heres code uniform sampler2D tex;
uniform sampler2D tex2;
uniform mat4 PrevMatrix;
varying vec2 vTexCoord;
void main(void)
{
vec4 color = texture2D(tex, vTexCoord);
vec4 zOverW = texture2D(tex2, vTexCoord);
// H is the viewport position at this pixel in the range -1 to 1.
vec4 H = vec4(vTexCoord.x * 2 - 1, (1 - vTexCoord.y) * 2 - 1, zOverW.g, 1);
// Transform by the view-projection inverse.
vec4 D = gl_ModelViewProjectionMatrixInverse * H;
// Divide by w to get the world position.
vec4 worldPos = D / vec4(D.w);
// Current viewport position
vec4 currentPos = H;
// Use the world position, and transform by the previous view-projection matrix.
vec4 previousPos = PrevMatrix * worldpos;
// Convert to nonhomogeneous points [-1,1] by dividing by w.
previousPos = previousPos / vec4(previousPos.w);
// Use this frame's position and last frame's to compute the pixel velocity.
vec2 velocity = vec2(currentPos.xy - previousPos.xy)/2.0;
//velocity = (velocity + 1.0 ) / 2.0;
gl_FragColor = vec4(velocity.x, velocity.y, color.g, 1.0);
}
