How to position a textured quad in screen coordinates? - slimdx

Question

I am experimenting with different matrices, studying their effect on a textured quad. So far I have implemented Scaling, Rotation, and Translation matrices fairly easily - by using the following method against my position vectors:
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for(int a=0;a<noOfVertices;a++)
{
myVectorPositions[a] = SlimDX.Vector3.TransformCoordinate(myVectorPositions[a],myPerspectiveMatrix);
}
However, I what I want to do is be able to position my vectors using world-space coordinates, not object-space.
At the moment my position vectors are declared thusly:
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myVectorPositions[0] = new Vector3(-0.1f, 0.1f, 0.5f);
myVectorPositions[1] = new Vector3(0.1f, 0.1f, 0.5f);
myVectorPositions[2] = new Vector3(-0.1f, -0.1f, 0.5f);
myVectorPositions[3] = new Vector3(0.1f, -0.1f, 0.5f);
On the other hand (and as part of learning about matrices) I have read that I need to apply a matrix to get to screen coordinates. I've been looking through the SlimDX API docs and can't seem to pin down the one I should be using.
In any case, hopefully the above makes sense and what I am trying to achieve is clear. I'm aiming for a simple 1024 x 768 window as my application area, and want to position a my textured quad at 10,10. How do I go about this? Most confused right now.

Answer 1

I am not familiar with slimdx, but in native DirectX, if you want to draw a quad in screen coordinates, you should define the vertex format as Translated, that is you specify the screen coordinates directly instead of using D3D transform engine to transform your vertex. the vertex definition as below
#define SCREEN_SPACE_FVF (D3DFVF_XYZRHW | D3DFVF_DIFFUSE)
and you can define your vertex like this
ScreenVertex Vertices[] =
{
// Triangle 1
{ 150.0f, 150.0f, 0, 1.0f, 0xffff0000, }, // x, y, z, rhw, color
{ 350.0f, 150.0f, 0, 1.0f, 0xff00ff00, },
{ 350.0f, 350.0f, 0, 1.0f, 0xff00ffff, },
// Triangle 2
{ 150.0f, 150.0f, 0, 1.0f, 0xffff0000, },
{ 350.0f, 350.0f, 0, 1.0f, 0xff00ffff, },
{ 150.0f, 350.0f, 0, 1.0f, 0xff00ffff, },
};

Answer 2

By default screen space in 3d systems is from -1 to 1 (where -1,-1 is bottom left corner and 1,1 top right).
To convert those unit to pixel values, you need to convert pixel values into this space. So for example pixel 10,30 on a screen of 1024*768 is:
position.x = 10.0f * (1.0f / 1024.0f); // maps to 0/1
position.x *= 2.0f; //maps to 0/2
position.x -= 1.0f; // Maps to -1/1
Now for y you do
position.y = 30.0f * (1.0f / 768.0f); // maps to 0/1
position.y = 1.0f - position.y; //Inverts y
position.y *= 2.0f; //maps to 0/2
position.y -= 1.0f; // Maps to -1/1
Also if you want to apply transforms to your quads, It is better to send the transformation to the shader (and do the vector transformation in the vertex shader), rather than doing the multiplications on the vertices, since you will not need to update your vertexbuffer every time.