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Vertex shader value is not interpolated in pixel shader


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#1 Kiristu   Members   -  Reputation: 135

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Posted 16 July 2014 - 06:14 PM

Hi,

 

I'm developing a shader that will display parallax occlusion mapping and some other effects. To have it working I need to use normals and tangents.

 

My vertices contain normal information in the NORMAL semantic (am I using correct word here ?) and the tangents are stored in TEXCOORD1.

 

Since the result is not what I expect, I display the normals and tangent values as the output color.

 

When the normals are displayed, I get what I expect: a gradient between various colors representing the normal values.

But when I do exactly the same thing with tangent values, there is not gradient and each face has a constant color (even if faces have different colors).

So it seems like something prevents the interpolation to occur when the tangent values are used.

I send the given values to pixel shader via COLOR1 or TEXCOORD2 and get the same result. I've also tried some other TEXCOORDs and COLORs.

 

The code is something like (copied from my code but without many other things):

struct VS_OUTPUT_DIR
{
	float4 Position : POSITION;
	float2 TexCoord : TEXCOORD0;
	float4 Diffuse : COLOR0;
	float4 specular : COLOR1;
};


VS_OUTPUT_DIR vertexMain(in float4 vPosition  : POSITION,
						 in float3 vNormal    : NORMAL,
						 in float3 Tangent 	  : TEXCOORD1,
						 float2 texCoord      : TEXCOORD0,
						 float4 diffuse		  : COLOR0)
{
	VS_OUTPUT_DIR Output;
...

	Output.specular = float4(Tangent, 1);
	return Output;
}

float4 pixelMain( VS_OUTPUT_DIR IN) : COLOR
{
        ...
	return abs(IN.specular);
}

So any clue of what I could test or check would be very appreciated.

 

Note: I use Intel GPA tools to debug the shaders.

 

Thanks,

K.

 



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#2 megadan   GDNet+   -  Reputation: 742

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Posted 16 July 2014 - 07:06 PM

Are your tangents normalized? What color are you seeing? You could try debugging your C++ code where they are generated to verify the values are what you are expecting (perpendicular to normal, magnitude 1).There is also a semantic you can use specifically for tangents: TANGENT.



#3 Pink Horror   Members   -  Reputation: 1799

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Posted 16 July 2014 - 08:19 PM

How do you compute your tangents? My guess is that your triangles all have unique vertices, even when they are adjacent, and the flat shading is an accurate reflection of the situation. I would dump the buffers out to a file and inspect manually.

#4 megadan   GDNet+   -  Reputation: 742

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Posted 16 July 2014 - 09:42 PM

Yeah, if the tangents are pointing in the same direction in a triangle, the colors will be the same.



#5 Kiristu   Members   -  Reputation: 135

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Posted 17 July 2014 - 03:59 PM

The tangents are normalized.

The colors are those from the coordinates of the tangents ( (1,1,1) => pure white, (1,0,0) => pure red etc...).

I created a simple model and debugged the computation and it seems to be correct.

 

However, I noticed all the vertices of my simple model (4 vertices) are duplicated because the exporter (B3D from Blender) exports the model face by face, writing all their vertices each time. So my model is now 9 vertices.

 

As you suggested, the tangents computation will be the same for every vertex on a given face, giving this result.

 

Something I did not say is that I'm using the Irrlicht 3D engine and, if the normals are bound to NORMAL semantic, the tangents (for some reason) are put into TEXCOORD1 (not into TANGENT).

 

I guess this makes a difference because even if vertices are duplicated, the normal interpolation still occurs while tangent does not.

 

Well, finally I'm not sure the problem is really with the shader. Having this heavy vertex duplication seems to be the origin of it, and might be also a performance issue.

I would still be interested to (try to) understand why normals are interpolated and texcoord1 is not.

 

Thanks for your help.



#6 JohnnyCode   Members   -  Reputation: 664

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Posted 18 July 2014 - 07:30 PM

for a single triangle, tangent vectors of its vertcies are identical 3d vectors. And if normals on the triangle's vertcies are smooth normals, you would spot a gradient happen between binormal vectors, since binormals are cross product of a (smoothed) normal and tangent.

 

 

 

Since the result is not what I expect, I display the normals and tangent values as the output color.

 

When the normals are displayed, I get what I expect: a gradient between various colors representing the normal values.

But when I do exactly the same thing with tangent values, there is not gradient and each face has a constant color

In case you are not seeing a smooth lighting for your mesh, I would make sure

 normals are smooth normals (what I suspect is true in your case), not the face normals, and if problem prevails, put your attention to computation of the binormal vector.



#7 Kiristu   Members   -  Reputation: 135

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Posted 23 July 2014 - 07:45 AM

Sorry to UP that topic.

 

I just want to confirm that my problem was with the model itself and the exporter duplicating vertices. I removed duplicated vertices and tangents are correctly interpolated, and my problem is solved.

 

Thanks for your help.






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