Cheap method for procedural vegetation animation

SpeedTree has all kinds of great ways to animate trees, but if you need a very, very cheap (in performance terms) solution without having to mark up the assets in any way, I used a few tricks back in 2003 on the game Corvette 50th Anniversary.  Back then it was vertex shader 1.0 in straight assembly (no hlsl) for Xbox and VU code on PlayStation 2.

The technique is essentially to do a little bit of morph target blending, but to create the target on the fly, and subtly change it every frame.  You also vary how the blend itself is performed every frame.  So you just need to pass a matrix and a blend vector to your shader.

 float3 localSpacePosition = In.pos;
 float3 targetPosition = mul(localSpacePosition, g_morphMatrix);
 float3 blendedPosition = lerp(localSpacePosition, targetPosition, g_morphBlend.xyz);
 worldSpaceVertex = mul(float4(blendedPosition, 1), matWorld);

So obviously this is very, very cheap in shader terms.  The magic is in how you construct your input parameters.

The morph matrix is an orthogonal matrix that will rotate an object around it’s up axis.  The up vector of that matrix is constructed with some tweakables, like this:

float3 up = normalize(float3(0.14, 4.0 + sin(vortexRate * time) * vortexScale), 0.14));

The trick comes from the fact if you normalize a non-unit vector the largest component is dominant – so by tweaking the Y value over time you can get a kind of ‘eccentric’ rotation that is still mostly around float3(0,1,0) but your eye won’t recognize this as an obvious linear rotation.  By tweaking x & z you can set a general ‘wind direction’.

The lateral and direction vectors of the matrix are formed just by advancing time around 2PI, you likely have an engine function to make this kind of matrix already:

g_morphMatrix.setFromAxisRotation(up, time * 2PI);

The blend vector controlling the morph is another tweakable, this is the other half of the trick because you can scale this vector so the morph is applied differently on each X Y Z axis.  This is really how you can tweak it to look natural – by applying more morph on the X & Z axes than Y vertices on branches that are further from the trunk will move more than the trunk itself.  So you can very cheaply simulate wind & gravity on these extremities while the trunk and inner pieces have a much softer sway.  Applying further damped sine waves to X & Z over time can simulate wind gusts, or even air drafts caused by objects flying by.

So, by setting up the morph target and the blend to suit, very cheap natural look motion can be achieved.