| Re: mesh clipping [message #31963 is a reply to message #31904] |
Wed, 28 August 2002 09:24  |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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"lyubo" <lzagorch@cs.wright.edu> wrote
> Rick, you were right. I really want to slice the mesh up interactively
> and that's why I was trying to clip it to a plane. I guess alpha blending
> will be faster but the question that I have here is how can I use alpha
> blending with a mesh? I thought that I can apply alpha blending only to
> texture mapped polygons, by using an alpha image as texture. With
> the mesh I don't have any texture. I will try to find examples on the net,
> I just wanted to thank you for your reply.
Ahh, you have a wire mesh....
You are *mostly* correct in thinking that you need to work with texture
mapped solid polygons to use alpha blending. In IDL 5.5 there is a bug that
allows you to texture wireframe models. But, before we go there, you need
to texture your polygon first...
For now, work with a solid polygon. Let's assume you want to draw your
polygon in grey. Create a instance of IDLgrImage with this texture data:
imagedat = [[180,180,180,255],[180,180,180,0]]
Use this image object to texture your polygon.
The trick will be setting up the texture coordinates. Your texcoords array
will be a 2xn array where n is the number of verticies in your mesh and each
coordinate pair maps a pixel in your image to a vertex in your mesh. So,
for verticies you want "on" you will give it a texcoord of [0,0] and for
verts you want off, [0,1] (or is it [1,0]? Well, you get the idea).
There are a few things to watch out for. One is that if I remember
correctly, I don't actually use texcoords of 0 or 1 to assign pixels at the
edge of my texture. I ended up using 0.001 and 0.999. Unfortunatly I
can't remember why...
A second issue will be that you will not have a cleanly defined edge along
your slices. IDL will blend from opaque to transparent giving you a "soft"
edge. This may be a result of the type of shading used though..
And then there is the order in which the polygon is drawn. It has to be
drawn back to front. And if you rotate it 180 degrees you draw it back to
front, which turns out to be front to back. I usually end up slicing my
mesh into a +z portion and -z portion and then keep track of where the
camera is and flip the two objects in my model when the camera crosses the
xy plane.
Ahh, the wire mesh... Like I said, IDL 5.5 has a bug where wire mesh
polygons can be textured. It just doesn't work as expected. But you should
be able to get it to work. Start with the solid and get that working...
> As far as my graphics adapter, I use Nvidia GeForce3 on a P4 2.0GHz
> dual processor with 512Mb Ram platform. Which graphic adapters
> support rendering of volumes?
That I can't answer. We don't do volumes so I haven't ever investigated
this. I can tell you that the high end consumer cards like your GF3 are
optimized for gaming. They concentrate on fill rate first, then polygon
count. If there is any support for volumes it is WAY down the list.
-Rick
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