Hi Robert,
I have been chasing a bug in our osg-based application where a color
change resulted in a broken geometry (which uses VertexBufferObjects).
The update of the VBO does not seem to work when "copyAll" is false
(i.e. total size did not change) and not all Arrays are modified:
VertexBufferObject::compileBuffer(State& state) const
{
[...]
unsigned int offset = 0;
for(BufferEntryArrayPairs::const_iterator itr =
_bufferEntryArrayPairs.begin();
itr != _bufferEntryArrayPairs.end();
++itr)
{
const BufferEntryArrayPair& bep = *itr;
const Array* de = bep.second;
if (de)
{
if (copyAll ||
bep.first.modifiedCount[contextID] !=
bep.second->getModifiedCount() ||
bep.first.dataSize != bep.second->getTotalDataSize())
{
// copy data across
bep.first.dataSize = bep.second->getTotalDataSize();
bep.first.modifiedCount[contextID] =
de->getModifiedCount();
if (copyAll)
{
bep.first.offset = offset;
de->setVertexBufferObjectOffset((GLvoid*)offset);
offset += bep.first.dataSize;
}
[...]
The "offset" variable is only updated when the particular Array
has changed. If only the second of two arrays changes it writes at
offset 0 (where the first array starts) so both arrays will have an
offset of 0.
I think a proper fix which also properly handles array
additions/removals would have to use free-lists (or some
other memory management scheme).
I can not think of a simple fix which still minimizes updates.
Regards,
Peter
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