   HLSL Billboarding question.
  Hello,I'm trying to understand the billboarding, I understand the principe : A quad that is always facing the camera.I have found an exemple using HLSL that I don't understand quite well :Here is the Shader code used to apply the billboarding :VS_SUNOUT SunVS(float4Pos: POSITION, float2Offset : TEXCOORD0, float2Tex : TEXCOORD1){VS_SUNOUTOutput;// do screen-aligned billboarding (see http://www.eisscholle.de/blog/?postid=7)Output.Position= mul(Pos, matWorldView);Output.Position.xy  += Offset;Output.Position= mul(Output.Position, matProj);Output.TexCoord= Tex;return Output;}And from the www.eisscholle.de this explanation : Let's recap what billboards are: screen aligned quads (we talk about spherical billboards here, axis aligned work similar). Now if we just could pull out a quad of the 4 points when they are already on-screen. We can! In eye-space everything but the projection is applied to the vertices. In OpenGL the x/y plane is parallel to the screen. So we could just apply our offsetting to x and y in eyespace to get our screen-aligned quad and do the projection afterwards. The code for this technique is below. All one have to do is to separate the ModelViewProjection transform into ModelView transform, offsetting and then projection.Now my questions :- How is it possible to make the Billboard Quad turn around axis just be adding an Offset value ?- Why wait to be in the "View space" to apply the Offset ? The View Space is a 3D space based on the camera position and other parameters isn't it ?- What's this offset ?Tx you !   