View Full Version : How to do a speed pass?

 CGOrange06 June 2009, 04:20 PMHey, I need to do a pass that shows the speed of an object. I tried doing a motion vector or a velocity pass but both seem to show the direction of the object in the color but I just need the speed of it so I dont want the color to change when the object changes direction. any help on this would be amazing. thanks in advance
Hezza
06 June 2009, 04:33 PM
Not sure if it'd work for what you want, but the red/green vector pass that gives you speed and direction, you could maybe split out the two channels, de-saturate them and recombine? Or maybe just take their luminence.....

What you planning to use it for?

CGOrange
06 June 2009, 04:46 PM
There seems to be yellow in the pass too, more like a yellow, green, red.. I think there are 3 channels.

The compositer wants a pass to use for making a kind of energy that is on the wings. But yeah, I need more of a 2 color pass then a 3. and for instance when the wing is going up the tip is red and when it goes down the tip is green... so it's getting a direction somewhere. I was wondering if there was a shader or something that just did the speed and not the direction. am I doing this totally wrong?

chippey
06 June 2009, 06:47 PM
There seems to be yellow in the pass too, more like a yellow, green, red.. I think there are 3 channels.

The compositer wants a pass to use for making a kind of energy that is on the wings. But yeah, I need more of a 2 color pass then a 3. and for instance when the wing is going up the tip is red and when it goes down the tip is green... so it's getting a direction somewhere. I was wondering if there was a shader or something that just did the speed and not the direction. am I doing this totally wrong?

Could your compositor not just take the magnitude of the motion vector pass? i.e. sqrt(red*red+green*green)

CGOrange
06 June 2009, 07:03 PM
I'm not sure, thats beyond my compositing knowledge.. but anyone know if there is a tutorial for what you are saying?

Her is an image of the up and down of the wing.. notice how the tip color changes.

http://www.cgorange.com/files/velocity.jpg

Sincrol
06 June 2009, 11:08 PM
well.... when i render a normal2d pass motion vector in Maya(toxic one...), it works absolutely PERFECT with Fusion's Motion Vector Blur node....

i dont need any other app or Plugin. no realSmart stuff or anything else.

Sincrol
06 June 2009, 11:12 PM
I'm not sure, thats beyond my compositing knowledge.. but anyone know if there is a tutorial for what you are saying?

Her is an image of the up and down of the wing.. notice how the tip color changes.

http://www.cgorange.com/files/velocity.jpg

that top thing is absolutely normal... the far the movement is from the pivot of motion, the more Motion distance it will have... so, it will have more motion blur, in that way, the color is more saturated. :)

the only thing you need to know is what exactly ur composing app needs... if it needs Y to Green... then make it green... if is blue, then... well, blue... :P

anyway, its a good tip to render the motion pass as 32bitEXR file... in my opinion, thats the best metod...

CGOrange
06 June 2009, 12:19 AM
Yeah umm, I know that it is a good motion vector pass and it works great as a motion blur. That wasn't the question.

Anyway. I brought it in ae and made the mode luminance and put a black solid under it. And it looks great. I did a track matte using the alpha but it has an outline. I'm guessing that it's because it was rendered with premiltiply.

Thanks hezz for the luminance tip.

Problem solved. Thanks for the posts

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06 June 2009, 12:19 AM
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