Tutorial: Selections and Masks -- Basic Concepts

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photoken
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Tutorial: Selections and Masks -- Basic Concepts

Post by photoken » Wed 03 Dec 2014 10:38

The PL Help file says that a lasso and its mask "only describe different points of view of the same thing." Here's a simple exercise that will illustrate the relationship between the lasso and its mask.

In a nutshell, the selection's mask is a raster image whose pixels can represent any value between fully opaque and fully transparent. The selection's lasso is a vector object which is drawn at the halfway point between the fully opaque pixels and the fully transparent ones. This lasso is what is shown as the selection marquee.

So, to start:
  1. Create an image 640px wide and 480px high, specifying a transparent background.
  2. Choose the Paint tool, set the foreground colour to Black, and set its brush to these values:
    ML brush settings.png
  3. Position your cursor as close to the middle of the image as you can, and click once. You'll get a nice, faded circle:
    ML painted circle.png
  4. Use Tool...Mask...Mask from Alpha to get this:
    ML mask from alpha.png
    Continued in next message...
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Last edited by photoken on Wed 03 Dec 2014 11:05, edited 1 time in total.
Ken
Yes, I think it can be eeeeeasily done....
Just take everything out on Highway 61.

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photoken
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Re: Tutorial: Selections and Masks -- Basic Concepts

Post by photoken » Wed 03 Dec 2014 10:53

...continued from previous message.
  • At the bottom of the Layers panel, use Mask...Create from Mask.
  • Drag that layer mask off the image layer and up to the top of the layer stack.
  • Right-click on the visibility icon of that former layer mask and choose "Show".
  • With that former layer mask still the active layer, use Tool...Color...Invert. You'll get an image that looks like this:
    ML mask inverted.png
    It is exactly the equivalent of the transparency of the original circle image, but now it's on a white background. This shows us what that Mask from Alpha command created -- a raster image that is essentially the same as the transparency of the original image.
Now we can see how the selection lasso (marquee) relates to the mask:
  1. Use Tool...Lasso...Redo Last Lasso to get back our original mask and marquee.
  2. Use Tool...Lasso...Convert Lasso to Layer to convert our selection to an independent vector object.
  3. Change the line colour of that newly created vector object to white, and you'll see that the original selection lasso (marquee) was created at a point exactly halfway between the fully opaque and the fully transparent pixels of the original image.
To really show all that in one awesome image :wink: , you can use the Rectangular selection tool to draw a rectangular mask over half the image of the circle on the white background, and use the Create from Mask command to mask off half of the white background circle we created:
ML final.png
The layers of this exercise wind up looking like this:
ML layers.png
Hope this helps to understand what's going on when selecting!
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Ken
Yes, I think it can be eeeeeasily done....
Just take everything out on Highway 61.