Calculate velocity in screen space

[sipi]

How would you calculate the velocity of a moving geometry (consistent topology, static camera) in screen space?

[haggi]

You have a vex function called toNDC where you can use your velocity information to transform it to normalized raster coordinates. Even if it says it's only well defined in shading context it seems to work in sop as well.

[sipi]

15 minutes ago, haggi said:

You have a vex function called toNDC where you can use your velocity information to transform it to normalized raster coordinates. Even if it says it's only well defined in shading context it seems to work in sop as well.

yeah, i guess i have to do something with the NDC stuff, but not clear to me how

Edited June 5, 2025 by sipi

[Librarian]

@sipi maybe
 

//box ndc
vector offset = {0.5, 0.5, 0};
vector cameraScale = set(ch("camera_X"), ch("camera_Y"), 1);
if (@group_near)  @P = set(@P.x, @P.y, ch("near")*-1); 
if (!@group_near) @P = set(@P.x, @P.y, ch("far")*-1);  
vector xformpos = @P *  maketransform(0,0, offset,{0,0,0},cameraScale,{0,0,0}); 
@P = fromNDC(chs("camera_path"), xformpos); 

 

Edited June 5, 2025 by Librarian

[sipi]

This is what I could come up with. Uses 1 qLib node (Camera Plane ql).

screen_space_velocity.hiplc

Edited June 5, 2025 by sipi
typo

[sipi]

I think I have found the perfect solution. This seems 100% correct. Takes into account the camera motion as well.

screen_space_velocity_2.hiplc