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Motion vector calculation problem

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Hello.

I'm having problems with my motion vector calculations.

The camera is only very barely moving, but as you can see the motion vectors of the bottom left tile are calculated incorrectly when the camera gets too close. I believe the problem happens due to the projection ending up behind the camera (at least one vertex is behind the camera).

I calculate my motion vectors for each vertex like this:

gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4(position, 1.0);
vec4 previousPosition = previousViewProjectionMatrix * previousModelMatrix * vec4(position, 1.0);
motionVector = gl_Position.xy / gl_Position.w - previousPosition.xy / previousPosition.w;

I am unsure how to fix this. The problem seems to come from the motion vectors becoming extremely large, large enough to cause the whole triangle to receive motion blur after interpolation. Any help would be appreciated!

Edited by theagentd

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Thank you! Your solution worked flawlessly! Since I'm only targeting PC and the additional pixel shader math did not cause a measurable decrease in performance (both run at 140 FPS) I've decided to simply go with accurate motion vectors.

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@osmanb

you might reduce the issue by dividing by abs(w), as most of the problem arises due to the flip of x and y when you divide by something negative. it's still not correct, but far less noticeable and you get away with per-vertex cost.

Interesting. Never thought to try that, but I might give it a whirl. Thanks.

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@osmanb

you might reduce the issue by dividing by abs(w), as most of the problem arises due to the flip of x and y when you divide by something negative. it's still not correct, but far less noticeable and you get away with per-vertex cost.

I tried that after I posted my first post, but it didn't help. I also tried with -abs(w). It seems like the problem is that the motion vector becomes extremely large when a vertex lies very close to (linear) Z=0. Direction isn't really the main problem; it's the length of the vector. I clamp my motion vectors so my motion blur doesn't explode, but the motion vectors are several magnitudes too large even for unnoticable movement.

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theagentd, you're right, division by zero (or close to zero) will in most cases lead to bad results. maybe clamping to the near plane might give a reasonable result? something like

xy/max(near_plane,abs(w))

?

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