decal.faceMe() method

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I'd like to use the 3D decals as sprites in a scene, but have them always 
rotated to face the camera. Image is something like a forest, with lots of 
sprite trees. i want to take advantage of the z-ordering and scaling but not 
have the trees rotate and look "flat".

so need a kind of inverse lookAt function eg 
{{{  sprite.faceMe(camera);   }}}

camera lookAt function is maybe not relevant

{{{
    public void lookAt(float x, float y, float z) { 
        direction.set(x, y, z).sub(position).nor();
    }
}}}

I guess its just a case of calculating the angle between camera and object, and 
rotating the decal by that amount, using this method:

{{{
    public void rotateY(float angle) {
        rotator.set(Y_AXIS, angle);
        rotation.mul(rotator);
        updated = false;
    }

}}}

Original issue reported on code.google.com by dc.pik...@gmail.com on 16 Aug 2011 at 8:56

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i guess you cant use wiki markup in tickets then!

Original comment by dc.pik...@gmail.com on 16 Aug 2011 at 9:01

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in theory:
To archieve this, the decal's normal must have the same orientation as the 
camera's direction vector.

If there is an issue with frontside/backside and therefore the decal being 
culled away the direction of the decal's normal must match the camera's 
negative direction, as well.

I don't know if decals have a concept of a normal, but they sure have vertices 
(or at least an origin, h and w dimensions, and another 3d attribute) from 
which the normals could be calculated?

Original comment by radiok...@gmx.de on 16 Aug 2011 at 9:21

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"To archieve this, the decal's normal must have the same orientation as the 
camera's direction vector."

-> craps...

To archieve this, the decal's normal must have the same orientation as the 
vector between the camera's position and the decal's center. This way the 
decal's offset from the camera's viewing center is taken into calculation as 
well.

Original comment by radiok...@gmx.de on 16 Aug 2011 at 9:26

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decals seem to be viewable from any angle (no back culling), so dont think the 
normals will be a big problem?

Original comment by dc.pik...@gmail.com on 16 Aug 2011 at 9:43

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right, forget the note on culling I mentioned.

The decal's normal is just for calculation of the needed rotation angle round 
the y-axis.

Original comment by radiok...@gmx.de on 16 Aug 2011 at 9:54

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Needed: rotation angle alpha around decal's center

In our case we just have to take the x,z plane into concideration.

I have attached a scan so we get the naming convention right to issue the 
formulas.

We could get the angle by using trigonometric laws for arbitrary angled 
triangles that are made up by the vectors shown in attached scan (decal's 
normal, a vector camposition->decalcenter, and the connection vector decal's 
normal->cam's position).

For trigonometric calculations only the vector's length are relevant which can 
be calculated by libGDX functions.

Original comment by radiok...@gmx.de on 16 Aug 2011 at 10:28

Attachments:

• decalRotationAngle.jpg

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[deleted comment]

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I mentioned DecalSprite's class having an empty method faceCamera which could 
be called in DecalWall demo by 

wall.faceCamera(oCam)

once it is filled with the maths above.

Original comment by radiok...@gmx.de on 16 Aug 2011 at 10:59

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is x/z all we need? potentially if the camera is a bit  from above maybe we 
need all three axes?

but for 95% of uses just the Y rotate should be fine i think...

Original comment by dc.pik...@gmail.com on 16 Aug 2011 at 11:04

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I thought the Decals should stay perpendicular to the floor and just rotate 
round y Axis to face the camera.

Calculating everythin in 3d would take care of facing the cam in all 3 axis.

Depends on what you'd like to have...

Original comment by radiok...@gmx.de on 16 Aug 2011 at 5:56

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In the end it turned out being a lot more down to earth as expected.

It's just 3 lines of code actually *sigh*

documented it here:
http://code.google.com/p/libgdx-users/wiki/decalsLookAt

and opened enhancement request here:
http://code.google.com/p/libgdx/issues/detail?id=393

/closed

Original comment by radiok...@gmx.de on 16 Aug 2011 at 7:37

• Changed state: Fixed