Hello. I am creatin own engine from scratch in d3d9. Lookin on forum and google i found the way to create a ray and check if it intersects given area. It works. But i cannot do it the other way. To feed a D3DXVECTOR3 and retrieve XY on the screen. I found several "solutions" on the internet but damn they give wrong results.

So from what i've read i have to:

1) create matrix from projection * view

2) use the new matrix on given D3DXVECTOR3 containing world location

3) the screen x will be (vector.x + 1) * 0.5 * width

But something is clearly wrong because i see the output values. Is it wrong alghoritm or should i show my code?

Thanks in advance

Greetz

Thanks for such a quick reply. I am tryin to do basic RTS game so i will obviously need to select units (so create a ray to check if it selects some object under cursor) but i will also need to draw HP bar for example over unit's head which requires the 3d unit location to be converted to screen, to draw the 2D bar just above.

So to achieve first i created a white box with edges on (-60, -13, -60), (60, -10, 60) and this piece of code detects perfectly if the ray hit (the code is called every frame when i move cusor so i see if the "ray hit" text disappears or shows up). "s" is mouse xy:

        D3DVIEWPORT9 vp;
	d3ddev->GetViewport(&vp);

	D3DXMATRIX view;
	D3DXMATRIX proj;
	d3ddev->GetTransform(D3DTS_VIEW, &view);
	d3ddev->GetTransform(D3DTS_PROJECTION, &proj);

	float px = (((2.0f * s.x) / vp.Width) - 1.0f) / proj(0, 0);
	float py = (((-2.0f * s.y) / vp.Height) + 1.0f) / proj(1, 1);

	Ray ray;
	ray._origin = D3DXVECTOR3(0.0f, 0.0f, 0.0f);
	ray._direction = D3DXVECTOR3(px, py, 1.0f);
		
	D3DXMATRIX viewInverse;
	D3DXMatrixInverse(&viewInverse, 0, &(view));
	D3DXVec3TransformCoord(&ray._origin, &ray._origin, &viewInverse);
	D3DXVec3TransformNormal(&ray._direction, &ray._direction, &viewInverse);

	D3DXVec3Normalize(&ray._direction, &ray._direction);

	if (D3DXBoxBoundProbe(&D3DXVECTOR3(-60, -13, -60), &D3DXVECTOR3(60, -10, 60), &ray._origin, &ray._direction))
	{
		RECT rect = { 20, 400, 220, 450 };
		font.printf(rect, D3DCOLOR_XRGB(123, 123, 123), "RAY HIT");
	}

I assume it is enough to loop through all objects and check if D3DXBoxBoundProbe returns true with their collision box and this way i will select unit (is it the optimal way tho?).

I understand its all about multiplying matrices, but I have to admit that i dont fully understand few d3d things. If we are in 3D world, why are matrices 4x4 and not 3x3? Lookin at functions "D3DXVec3TransformNormal" and "D3DXVec3TransformCoord" i see they differ only by "w" value 0/1. What is the magical "w" (msdn doesnt help )?

Greetz

Edit: Yeah the matrix multiplication order was messed in 3d to screen :) I read also on the xyzw, i get everything now :) Thanks a lot

How about this? offset adjusts the world position forward or backward relative to the camera and is probably not necessary for what you need.

float4 WorldToScreen(const float4& v, 
    float offset,
    float4x4 &orientation,
    float4x4 &modelView,
    float4x4 &projection,
    float width, float height, float _near, float _far)
{
    
    float4 result = modelView * (orientation * v);
    result.z -= offset;
    result = projection * result;



    // Homogeneous divide
    const double rhw = 1 / result.w;

    return float4(
        (1 + result.x * rhw) * width / 2,
        (1 + result.y * rhw) * height / 2,  // or (1 - result.y * rhw) * height / 2,
        (result.z * rhw) * (_far - _near) + _near,
        rhw);
}