View Full Version : Using Premultiplied Images and unpremultiplying them

12 December 2006, 12:25 PM
Is there any way to un-premultiply rendered images in Photoshop?

Most 3D applications write images where the rgb-image is premultiplied with the alpha channel. The result canīt be used in Photoshop, since photoshop wants a clean (not-premultiplied) edge and cuts it with the alpha on its own. The result is, that if the 3D element is rendered against black background and you put a white background behind the object in photoshop, you end up in dark edges around the object. Compositing tools like AfterEffects handle this with ease (they automatically unpremultiply).

So .. in formula, what 3D delivers (using finalrender, actually no straight mode available):

RGB(premultiplied) = RGB(rendered) * Alpha

What i need:

RGB(rendered) = RGB(premultiplied) / Alpha

quite easy formula, but how can this be done in Photoshop? Or is there a filter available that calculates this?

12 December 2006, 04:39 PM
Layer, Matting, Remove Black Matte.

To use this your layer has to have transparency, not just a mask. In other words you have to add your alpha as a layer mask, then apply the layer mask, then use Remove Black Matte.

12 December 2006, 05:02 PM
THANKS! .. i thought this would only pixel-shrink the transparency mask, but i compared with a straight AfterEffects export and the results match perfectly. great :)

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12 December 2006, 05:02 PM
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