Canvas as VisualBrush on GeometryModel3D : z-index problem

I took the TNF Video Carousel example and modified it (slightly) to
use a VisualBrush of a Canvas on each of the 3D planes instead of
video clips. Works fine except that I'm noticing some sort of z-index
problem.

I have 3 ListBox3DItem's each using a different jpg as the background
of their canvas and I set the opacity of the canvas to .5 and add them
to the Carousel. When I run the app I notice that each canvas is only
transparent to a canvas added prior to itself.

In other words:

Canvas c1 = new Canvas();
c1.Background = new ImageBrush((ImageSource)new
ImageSourceConverter().ConvertFromString(@"C:\background1.jpg"));
c1.Height = 1120;
c1.Width = 624;
c1.Opacity = .5;
Carousel.Add(c1);

Canvas c2 = new Canvas();
c2.Background = new ImageBrush((ImageSource)new
ImageSourceConverter().ConvertFromString(@"C:\background2.jpg"));
c2.Height = 1120;
c2.Width = 624;
c2.Opacity = .5;
Carousel.Add(c2);

Canvas c3 = new Canvas();
c3.Background = new ImageBrush((ImageSource)new
ImageSourceConverter().ConvertFromString(@"C:\background3.jpg"));
c3.Height = 1120;
c3.Width = 624;
c3.Opacity = .5;
Carousel.Add(c3);

Running this example:

C1 will be completely solid when it is in the foreground and C2 and C3
are behind it
C2 will be transparent (50%) when it is in the foreground and you
will see C1 through it as it rotates, but not C3
C3 will be transparent (50%) when it is in the foreground and you
will see C1 and C2 through it as it rotates

I've tried Canvas.SetZIndex(C3) numerous times trying to give the last-
added canvas a higher z-order thinking that might do the trick but no
luck.

Any ideas?

..

Forgot to mention, I've also tried setting the opacity of the
VisualBrush directly, instead of the Canvas that it uses, but that has
the same effect as the above example.

I solved the problem by removing and re-adding the Model3DGroup to the
main group. This forces the geometry to render on top of the other
children.

For future reference, I modified this in ListBox3D.cs:

private ListBox3DItem GetFrontmostItem()
{
ListBox3DItem current = this.Items[_FrontmostItemIndex] as
ListBox3DItem;

// force the object to render in front of everything else
// by adding it on top of the other children each time
_ModelItems.Children.Remove(current.ItemGroup);
_ModelItems.Children.Add(current.ItemGroup);

return current;
}

Hope this saves someone else a bit of trouble!

I'm not familiar with the video carousel example, but you're encountering
transparency issues depending on the order of the objects, right? This is a
general problem in 3D graphics having to do with the depth buffer (which
DiffuseMaterial writes to). For example, say you have three overlapping
transparent DiffuseMaterial models and you draw them back to front, then
everything is good. If you draw them front to back, the front one will block
all of the others because it was drawn first. You need to sort the objects
in your scene.

Also, Canvas.ZIndex is only relevant in 2D.

Jordan

"znelson" <znelson@gmail.com> wrote in message
news:1173327242.575443.324630@h3g2000cwc.googlegroups.com...
>I solved the problem by removing and re-adding the Model3DGroup to the
> main group. This forces the geometry to render on top of the other
> children.
>
> For future reference, I modified this in ListBox3D.cs:
>
> private ListBox3DItem GetFrontmostItem()
> {
> ListBox3DItem current = this.Items[_FrontmostItemIndex] as
> ListBox3DItem;
>
> // force the object to render in front of everything else
> // by adding it on top of the other children each time
> _ModelItems.Children.Remove(current.ItemGroup);
> _ModelItems.Children.Add(current.ItemGroup);
>
> return current;
> }
>
> Hope this saves someone else a bit of trouble!
>

That's exactly the problem I was having. Unfortunately the 3D objects
are not treated equal when added to the group. The depth buffer you
mention is definately taking into consideration the order in which the
objects are added to the group. So by adding the object I want to be
transparent to everything else in the scene, I have to move it from
where it is in the group to the last object in the group. Then all is
good.

3D software programs such as Lightwave 3D aren't affected by this.
Each object in the scene is on the same level as the next. So if every
object is 50% transparent, every object could be seen through any
other object. I couldn't imagine having to juggle objects around in a
complex scene just to get them to be truly transparent so you can see
earlier-added objects through them. Oh well...

It's possible that Lightwave is sorting before drawing or that they are
using different depth options than we are. I was referring more to OpenGL,
Direct3D, XNA, etc.

I forgot to mention that EmissiveMaterial and SpecularMaterial don't touch
the depth buffer so if you just use them you'll be fine. However, you might
not get the look you're going for.

Jordan

"znelson" <znelson@gmail.com> wrote in message
news:1173835539.439217.310200@b75g2000hsg.googlegroups.com...
> That's exactly the problem I was having. Unfortunately the 3D objects
> are not treated equal when added to the group. The depth buffer you
> mention is definately taking into consideration the order in which the
> objects are added to the group. So by adding the object I want to be
> transparent to everything else in the scene, I have to move it from
> where it is in the group to the last object in the group. Then all is
> good.
>
> 3D software programs such as Lightwave 3D aren't affected by this.
> Each object in the scene is on the same level as the next. So if every
> object is 50% transparent, every object could be seen through any
> other object. I couldn't imagine having to juggle objects around in a
> complex scene just to get them to be truly transparent so you can see
> earlier-added objects through them. Oh well...
>