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Is there a way to adjust the transparency of an image in the compositor with a percentage of the alpha of the original image?

(So images that pixels that are completely transparent stay that way, and pixels that are partially transparent stay the same relative to the rest of the image)

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Yes by using math.

The alpha channel is a value from 0.0=transparent to 1.0=solid. A math node can be used to multiply alpha values by the value you want to use. Which you then feed into the alpha over Fac to combine the images. You can also use a CombineRGBA to join together depending on your needs.

While multiplying a zero value by 0.5 leaves it zero you can use math node to get values greater than 0.99 if you wish to only effect a non-transparent area.

You can also use less than to pass through unaltered values and add them together before the alpha over.

enter image description here

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  • $\begingroup$ What exactly is the application in which you want to setup you show in the screenshot? Why would I want all alpha values below 0.9 to be multiplied by 1 and all above 0.9 to be multiplied by 0.45-0.5? $\endgroup$
    – JulianHzg
    Commented Aug 10, 2013 at 10:24
  • $\begingroup$ @JulianHzg - normally you would only want to use a single multiply node to adjust the alpha value. I think it was the question specifying not touching transparent while adjusting semi-transparent that made me add the filtering. While I show an extreme case to show that you can control what you adjust, the only real use I know of for filtering only part of the alpha value is if you have a bad alpha channel. A single image is easy to fix but a sequence is easier to fix in composite. $\endgroup$
    – sambler
    Commented Aug 11, 2013 at 8:27

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