Bluefish444 Create3D Bedienungsanleitung PDF herunterladen (Seite 110)

10. A few additional words on blending and transparency:

It may seem odd, especially to those who are not familiar with OpenGI or other 3D graphics

development environments, that transparency is one of the most difficult tasks to be solved in

real time 3D graphics. As already mentioned, OpenGI uses the so-called Depth Buffer to keep

track of the distance of each pixel (fragment) of the image from the point of view of the viewer, ie.

the camera.

If in the course of the calculations a given fragment (the smallest element calculated in OpenGl is

called a fragment) of the object about to be shown receives a depth value that is bigger than the

corresponding value already in the buffer, it means that there is already a fragment closer than

the new one, so the new one can be discarded. This method works perfectly as long as the object

that is closer is not transparent at all. Because if it is, and we can see through it, we should be

able to see the object behind it as well.

OpenGI has a separate setting for taking transparency into consideration, for deciding whether to

calculate or not the blending of a fragment already rendered with one that is just being calculated

and is at least partially transparent. This process is called Blending. When Blending is switched

off, OpenGI will not calculate transparency, everything will remain non transparent and will cover

everything behind it. When Blending is on, the Depth Buffer cannot be used or can be used only

in part. In this case how do we calculate which object should cover the other? As a matter of fact

no hardware assisted methods are available for this. The order in which you place your Objects

on a Page determines the order in which they are being drawn. In Nemo this means that the

closer the Object is to the camera the lower it should be in the tree view list of the Objects panel.

Consequently care should always be taken to place Objects in the right order, according to their

distance from the camera, particularly those Objects which have transparent parts. This is the

main purpose of the possibility of dragging and dropping objects to change their order.

Even bearing this in mind we have to overcome another problem. First, we have to know whether

an object has a transparent part. In the simplest case the value of Transparency, one of the

common properties, is smaller than 1.

But there is a Transparency property for Materials, too and an Object can have several Materials

applied on it. More complicated still if neither the Object nor any of its Materials have

Transparency values set below one, but one of the Textures attached to one of the Materials

have pixels with alpha values that make them part transparent. These pixels cannot be checked

automatically by NemoEngine all the time, you have to intervene manually.

Using this manual solution is much simpler than explaining the reason why you need to use it. If

you apply 32-bit texture, about which you know that it has transparent pixels, you should set

either the Transparency property of the Material or the Object itself to a value very slightly less

than 1 (0.998 is perfect). This way NemoEngine is instructed to switch to Blending mode and do

not cover the Objects behind with this one. Still, this value being almost 1 ensures that if you don't

really want to make the whole object transparent it will still look opaque.

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Bluefish444 Create3D Bedienungsanleitung Seite 110