# where is this shader error from?

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I am trying to implement animation using vertex shader. And it ALMOST works. I mean I want my object to move smoothly with time. And I basically does this but there is some small artifact that I cannot track down. Here are the details: Every position is described by a 3x4 matrix - every vertex of an object must be multiplied by this matrix for this object to be in this position. I have 3 such matrices - one at each point of time. matrixA = {1,0,0,0, 0,1,0,0, 0,0,1,0}; at point in time 0 ms matrixB = {1,0,0,0, 0,1,0,1, 0,0,1,0}; at point in time 1000 ms matrixC = {1,0,0,0, 0,1,0,2, 0,0,1,0}; at point in time 2000 ms My app keeps track of current time and updates shader parameters accordingly. These 3 (uniform in Cg) parameters are: 2 matrices and a weight factor. The weight factor is used to linearly interpolate between two matrices, so it changes it's value from 0.0 to 1.0 during the flow of animation from 0 ms to 1000 ms, and again from 0.0 to 1.0 during animation from 1000 ms to 2000 ms. The 2 matrices send to a vertex shader are updated when 1000 ms have passed - so it is now blending between matrixB and matrixC. Such interpolation between matrices in a vertex shader guarantees that the motion is smooth. And it is smooth. Except for one small problem: at the point in time when matrices are updated (1000 ms) the animated object for a fraction of second takes the position of the next matrix - that is, at time 1000 ms the object is for a fraction of second at position implied by matrixC, then instantly comes back to a position close to matrixB (which is the right position at this point in time) and continues to move smoothly towards position implied by matrixC. Here is my shader code:
struct vertex_input
{
float4 position : POSITION;
};

struct vertex_output
{
float4 position : POSITION;
};

vertex_output vertex_program(vertex_input IN, uniform float4x4 model_view_proj, uniform float3x4 previous_matrix, uniform float3x4 next_matrix, uniform float weight)
{
vertex_output OUT;
float3 previous_position = mul( previous_matrix, IN.position );
float3 next_position = mul( next_matrix, IN.position );
float3 position = lerp(previous_position, next_position, weight);//here is the interpolation
OUT.position = mul(model_view_proj, float4(position, 1.0f));
return OUT;
}

I thought I pass in parameters with wrong values so I printed them out just before sending them to the shader. Here is a fragment of the output at this problematic moment around 1000 ms. previous_matrix : 1000 0100 0010 next_matrix : 1000 0101 0010 weight : 0.982 previous_matrix : 1000 0100 0010 next_matrix : 1000 0101 0010 weight : 0.999 previous_matrix : 1000 0101 0010 next_matrix : 1000 0102 0010 weight : 0.016 previous_matrix : 1000 0101 0010 next_matrix : 1000 0102 0010 weight : 0.034 You see it's correct!! Please, I want to hear that I did some very stupid mistake!

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A possibility is that it switched matrices but not weights. You should re-examine your code more thuroughly. Perhaps somehow it's using the last weight. It sounds like it's in where you switch the weight in relation to where you switch the matrices in your code.

I had a similar problem interpolating between alpha's in my particle engine. But finally figured it out, to bad I can't remember how :(

HTH,

Devin

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Hmm, I'm pretty sure I pass in the correct values cause I printed them every rendering loop just before sending them to the shader. So is this possible that the weight was not set despite of calling cgGLSetParameter1f() ?

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You could debug the shader with PIX to find out exactly what values are being passed, and better, used.

-Devin

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I can't use PIX since I use OpenGL. (I posted this in this forum cause Cg is like HLSL;) Hopefully Nvidia has some OpenGL shader debuging tool.
(But will it run on my ATi card?)

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