world object location to screen space transform?

[jeskeca]

How do I get the screen X/Y location for an object position?

I would like to put up HUD "target indicators" that are drawn in HUD_PROJECTION but are placed on the screen to fit over an object position. I don't simply want them billboarded, as I want them to be constant size in the view.

My current strategy was to compute the screen X/Y projection of an object position, and use this to set the X/Y position of a HUD_PROJECTION element. However, I have only limited understanding of how the rendering/camera transform works. I tried to mirror what I see in BrumeMesh ComputeScreenQuad/ComputePoint to compute a screen-space X/Y, but either that's the wrong thing to do, or I'm doing something wrong.

I considered using billboarding and figuring out how to set the size of the billboard based on the object position so the billboard would stay constant-size in screen-space. I would also like to know how to do this, as it will allow me to make target indicators that are constant size but properly occluded by world-objects.

[jeskeca]

Exciting, I was able to answer my first question by inverting the transform in the mouse ray-casting code.

This code seems to transform obj's position into screen/HUD coordinates...

Code: Select all

// compute camera coordinate for model
BrumeMatrix camLocal = brume.ActiveCamera.ViewMatrix *
             brume.ActiveCamera.ComputeProjection(BrumeRenderParams.PERSPECTIVE_DEFAULT);
BrumeVector newPos = BrumeVector.Transform(obj.Pos, camLocal);

// newPos is in -1 < x < 1 viewport space, convert to HUD coordinates,
// casting to an int removes the sub-pixel precision which we don't want for HUD textures
this.Pos.X = (float)  (int)(newPos.X * brume.BrumeWidth/2);
this.Pos.Y = (float) (int)(newPos.Y * brume.BrumeHeight/2)


Now I've been thinking about the "screen-sized" billboards, and it seems like it would be better to do that by handling it in the rendering code size transform than by calculating the necessary size to make the distant object translate to a fixed screen-size. Am i right in thinking this?

[Silmaryls]

Well I think it would be nice to have that feature into rendering code for ease of use.
But as the feature will not be available soon you can start with computing the size for the moment.