# Imposter Lighting

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Hi,

I am trying to implement an imposter lighting scheme where I record a texture atlas of my model taken at various Y axis rotation angles (to pre-calculate  a set of textures I can render as imposters, lerping between them). I have a system where I write a second texture atlas containing the model normals instead of the texture values, as a kind of deferred rendering process in my pipeline.

Aside from having some trouble using my low grade maths skills to rotate the normal stored in the appropriate pixel, I get some reasonable results; i.e. the lighting on the 3D model somewhat smoothly interpolates into the imposter, which lights itself using the normals stored in the normals texture atlas.

I am having one issue though and am looking for help. In my Imposter VS I pass in a radian angle of rotation which matches the angle I will use in my Model matrix when rendering the full 3D model. I use this to select the appropriate texture and normal pixels from my texture atlas. This works OK. In order for my Normal to work I need to rotate my normal in the PS from the pre-baked model normal value rotated via the Y-axis rotation value (Y is UP in my world) - this should then present the same value for the normal as though I'd read it via more standard means through the 3D model VS input structure rotated using a Model matrix.

My code fragments are;

imposter VS;

// Matrix def from http://gamedev.stackexchange.com/questions/103002/how-to-rotate-a-3d-instance-using-an-hlsl-shader
output.ModelRotation =
float3x3(
cos(modelRotation), 0.0f, -sin(modelRotation),
0.0f, 1.0f, 0.0f,
sin(modelRotation), 0.0f, cos(modelRotation));



imposter PS;

// LERP between my two possible pre-baked normal textures. These are stored in model space, not tangent space.
float3 normalSample =
((tex2D(TextureSampler1, ps_input.TextureCoordinate0.xy) * ps_input.TextureCoordinate0.z) +
(tex2D(TextureSampler1, ps_input.TextureCoordinate1.xy) * ps_input.TextureCoordinate1.z)).rgb;

// Correct them into -1->1 range.
float3 normal = (2 * (normalSample)) - 1.0f;

// The normal is in model space. We need to apply the specific model rotation on top of it, for this particular instance of the imposter
normal = normalize(mul(normal, ps_input.ModelRotation)); // Rotate


I then use the standard lighting calculation I use elsewhere to light the pixel using the normal.

My problem is that I think my Y-axis rotation matrix is incorrect, but I struggle with the row-vs-column ordering concepts in HLSL vs. DirectX, so cannot easily verify the matrix has the correct effect on the Normal via a C# unit test. If anyone can guide me to the correct method of constructing that matrix, I'd be grateful.

Any other comments on the basic methods also gratefully received.

Phillip

Edited by PhillipHamlyn

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