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UV mirror issues with normalmap , using nonprecomputed tangent space calculating


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#1 Tunisia hawker   Members   -  Reputation: 160

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Posted 05 November 2012 - 06:24 AM

Hi,I'm trying to do normalmap using directx9.After reading 2.6 Normal Mapping without Pre-Computed Tangents by Christian Schueler ,I deside to use this.
[source lang="cpp"]float3x3 ComputeTangentFrame(float3 N,float3 p,float2 uv){ float3 dp1 = ddx(p); float3 dp2 = ddy(p); float2 duv1 = ddx(uv); float2 duv2 = ddy(uv); float3x3 M = float3x3(dp1,dp2,cross(dp1,dp2)); float2x3 inversetransposeM = float2x3(cross(M[1],M[2]),cross(M[2],M[0])); float3 T = mul(float2(duv1.x,duv2.x),inversetransposeM); float3 B = mul(float2(duv1.y,duv2.y),inversetransposeM); return float3x3(normalize(T),normalize(B),N);}[/source]
It seems good and simple,but soon I get punished at little seams with UV mirrors.The whole model looks right under light,but at those seams,normal gets wrong and light reflect in unproper way,so a sudden change or miss lighting on surface appears.I think this is because when current pixel is shared by different triangles with different directions of UV,the ddx and ddy instruction will get wrong answers.
So,I use
[source lang="cpp"]float r = (dot(cross(T,B),N)<0.0f) ? -1.0f : 1.0f;[/source]
to judge whether UV was mirrored.Then I realized I can't change tangent or binormal or normal,for most part of the mesh already seems right.On the other hand,I cannot know whether it's U mirror or V mirror.It's hard to tell the right direction of two axes(tangent and binormal) when you only know one axes(Normal).The author do not give futher information.
Btw,because I'm using dx9,so I can't split vertexes in shader.
So I want to know is there a possible way to solve this problem?Thanks for any reply :)

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#2 Ashaman73   Crossbones+   -  Reputation: 7416

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Posted 05 November 2012 - 06:41 AM

The problem at mirrored seams is, that the tangentspace changes the handness at the mirrored faces (=> dot(cross(T,B),N)<0.0f is a way to determine this). When using shared vertices you only save one tangent space, so one face will use the wrong one often resulting in inverted lighting. The solution is quite simple, just duplicate the vertices when you detect a change in the handness of the tanget space much like you would duplicate a vertex when the uv coords change.

Edited by Ashaman73, 05 November 2012 - 06:44 AM.


#3 Tunisia hawker   Members   -  Reputation: 160

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Posted 13 November 2012 - 09:59 PM

Thanks for replying my problem.
Things are not the way I thought.The problem is not about UV seams at all,but the UV directions.The truth is I was using directx sampling texture directions to calculate my Binormal,that is having a V direction from top to down.But I haven't check my resources.They are bulit in Maya,which have a down-to-top V direction.So it's OK with color and fails correct normal mapping.Without my artist,I could never know this.Posted Image




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