes_stride, attribute_weights, attribute_count, vertex_lock, target_index_count, target_error, options, result_error);
return meshopt_simplifyWithAttributes(out.data, in.data, index_count, vertex_positions, vertex_count, vertex_positions_stride, vertex_attributes, vertex_attributes_stride, attribute_weights, attribute_count, vertex_lock, target_index_count, target_error, options, result_error);
}
template <typename T>

149
3rdparty/meshoptimizer/src/simplifier.cpp vendored
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@ -111,10 +111,12 @@ struct PositionHasher
{
const float* vertex_positions;
size_t vertex_stride_float;
const unsigned int* sparse_remap;
size_t hash(unsigned int index) const
{
const unsigned int* key = reinterpret_cast<const unsigned int*>(vertex_positions + index * vertex_stride_float);
unsigned int ri = sparse_remap ? sparse_remap[index] : index;
const unsigned int* key = reinterpret_cast<const unsigned int*>(vertex_positions + ri * vertex_stride_float);
// scramble bits to make sure that integer coordinates have entropy in lower bits
unsigned int x = key[0] ^ (key[0] >> 17);
@ -127,7 +129,25 @@ struct PositionHasher
bool equal(unsigned int lhs, unsigned int rhs) const
{
return memcmp(vertex_positions + lhs * vertex_stride_float, vertex_positions + rhs * vertex_stride_float, sizeof(float) * 3) == 0;
unsigned int li = sparse_remap ? sparse_remap[lhs] : lhs;
unsigned int ri = sparse_remap ? sparse_remap[rhs] : rhs;
return memcmp(vertex_positions + li * vertex_stride_float, vertex_positions + ri * vertex_stride_float, sizeof(float) * 3) == 0;
}
};
struct RemapHasher
{
unsigned int* remap;
size_t hash(unsigned int id) const
{
return id * 0x5bd1e995;
}
bool equal(unsigned int lhs, unsigned int rhs) const
{
return remap[lhs] == rhs;
}
};
@ -167,9 +187,9 @@ static T* hashLookup2(T* table, size_t buckets, const Hash& hash, const T& key,
return NULL;
}
static void buildPositionRemap(unsigned int* remap, unsigned int* wedge, const float* vertex_positions_data, size_t vertex_count, size_t vertex_positions_stride, meshopt_Allocator& allocator)
static void buildPositionRemap(unsigned int* remap, unsigned int* wedge, const float* vertex_positions_data, size_t vertex_count, size_t vertex_positions_stride, const unsigned int* sparse_remap, meshopt_Allocator& allocator)
{
PositionHasher hasher = {vertex_positions_data, vertex_positions_stride / sizeof(float)};
PositionHasher hasher = {vertex_positions_data, vertex_positions_stride / sizeof(float), sparse_remap};
size_t table_size = hashBuckets2(vertex_count);
unsigned int* table = allocator.allocate<unsigned int>(table_size);
@ -205,6 +225,57 @@ static void buildPositionRemap(unsigned int* remap, unsigned int* wedge, const f
allocator.deallocate(table);
}
static unsigned int* buildSparseRemap(unsigned int* indices, size_t index_count, size_t vertex_count, size_t* out_vertex_count, meshopt_Allocator& allocator)
{
// use a bit set to compute the precise number of unique vertices
unsigned char* filter = allocator.allocate<unsigned char>((vertex_count + 7) / 8);
memset(filter, 0, (vertex_count + 7) / 8);
size_t unique = 0;
for (size_t i = 0; i < index_count; ++i)
{
unsigned int index = indices[i];
assert(index < vertex_count);
unique += (filter[index / 8] & (1 << (index % 8))) == 0;
filter[index / 8] |= 1 << (index % 8);
}
unsigned int* remap = allocator.allocate<unsigned int>(unique);
size_t offset = 0;
// temporary map dense => sparse; we allocate it last so that we can deallocate it
size_t revremap_size = hashBuckets2(unique);
unsigned int* revremap = allocator.allocate<unsigned int>(revremap_size);
memset(revremap, -1, revremap_size * sizeof(unsigned int));
// fill remap, using revremap as a helper, and rewrite indices in the same pass
RemapHasher hasher = {remap};
for (size_t i = 0; i < index_count; ++i)
{
unsigned int index = indices[i];
unsigned int* entry = hashLookup2(revremap, revremap_size, hasher, index, ~0u);
if (*entry == ~0u)
{
remap[offset] = index;
*entry = unsigned(offset);
offset++;
}
indices[i] = *entry;
}
allocator.deallocate(revremap);
assert(offset == unique);
*out_vertex_count = unique;
return remap;
}
enum VertexKind
{
Kind_Manifold, // not on an attribute seam, not on any boundary
@ -252,7 +323,7 @@ static bool hasEdge(const EdgeAdjacency& adjacency, unsigned int a, unsigned int
return false;
}
static void classifyVertices(unsigned char* result, unsigned int* loop, unsigned int* loopback, size_t vertex_count, const EdgeAdjacency& adjacency, const unsigned int* remap, const unsigned int* wedge, const unsigned char* vertex_lock, unsigned int options)
static void classifyVertices(unsigned char* result, unsigned int* loop, unsigned int* loopback, size_t vertex_count, const EdgeAdjacency& adjacency, const unsigned int* remap, const unsigned int* wedge, const unsigned char* vertex_lock, const unsigned int* sparse_remap, unsigned int options)
{
memset(loop, -1, vertex_count * sizeof(unsigned int));
memset(loopback, -1, vertex_count * sizeof(unsigned int));
@ -298,7 +369,7 @@ static void classifyVertices(unsigned char* result, unsigned int* loop, unsigned
{
if (remap[i] == i)
{
if (vertex_lock && vertex_lock[i])
if (vertex_lock && vertex_lock[sparse_remap ? sparse_remap[i] : i])
{
// vertex is explicitly locked
result[i] = Kind_Locked;
@ -383,7 +454,7 @@ struct Vector3
float x, y, z;
};
static float rescalePositions(Vector3* result, const float* vertex_positions_data, size_t vertex_count, size_t vertex_positions_stride)
static float rescalePositions(Vector3* result, const float* vertex_positions_data, size_t vertex_count, size_t vertex_positions_stride, const unsigned int* sparse_remap = NULL)
{
size_t vertex_stride_float = vertex_positions_stride / sizeof(float);
@ -392,7 +463,8 @@ static float rescalePositions(Vector3* result, const float* vertex_positions_dat
for (size_t i = 0; i < vertex_count; ++i)
{
const float* v = vertex_positions_data + i * vertex_stride_float;
unsigned int ri = sparse_remap ? sparse_remap[i] : unsigned(i);
const float* v = vertex_positions_data + ri * vertex_stride_float;
if (result)
{
@ -431,15 +503,17 @@ static float rescalePositions(Vector3* result, const float* vertex_positions_dat
return extent;
}
static void rescaleAttributes(float* result, const float* vertex_attributes_data, size_t vertex_count, size_t vertex_attributes_stride, const float* attribute_weights, size_t attribute_count)
static void rescaleAttributes(float* result, const float* vertex_attributes_data, size_t vertex_count, size_t vertex_attributes_stride, const float* attribute_weights, size_t attribute_count, const unsigned int* sparse_remap)
{
size_t vertex_attributes_stride_float = vertex_attributes_stride / sizeof(float);
for (size_t i = 0; i < vertex_count; ++i)
{
unsigned int ri = sparse_remap ? sparse_remap[i] : unsigned(i);
for (size_t k = 0; k < attribute_count; ++k)
{
float a = vertex_attributes_data[i * vertex_attributes_stride_float + k];
float a = vertex_attributes_data[ri * vertex_attributes_stride_float + k];
result[i * attribute_count + k] = a * attribute_weights[k];
}
@ -818,7 +892,13 @@ static bool hasTriangleFlip(const Vector3& a, const Vector3& b, const Vector3& c
Vector3 nbc = {eb.y * ec.z - eb.z * ec.y, eb.z * ec.x - eb.x * ec.z, eb.x * ec.y - eb.y * ec.x};
Vector3 nbd = {eb.y * ed.z - eb.z * ed.y, eb.z * ed.x - eb.x * ed.z, eb.x * ed.y - eb.y * ed.x};
return nbc.x * nbd.x + nbc.y * nbd.y + nbc.z * nbd.z <= 0;
float ndp = nbc.x * nbd.x + nbc.y * nbd.y + nbc.z * nbd.z;
float abc = nbc.x * nbc.x + nbc.y * nbc.y + nbc.z * nbc.z;
float abd = nbd.x * nbd.x + nbd.y * nbd.y + nbd.z * nbd.z;
// scale is cos(angle); somewhat arbitrarily set to ~75 degrees
// note that the "pure" check is ndp <= 0 (90 degree cutoff) but that allows flipping through a series of close-to-90 collapses
return ndp <= 0.25f * sqrtf(abc * abd);
}
static bool hasTriangleFlips(const EdgeAdjacency& adjacency, const Vector3* vertex_positions, const unsigned int* collapse_remap, unsigned int i0, unsigned int i1)
@ -1481,7 +1561,7 @@ size_t meshopt_simplifyEdge(unsigned int* destination, const unsigned int* indic
assert(vertex_positions_stride >= 12 && vertex_positions_stride <= 256);
assert(vertex_positions_stride % sizeof(float) == 0);
assert(target_index_count <= index_count);
assert((options & ~(meshopt_SimplifyLockBorder)) == 0);
assert((options & ~(meshopt_SimplifyLockBorder | meshopt_SimplifySparse | meshopt_SimplifyErrorAbsolute)) == 0);
assert(vertex_attributes_stride >= attribute_count * sizeof(float) && vertex_attributes_stride <= 256);
assert(vertex_attributes_stride % sizeof(float) == 0);
assert(attribute_count <= kMaxAttributes);
@ -1489,22 +1569,30 @@ size_t meshopt_simplifyEdge(unsigned int* destination, const unsigned int* indic
meshopt_Allocator allocator;
unsigned int* result = destination;
if (result != indices)
memcpy(result, indices, index_count * sizeof(unsigned int));
// build an index remap and update indices/vertex_count to minimize the subsequent work
// note: as a consequence, errors will be computed relative to the subset extent
unsigned int* sparse_remap = NULL;
if (options & meshopt_SimplifySparse)
sparse_remap = buildSparseRemap(result, index_count, vertex_count, &vertex_count, allocator);
// build adjacency information
EdgeAdjacency adjacency = {};
prepareEdgeAdjacency(adjacency, index_count, vertex_count, allocator);
updateEdgeAdjacency(adjacency, indices, index_count, vertex_count, NULL);
updateEdgeAdjacency(adjacency, result, index_count, vertex_count, NULL);
// build position remap that maps each vertex to the one with identical position
unsigned int* remap = allocator.allocate<unsigned int>(vertex_count);
unsigned int* wedge = allocator.allocate<unsigned int>(vertex_count);
buildPositionRemap(remap, wedge, vertex_positions_data, vertex_count, vertex_positions_stride, allocator);
buildPositionRemap(remap, wedge, vertex_positions_data, vertex_count, vertex_positions_stride, sparse_remap, allocator);
// classify vertices; vertex kind determines collapse rules, see kCanCollapse
unsigned char* vertex_kind = allocator.allocate<unsigned char>(vertex_count);
unsigned int* loop = allocator.allocate<unsigned int>(vertex_count);
unsigned int* loopback = allocator.allocate<unsigned int>(vertex_count);
classifyVertices(vertex_kind, loop, loopback, vertex_count, adjacency, remap, wedge, vertex_lock, options);
classifyVertices(vertex_kind, loop, loopback, vertex_count, adjacency, remap, wedge, vertex_lock, sparse_remap, options);
#if TRACE
size_t unique_positions = 0;
@ -1522,14 +1610,14 @@ size_t meshopt_simplifyEdge(unsigned int* destination, const unsigned int* indic
#endif
Vector3* vertex_positions = allocator.allocate<Vector3>(vertex_count);
rescalePositions(vertex_positions, vertex_positions_data, vertex_count, vertex_positions_stride);
float vertex_scale = rescalePositions(vertex_positions, vertex_positions_data, vertex_count, vertex_positions_stride, sparse_remap);
float* vertex_attributes = NULL;
if (attribute_count)
{
vertex_attributes = allocator.allocate<float>(vertex_count * attribute_count);
rescaleAttributes(vertex_attributes, vertex_attributes_data, vertex_count, vertex_attributes_stride, attribute_weights, attribute_count);
rescaleAttributes(vertex_attributes, vertex_attributes_data, vertex_count, vertex_attributes_stride, attribute_weights, attribute_count, sparse_remap);
}
Quadric* vertex_quadrics = allocator.allocate<Quadric>(vertex_count);
@ -1547,14 +1635,11 @@ size_t meshopt_simplifyEdge(unsigned int* destination, const unsigned int* indic
memset(attribute_gradients, 0, vertex_count * attribute_count * sizeof(QuadricGrad));
}
fillFaceQuadrics(vertex_quadrics, indices, index_count, vertex_positions, remap);
fillEdgeQuadrics(vertex_quadrics, indices, index_count, vertex_positions, remap, vertex_kind, loop, loopback);
fillFaceQuadrics(vertex_quadrics, result, index_count, vertex_positions, remap);
fillEdgeQuadrics(vertex_quadrics, result, index_count, vertex_positions, remap, vertex_kind, loop, loopback);
if (attribute_count)
fillAttributeQuadrics(attribute_quadrics, attribute_gradients, indices, index_count, vertex_positions, vertex_attributes, attribute_count, remap);
if (result != indices)
memcpy(result, indices, index_count * sizeof(unsigned int));
fillAttributeQuadrics(attribute_quadrics, attribute_gradients, result, index_count, vertex_positions, vertex_attributes, attribute_count, remap);
#if TRACE
size_t pass_count = 0;
@ -1571,7 +1656,8 @@ size_t meshopt_simplifyEdge(unsigned int* destination, const unsigned int* indic
float result_error = 0;
// target_error input is linear; we need to adjust it to match quadricError units
float error_limit = target_error * target_error;
float error_scale = (options & meshopt_SimplifyErrorAbsolute) ? vertex_scale : 1.f;
float error_limit = (target_error * target_error) / (error_scale * error_scale);
while (result_count > target_index_count)
{
@ -1630,9 +1716,14 @@ size_t meshopt_simplifyEdge(unsigned int* destination, const unsigned int* indic
memcpy(meshopt_simplifyDebugLoopBack, loopback, vertex_count * sizeof(unsigned int));
#endif
// convert resulting indices back into the dense space of the larger mesh
if (sparse_remap)
for (size_t i = 0; i < result_count; ++i)
result[i] = sparse_remap[result[i]];
// result_error is quadratic; we need to remap it back to linear
if (out_result_error)
*out_result_error = sqrtf(result_error);
*out_result_error = sqrtf(result_error) * error_scale;
return result_count;
}
@ -1697,14 +1788,14 @@ size_t meshopt_simplifySloppy(unsigned int* destination, const unsigned int* ind
// we clamp the prediction of the grid size to make sure that the search converges
int grid_size = next_grid_size;
grid_size = (grid_size <= min_grid) ? min_grid + 1 : (grid_size >= max_grid) ? max_grid - 1 : grid_size;
grid_size = (grid_size <= min_grid) ? min_grid + 1 : (grid_size >= max_grid ? max_grid - 1 : grid_size);
computeVertexIds(vertex_ids, vertex_positions, vertex_count, grid_size);
size_t triangles = countTriangles(vertex_ids, indices, index_count);
#if TRACE
printf("pass %d (%s): grid size %d, triangles %d, %s\n",
pass, (pass == 0) ? "guess" : (pass <= kInterpolationPasses) ? "lerp" : "binary",
pass, (pass == 0) ? "guess" : (pass <= kInterpolationPasses ? "lerp" : "binary"),
grid_size, int(triangles),
(triangles <= target_index_count / 3) ? "under" : "over");
#endif
@ -1829,14 +1920,14 @@ size_t meshopt_simplifyPoints(unsigned int* destination, const float* vertex_pos
// we clamp the prediction of the grid size to make sure that the search converges
int grid_size = next_grid_size;
grid_size = (grid_size <= min_grid) ? min_grid + 1 : (grid_size >= max_grid) ? max_grid - 1 : grid_size;
grid_size = (grid_size <= min_grid) ? min_grid + 1 : (grid_size >= max_grid ? max_grid - 1 : grid_size);
computeVertexIds(vertex_ids, vertex_positions, vertex_count, grid_size);
size_t vertices = countVertexCells(table, table_size, vertex_ids, vertex_count);
#if TRACE
printf("pass %d (%s): grid size %d, vertices %d, %s\n",
pass, (pass == 0) ? "guess" : (pass <= kInterpolationPasses) ? "lerp" : "binary",
pass, (pass == 0) ? "guess" : (pass <= kInterpolationPasses ? "lerp" : "binary"),
grid_size, int(vertices),
(vertices <= target_vertex_count) ? "under" : "over");
#endif

2
3rdparty/meshoptimizer/src/stripifier.cpp vendored
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@ -18,7 +18,7 @@ static unsigned int findStripFirst(const unsigned int buffer[][3], unsigned int
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;
unsigned int v = (va < vb && va < vc) ? va : (vb < vc ? vb : vc);
if (v < iv)
{

2
3rdparty/meshoptimizer/src/vertexcodec.cpp vendored
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@ -245,7 +245,7 @@ static unsigned char* encodeBytes(unsigned char* data, unsigned char* data_end,
}
}
int bitslog2 = (best_bits == 1) ? 0 : (best_bits == 2) ? 1 : (best_bits == 4) ? 2 : 3;
int bitslog2 = (best_bits == 1) ? 0 : (best_bits == 2 ? 1 : (best_bits == 4 ? 2 : 3));
assert((1 << bitslog2) == best_bits);
size_t header_offset = i / kByteGroupSize;