[Interest] [Qt3D] Update vertex buffer data together with increasing count of vertices leads to artifacts

Sean Harmer sean.harmer at kdab.com
Sat Apr 22 10:16:31 CEST 2017


could you file a JIRA with a test case please? These changes are 
supposed to be batched within a spin of the event loop before sent to to 
the backend. And on the backend, the geometry buffer update should 
happen within the same frame as the count is adjusted. So, looks like 
we've missed something from what you describe.



On 22/04/2017 00:04, Oleg Evseev wrote:
> Hi,
> I frequently update geometry with help of updateData function of vertex
> QBuffer, increasing vertices count with help of setCount function of
> geometry position QAttribute like this:
>     uint vertexCount = positionAttribute->count();
> vertexBuffer->updateData(vertexCount*3*sizeof(VERTEX_TYPE),appendVertexArray);
>     positionAttribute->setCount(vertexCount+2);
> (initial data of vertexBuffer of course has enough space for updates)
> Sometimes (especially with high frequency updates) I observe temporary
> very short time visual artifacts - triangles (faces) based on newly
> added vertices jump somewhere aside, but then "come back" to the place
> they should be.
> As for me it all looks like setCount get to qt3d rendering backend and
> "goes down" to opengl calls before updateData, so data that is in
> current (not yet updated) vertex buffer on the place of new vertices
> (some scrap) is used in rendering and leads to artifacts. And only after
> that buffer data is finally updates.
> (sorry if I'm not correct in qt3d backend work aspects)
> Also finally when I don't need updates anymore, I want to reduce size of
> vertexBuffer data like this:
> QByteArray vertexArray = vertexBuffer->data();
> vertexArray.detach();
> vertexArray.resize((vertexCount+2)*3*sizeof(VERTEX_TYPE));
> ...
> vertexBuffer->setData(vertexArray);
> positionAttribute->setCount(vertexCount+2);
> This one leads sometimes (not always) to artifacts that remains, that is
> faces based on new vertexes are goes somewhere out of the screen
> (probably to 0 coordinate) and do not change anymore.
> Also in contrast with updateData I can't understand and trace chain how
> changing count of vertex attribute goes down finally to opengl calls.
> Thank in advance for help.
> --
> With regards, Oleg.
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