Vector blur node produces white artifact from alpha image during movement.

Hi,

So as you can see in my image, I’m trying to composite an image of the famous “Kurnan Noiskar” from Star Trek the next generation, in a shot to make it look like Picard is pushing it off the table.

I’m moving the Kurnan Noiskar by using the x and y info from a track with a transform node, which works nicely.
However I tried using the x-value of the track for the “speed” input in a vector node, on the Kurnan Noiskar as well as its alpha image (a seperate image). This creates a nasty artifact on the blurred alpha, which I don’t really understand. You can see it in the attached image (indicated by red arrows).

Any help would be appreciated!

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Have you tried vectorblurring the fg element image and alphaovering it with BG footage? What is the reasoning behind transforming and blurring the element and its mask separately and mixing it with the BG?

The fg element seems to already have alpha channel (checker background visible in thumbnail), so why do you need the mask from separate source? And if you do need the mask from separate source, why not push it into the fg element pipe with Set Alpha node and then transforming and blurring?

Yeah, you’re definitely going to need to blur the image … to do whatever it is that you want to do with it … before you alpha-over it into the scene. The blurred output itself, after being converted to grayscale and adjusted in contrast etc. as needs be, must be the source of the alpha-mask.

(You can also do other things to the mask, for additional visual interest. Such as, say, modulating it with a fat, pudgy, random-noise texture, whose influence is constrained by an appropriate curves-node so that you can maintain control.)

Thanks for the response! I used After Effects for the effect (see the result here: https://www.youtube.com/watch?v=jvL7EBAkFz0)

I have tried to vectorblur the FG image before an alpha-over node, but that gives the same result. I wanted to seperate the elements to be able to find the problem, to no avail.

I’d still like to know how to do this in Blender, it’s a real head-scratcher. (And so easy in After Effects, haha)

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Upload your blend file (and pack textures), it is hard to say what is wrong without seeing the material.

The problem lies with the way you are using vector blur…

the speed input, requires a RGBA, based off the following calculations

“The image (RGBA) is based on screen space pixel movement of vertices… R being the x direction from current frame to previous frame, G being y direction from current frame to next frame, B being x direction from current frame to next frame, A being y direction from current frame to next frame.”

from here – http://www.blenderartists.org/forum/showthread.php?265887-Functionality-expansi-Custom-motion-vector-export-capability-willing-to-pay-donate&highlight=

Right now, you are using the X movement, as the previous movement, after movement, in both axis.

This isnt going to work at all.

You need to update to 2.77, it will expose the speed vector properlly on the track position node… this will give you a much more accurate speed vector to use for your vector blur.

One other thing, when you go for final render, instead of using nearest you shoudl be using bicubic or bilinear