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Member Since 24 Apr 2012
Offline Last Active Sep 09 2013 02:33 PM

Topics I've Started

Rotate A Quad Along Axis To Face Camera

23 January 2013 - 12:49 PM

I am having some success aligning a particle to a direction and then rotating it along its axis. At the moment I am just constantly rotating it. How do I calculate the correct rotation so it always faces the camera?
Please have a look at this video: Quad Slowly Rotating Along Axis
I am using DPSF. I have modified the "FlyingSparks" demo.
To calculate the orientation Quaternion I run this code:


private void UpdateOrientation(DefaultTextureQuadTextureCoordinatesParticle cparticle, float felapsedtimeinseconds)
   var normalized = cparticle.Velocity;
   cparticle.Orientation = Quaternion.Identity;
   cparticle.Right = -normalized;
   _rotation += 0.03f;
   cparticle.Orientation = Quaternion.CreateFromAxisAngle(normalized, _rotation) * cparticle.Orientation;


After some trial and error the above code produces what you see in the video.
Line 5: resets the orientation.
Line 6: aligns the quad to the velocity direction.
Line 7: increments the rotation.
Line 8: further rotates the quad along its axis by the rotation.
What I am asking is how do I calculate the rotation?
I am not a maths expert. I can guess that I need the normal of the particle, calculated within line 6 and then use that to determine it's difference from the camera direction. I have looked at some billboarding techniques but they don't offer exactly what I want.
I am very close to finishing my game and this is the last hurdle, any help or pointers would be greatly appreciated, thank you for taking the time to look at this smile.png

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

24 April 2012 - 07:58 AM

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.