Rect redrawing

In all the samples of dx 8.0 whole render target(window or full screen) is redrawn. If we want to redraw only the rect enclosing the object. How can wee determine the coordinate of rect. Better explain with reference to any of sample.
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GuyJohnstonConnect With a Mentor Commented:
You might consider applying transformation matrix
directly to the object verticies rather than use the
world transformation matrix. This way the verticies
are transformed directly. You can then get the bounding
box from the transformed verticies.

Depending on what you are doing, you may have to create
a reference vertex array to compute your transformations

I used this method in a flying saucer game and it worked
quite well.
Do you want to draw a bounding box rectangle around an object or do you want to redraw just the rectangular area containing the object. Is this a 3d or a 2D application ie is this a direct3d or a directdraw query ? I am sorry ... I dont understand the question fully ... whould you please rephrase it and give more detail. The question can be answered in various ways ... but I want to be sure of the context of the question first.
SumiaAuthor Commented:
I only want to determine the bounding rectangle in directx graphics or you can say direct3d in a 3d application. I have given the untransformed vertices and I want to reach the vertices used by the rasterrizer of directx graphics.
Have u looked at the computeboundingbox command ?

Computes a bounding box.

object.ComputeBoundingBox( _
    PointsFVF As Any, _
    NumVertices As Long, _
    FVF As Long, _
    MinArray As D3DVECTOR, _
    MaxArray As D3DVECTOR)

Object expression that resolves to a D3DX8 object.
Buffer containing the vertex data around which to calculate the bounding box.
Number of vertices.
Combination of flexible vertex format flags that describes the vertex format.
D3DVECTOR type, describing the returned lower-left corner of the bounding box.  
D3DVECTOR type, describing the returned upper-right corner of the bounding box.  

Or do u want the actual algorithm to do it ?
SumiaAuthor Commented:
dx graphics does not apply the transformations unless drawprimitive is called. As I have explained earlier that I want to know the transformed vertices(screen cordinates) when I have supplied the world co-ordinates . With compute bounding box the problem remains the same that it performs the calculations on vertex buffer we give to it. But how to obtain the vertex buffer that contains the transformed co-ordintate. I have seen a function ProcessVertices but I can not understand how it works.
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