[osg-users] Get current Billboard ModelView matrix?

[Sebastian Messerschmidt]

Hi Robert, Phillip
> Hi Phillip,
>
> On 30 June 2016 at 12:22, Philipp Meyer <philipp.meyer at fh-bielefeld.de> wrote:
>> unfortunately some objects move in my scene, so its not enough to only hold the old view matrix.
>>
>> The camera AND any object can move.
> OK.
>
> You haven't ever said what you are trying to do with current vs
> previous frame data.  I don't recall anyone ask for this specific
> requirement before.  The OSG community have been around quite a long
> time now...  So you are either doing something very novel or a trying
> to solve a common problem in a very awkward way.
That's not entirely correct. I asked for something like this some years 
ago to implement a per-object motion blur in a deferred pipeline.
Basically one would write per-fragment velocity vectors to a render 
target to later selectively blur in a post-processing step. The velocity 
vectors were based on the change in the view-matrix + the changes per 
object.
I remember experimenting with callbacks and holding the last 
model-matrix in the objects user-data which was used to fill an uniform 
each frame, but I ultimately switched to a solution which didn't 
consider the blur on a per-object base.
So basically there are use-cases, but I never found a satisfactory 
solution (I think the problem was performance, due to the many uniform 
updates/statesets).
May be there is a more advanced solution to the problem that haven't 
thought of.

Cheers
Sebastian
>
> One thing to also considering in your pondering is that not all
> objects will actually be in the view frustum over subsequent frames,
> especially as you pan around, new objects will appear in the view
> frustum for the first time so won't have a previous modelview matrix
> associated with them.
>
> Since you haven't said what this is all in aid of I can't say whether
> this later issue has any baring, it will however, if you really need
> to do what you say you need to do will introduce it's own set of
> issues.
>
>
> Robert.
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