How to get previous position of a mesh

Question

I'm trying to write a shader for doing the motionBlur effect in WebGL using three.js frameworks. I'm trying to adapt this tutorial for WebGL: http://john-chapman-graphics.blogspot.fr/2013/01/per-object-motion-blur.html and getting the velocity value with GPUComputeRenderer.

But i have problem for this line :

Position = uModelViewProjectionMat * gl_Vertex;
vPrevPosition = uPrevModelViewProjectionMat * gl_Vertex;

How can i get or calculate the PrevModelViewProjectionMat ? I know it's projectionMatrice * modelViewMatrix * vec4(position, 1.0) but don't know how to get the previous projMatrix or previous modelViewMatrix. Can someone help?

Answer 1

The previous projection matrix is probably going to be the same unless you modified the camera (near,far,fov).

The previous modelView matrix you need to calculate or save from the previous frame. Then provide it to the shader as a uniform, and do the transformation.