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view-frustum culling implementation with jit.gen & matrices

Jun 12 2012 | 8:31 pm

Pointed there by Robert R., i'm trying to implement view-frustum culling into my system.
I'm using : which seems to be a great reference (I saw this link a bit everywhere)

I'm currently into the Geometric Approach and, for sure, I need some generous hands to help me.

I understood the demo there :
This is the global description.

Implementation is a bit more tricky (for me)

As attached, I'm okay (I guess) with the setCamInternals.

I'm stuck in implementing the setCamDef which has to compute 3 vectors and to render the 8 points of the frustum (=the 6 planes used for checking which object has to be sent to the openGL pipeline)

I'm stuck because I don't know how to grab those 3 vectors required.

The first one is ok, this is the position of the cam.
The second one is "a point to where the camera is pointing" … How can I find/calculate it ?
The third is the up vector, same .. . how can I grab it ??

ANY help would be amazing for me.


  1. setCamInternals.PNG


Jun 13 2012 | 12:03 am

"The second one is "a point to where the camera is pointing" … How can I find/calculate it ?
The third is the up vector, same .. . how can I grab it ??"

both of these attributes can be retrieved.
you have to attach a jit.anim.node to your in order to make it work. (this requirement will be removed in an upcoming release)

the camera-point-position is the @lookat attribute of jit.anim.node.

the up vector can be calculated from the anim.node’s @quat attribute, by sending it to a jit.quat object, and querrying the yaxis attribute. (the up vector is the Y axis transformed by the camera’s current quaternion)

check out the patch below.

-- Pasted Max Patch, click to expand. --

Jun 13 2012 | 9:59 am

Hi Robert,
a big thanks to you.
I even understood how jit.anim.node object worked :)

implementing my calculation about view-frustum culling in next hours and posting here the result!!

Jun 13 2012 | 6:22 pm

I finally have little problem to retrieve ratio parameter :-/ (in setCamInternals :

I guess it is the ratio between height and wide of frustum?!

Jun 14 2012 | 9:43 am

so, I have some difficulties about 2 things:
– to retrieve the ratio parameter in order to calculate my 8 points defining my 6 planes for the culling
– to put that into my jit.gen.

basically, here is my current implementation on jit.gen
ratio & planes implementation are missing, and of course, the objects matching test.

probably, I'll create a matrix with my 8 points, then another jit.gen which would be fed by objects positions matrix + my 8 points matrix, calculating which objects is in or out of scope…

would you help me with that Robert, or Wes or ?


  1. viewfrustuCullingJitGen.PNG


Jun 14 2012 | 6:15 pm

ratio is window-width divided by window-height.

Jun 14 2012 | 9:29 pm

Hi Julien

a month ago I solved the modelviewprojection transformation with a max patch:

this patch calculates the xy-pos on the screen for one vertice. maybe it could be used to do what you wanna achieve?



Jun 15 2012 | 12:07 pm

@Robert.. thanks for this obvious thing I didn’t even notice :-/

@maybites: thanks a lot for your link.
I don’t know how to apply your nice patch (which I understood btw) to my "case"
I’m trying to implement

Jun 15 2012 | 12:45 pm

if I sent an n planes matrix to a jit.gen, what is the rule to retrieve a particular plane ??

I’m currently sending an 1×1 matrix with 4 planes to the first inlet of my jit.gen
I’m mapping the in 1 to a codebox, on the first inlet too
so in1.x, .y, .z are the first 3 planes

how can I have the fourth ?

Jun 16 2012 | 11:43 pm

so here is where I went.

I have that huge jit.gen (finally not that big)
It takes some parameters as input:
– position of the cam
– a point on the view direction of the cam (lookat)
– the up vector

It takes also cam parameters like:
– lens angle
– ratio of the view (w/h)
– nearClip distance
– farClip distance

it processes the whole parameters to render the 8 points of the view frustum.
As shown there:

The calculation is there:

first question:
I need to normalize a vector inside the codebox.
I did the basic calculation including sqrt()
does it exist a better way inside GenExpr / codebox ?

Now, I have my eight point defining my view.
They defines my 6 planes too.

I have to "test" all my object according to those planes in order to have a list of objects I have to inactivate and activate, according to the position & orientation of my cam.

That part is done in the second codebox and has to be done for the 6 planes.

Knowing I have a matrix with all my objects' positions, how can I test each one against those 6 planes ??

any help would be appreciate.
I don't know if this is the best implementation I could make of that concept.


  1. viewfrustumCulling.PNG


Jul 05 2012 | 11:17 am

Only to clarify my approach (which is quick and dirty compare to the road you are taking).

As far as I understand you wanna have a tool that tells you if your object is inside the viewing frustum. my patch does that insofar, that it calculates the x-y screen-coordinates of a 3d vertex. if the x-y coordinates are not inside the screen, the vertex would be outside the frustum.

But I might have missunderstood your problem.

Jul 05 2012 | 11:20 am

hi there,
x,y aren’t enough because my cam doesn’t have to be locked in only one plane.

Jul 06 2012 | 4:27 pm

if I understand you correctly: my patch works with all sorts of camera-transformations.

Jul 06 2012 | 7:10 pm

this would be nice if I could test it.

I’m currently trying to make a calculation for all objects around the cam for pure sound purpose. I already need that.
The position of the object relatively to the cam would be defined by azimuth/elevation/distance.

Then, I’d like to make a one pass calculation and calculate if the object has to be activated or not according to azimuth/elevation.

Indeed, all in one could be nice & efficient.

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