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Motion Blur and Alpha Channel

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Zach Youngblood
Motion Blur and Alpha Channel
on Dec 11, 2013 at 6:10:02 pm

I am creating motion graphics in AE using vector AI files. When I apply motion blur within AE it looks fine. However, when I render the animation out with an alpha channel and place it on a different background, the motion blur is a solid gray color (see image, objects on the right side are moving). Not only that but all of the vector objects have an ultra thin pixelated line around them (which you can see on the left hand side on the scale). Any suggestions?



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Roland R. Kahlenberg
Re: Motion Blur and Alpha Channel
on Dec 11, 2013 at 7:21:05 pm

Try rendering with Straight Alpha Channels.

HTH
RoRK

Intensive mocha & AE Training in Singapore and Other Dangerous Locations

Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects CS6 ACE/ACI


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Zach Youngblood
Re: Motion Blur and Alpha Channel
on Dec 11, 2013 at 7:54:39 pm

Straight instead of Premultiplied in the Color settings?


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Darby Edelen
Re: Motion Blur and Alpha Channel
on Dec 11, 2013 at 10:58:34 pm

[Zach Youngblood] "Straight instead of Premultiplied in the Color settings?"

Yes, either render it with Straight Alpha or interpret the imported footage as Premultiplied.

Darby Edelen


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Roland R. Kahlenberg
Re: Motion Blur and Alpha Channel
on Dec 11, 2013 at 11:23:09 pm

[Darby Edelen] "Yes, either render it with Straight Alpha or interpret the imported footage as Premultiplied."

Hi Darby, I haven't used a Premult Alpha for about a decade. I didn't think that interpreting the Alpha correctly was a solution - wouldn't the pre-mult alpha contain some RGB pixels of the rendered comp?

Cheers
RoRK

Intensive mocha & AE Training in Singapore and Other Dangerous Locations

Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects CS6 ACE/ACI


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Darby Edelen
Re: Motion Blur and Alpha Channel
on Dec 12, 2013 at 8:05:22 pm

[Roland R. Kahlenberg] "I didn't think that interpreting the Alpha correctly was a solution - wouldn't the pre-mult alpha contain some RGB pixels of the rendered comp?"

You probably know this already, but in the interest of other readers I'll try to be comprehensive :)

A Straight and Premultiplied render will both have the exact same alpha. The difference is in the RGB.

If you render straight then the RGB channels extend beyond the edges of the alpha channel as they will be masked by the alpha in the composite.

If you render premultiplied then these 'extended' straight RGB channels are multiplied by the alpha. The fully opaque areas are multiplied by white so they look the same. The transparent areas are multiplied by black (or some other arbitrary color, but usually black) and the semi-transparent areas become a mix of black and the original RGB values.

If you interpret premultiplied footage correctly then AE will unpremultiply (divide) the RGB values by the alpha and you'll end up with the original RGB values, the same ones you'd get from a straight alpha, since division (or 'un-multiplication'... glad they didn't call it that in grade school) is the inverse of multiplication.

I'm of the opinion that neither straight or premultiplied is necessarily the better option, the workflow just needs to match the chosen method.

Darby Edelen


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Zach Youngblood
Re: Motion Blur and Alpha Channel
on Dec 12, 2013 at 3:35:23 pm

Success!!! Thank you!


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