How to separate image from alpha using Quartz Composer?

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Level 1

121 points

How to separate image from alpha using Quartz Composer?

Apologies in advance if this isn't the correct forum to pose questions about Quartz Composer.

I'm attempting to construct a Quartz Composer composition that will take an image containing transparency, and — within the structure of that composition — disect the image so that I can perform separate operations on the alpha mask and the full image (i.e. as if the mask did not exist). This would be akin to opening a PNG or TIFF in GraphicConverter, then accessing the images I want via Show Alpha Channel and Remove Alpha Channel.

I'm able to easily access the alpha channel by applying a Color Martix patch to the image, but I can't seem to figure out how to get to the full "unmasked" image.

Can anyone nudge me in the right direction as to how I could remove the alpha mask from an image?

Thanks!!

John

iMac and MacBook, Mac OS X (10.6.6), 20th Anniversary Mac, 128K Mac, original Apple II

Posted on Feb 22, 2012 4:15 PM

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Answer 1

Feb 22, 2012 6:15 PM in response to John Purlia

Can't answer your question but just wanted to let you know that there is an Apple mailing list for Quartz Composer

http://prod.lists.apple.com/archives/quartzcomposer-dev/2012/Feb/index.html

Might be worth posting there also.

Good luck

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Answer 2

N.P.

Level 1

15 points

Mar 7, 2012 5:14 AM in response to John Purlia

Hi John,

You could use the Core Image Filter to extract the Alpha channel as a mask, and then other filters to use that mask.

In the 'Settings' pane of 'Patch Inspector' for the 'Core Image Filter' place the following Core Image Kernel routine code:

/*

A Core Image kernel routine

Alpha to Mask White Opaque, Black Transparent

*/

kernel vec4 alphaToBlackWhite(sampler image)

vec4 pixelValue = sample(image, samplerCoord(image)); // Get a pixel

          unpremultiply(pixelValue);

          pixelValue.r = pixelValue.a; // Red channel

          pixelValue.g = pixelValue.a; // Green channel

          pixelValue.b = pixelValue.a; // Blue channel

           pixelValue.a = 1.0; // Alpha Channel make Opaque

          return premultiply(pixelValue);

Alternatively you could rewrite the above Kernel, more details on writing Core Image Kernels is in the developer documentation 'Image Tutorial: Writing Kernels'

Hope this helps,

Neil.

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Answer 3

John Purlia Author

Level 1

121 points

Mar 7, 2012 9:12 AM in response to N.P.

Thank you!!

I was not aware that you could put code directly into Quartz Composure. This opens up a whole range of possibilities. (Which is actually kind of scary because I retired as a software engineer two years ago to focus on art, but if I can inject my own code to manipulate images...)

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Answer 4

N.P.

Level 1

15 points

Mar 7, 2012 9:51 AM in response to John Purlia

I stumbled across your post today whilst searching for a solution to similar problem I had myself so glad it helped us both.

I wouldn't want to bring you out of retirement but, there are more patches that you can program...

In the 'Library' window from the popup menu – which by default is set to 'Library' – choose 'Program'.

This will filter the patches down to:

Core Image Filter (as mentioned above);
GLSL Shader;
JavaScript;
OpenCL Kernel.

In case you haven't found this, another useful patch is 'Composite' allows you to change standard blending filters from a popup menu, rather than dropping in a new patch and having to wire it up each time.

Have fun with your art.

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Answer 5

Sep 11, 2015 3:25 AM in response to N.P.

Hi Neil,

I've just found this while trying to make an alpha -> luma plugin for live video, and your code works a treat.

Thank you so much.

James

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Answer 6

Sep 12, 2015 11:24 PM in response to Basic Monkey

i am trying to trigger the itunes visualizer through the audio input on my Macbook. Either by accessing it's code in quartz composer or through a hotkey command. Can i do this? http://www.trainingintambaram.in/salesforce-training-in-chennai.html

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