Can Apple's Preview app be used to reduce the opacity of an existing image?

I might be asking this wrong. To be clear, I've Googled it extensively but Google seems to think what I want is to remove a solid background from around my image, for example making a the black background/backdrop transparent in an image that's of a car. However, my background is already transparent and what I want is for the car to be at about 90% Opacity.

  • Which car? We can't see your picture. – Mark Setchell Aug 31 '16 at 21:58
  • The transparent one. – dan Aug 31 '16 at 22:54

Preview can't do this.

ImageMagick can, and nearly anything you can imagine.

You can easily install it with MacPorts (see MacPorts Home):

port install imagemagick

Let's say your original image is porsche-911.png: Porsche 911

Enter in Terminal the following command:

convert porsche-911.png -alpha set -channel A -evaluate set 50% porsche-911-50%.png

which will compute from your original input porsche-911.png the output porsche-911-50%.png:

Porsche 911 50% transparent

If you want to learn more about this great free tool, read:

ImageMagick home.

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