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# I want to align my bullet hole decal with the surface hit

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HI, this is my first post and would appreciate any help.

Here's a video of the problem

I am using the particle system from the "Particles3D" XNA example. Currently the texture is always aligned with the camera. I want it to be aligned with the surface hit.

In the video I have displayed the location and normal of each collision.

The sample particle system has a lot of redundant code that I don't need for the bullet holes. For example I don't need the particles to move, rotate, change size or color.

The vertex shader input contains a float3, Velocity, that I could presumably replace with the normal.

in the ParticleVertexShader the output.Position is calculated in ComputeParticlePosition.

 // Custom vertex shader animates particles entirely on the GPU. VertexShaderOutput ParticleVertexShader(VertexShaderInput input) { VertexShaderOutput output; // Compute the age of the particle. float age = CurrentTime - input.Time; // Apply a random factor to make different particles age at different rates. age *= 1 + input.Random.x * DurationRandomness; // Normalize the age into the range zero to one. float normalizedAge = saturate(age / Duration); // Compute the particle position, size, color, and rotation. output.Position = ComputeParticlePosition(input.Position, input.Velocity, age, normalizedAge); float size = ComputeParticleSize(input.Random.y, normalizedAge); float2x2 rotation = ComputeParticleRotation(input.Random.w, age); output.Position.xy += mul(input.Corner, rotation) * size * ViewportScale; output.Color = ComputeParticleColor(output.Position, input.Random.z, normalizedAge); output.TextureCoordinate = (input.Corner + 1) / 2; return output; } 

Inside ComputeParticlePosition the position is multiplied by the view and projection matrix.

 // Vertex shader helper for computing the position of a particle. float4 ComputeParticlePosition(float3 position, float3 velocity, float age, float normalizedAge) { float startVelocity = length(velocity); // Work out how fast the particle should be moving at the end of its life, // by applying a constant scaling factor to its starting velocity. float endVelocity = startVelocity * EndVelocity; // Our particles have constant acceleration, so given a starting velocity // S and ending velocity E, at time T their velocity should be S + (E-S)*T. // The particle position is the sum of this velocity over the range 0 to T. // To compute the position directly, we must integrate the velocity // equation. Integrating S + (E-S)*T for T produces S*T + (E-S)*T*T/2. float velocityIntegral = startVelocity * normalizedAge + (endVelocity - startVelocity) * normalizedAge * normalizedAge / 2; position += normalize(velocity) * velocityIntegral * Duration; // Apply the gravitational force. position += Gravity * age * normalizedAge; // Apply the camera view and projection transforms. return mul(mul(float4(position, 1), View), Projection); } 

So in summary, I don't need most of this code, I have only included it so you see what is currently happening. The input velocity is zero.

How do I transform the position so that it aligns with the normal?

Any help or pointers would be great.

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Here's a video of the problem fixed!

I ditched the idea of using a trianglelist based particle system. Instead I use a quad model circular array. I can manipulate the matrix up vector using the normal of the surface hit.

Thanks for looking.

BTW: I think I should have posted this in the XNA section, sorry about that, I didn't realise till after posting.

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