HLSL projection shader

Question

I am rendering a scene to a texture and then drawing a water plane with that texture using projection. Most of the samples I have seen on the web pass in a view/projection matrix to the vertex shader and transform the vertex, then in the pixel shader they do :

projection.x = input.projectionCoords.x / input.projectionCoords.w / 2.0 + 0.5;
projection.y = input.projectionCoords.y / input.projectionCoords.w / 2.0 + 0.5;

I have a shader that does the following and it works too but I don't know where I found this particular code or how it gives the same results with the above code. Here is the code I have

Vertex shader : (world is an identityMatrix, viewProj is my cameras combined viewProjection matrix):

output.position3D =  mul(float4(vsin.position,1.0), world).xyz;
output.position = mul(float4(output.position3D,1.0), viewProj);

output.reflectionTexCoord.x = 0.5 * (output.position.w + output.position.x);
output.reflectionTexCoord.y = 0.5 * (output.position.w - output.position.y);
output.reflectionTexCoord.z = output.position.w;

Pixel shader :

float2 projectedTexCoord = (input.reflectionTexCoord.xy / input.reflectionTexCoord.z);

What confuses me is the usage of 0.5 * (output.position.w + output.position.x) and 0.5 * (output.position.w - output.position.y). How does this have the same effect and what does the w component mean here?

Barış Uşaklı Barış Uşaklı · Accepted Answer · 2013-02-26T13:36:09

After a while I realized that it ends up being the same thing :

0.5 * (output.position.w + output.position.x);

0.5 * output.position.w + 0.5 * output.position.x

Then in the pixel shader :

(0.5 * output.position.w + 0.5 * output.position.x) / output.position.w

(0.5 * output.position.w)/output.position.w + (0.5 * output.position.x) / output.position.w

The first part becomes 0.5 :

0.5 + 0.5 * ( output.position.x / output.position.w)

This is equal to :

(output.position.x / output.position.w) / 2 + 0.5

I believe moving this calculation to the vertex shader is more efficient so I will just leave it there.

To completely move the calculation out of the shader the matrix can be calculated on the client side :

XMMATRIX v = XMLoadFloat4x4(&view);
XMMATRIX p = XMLoadFloat4x4(&projection);

XMMATRIX t(
    0.5f, 0.0f, 0.0f, 0.0f,
    0.0f, -0.5f, 0.0f, 0.0f,
    0.0f, 0.0f, 1.0f, 0.0f,
    0.5f, 0.5f, 0.0f, 1.0f);

XMMATRIX reflectionTransform = v * p * t;
XMStoreFloat4x4(&_reflectionTransform, XMMatrixTranspose(reflectionTransform));

Then all the vertex shader has to do is :

output.reflectionTexCoord = mul(float4(output.position3D,1.0), reflectionProjectionMatrix);

HLSL projection shader

1 Answers