Alpha Masks - please help

[lpbh25]

I keep reading about alpha masks and alpha channels and how they can help with cutting out images, without making cut-out results pixellated. Why do we need alpha channels and masks? How do I cut out images using alpha masks and channels - can someone help explain this to me? Thanks.

[Kemaru]

Hi, I'll write you a small tutorial on Alpha Mask Import and how to use masks.

Alpha Mask plugin.

A mask is a grayscale image that is used to modify the transparency of an image. Depending on the settings, full black can either be completely transparent (Alpha is 0) or opaque (Alpha is 255, or 100% in some software).

Paint.NET doesn't have Layer Masks the same way PhotoShop does. Fortunately, Alpha Mask Import works almost identically except for some minor differences.

The Alpha Mask Import plugin lets you use an image on your clipboard as a mask. Your mask should be grayscale with no transparency. The plugin also works if you have black on a transparent background. Oddly, though, the plugin doesn't (or at least it didn't the last time I tried) recognise white objects on a transparent background.

So, let's say you want to cut out this heart shape (made with the font tool by pressing ALT+3 with num lock):

From this pattern:

Click on the Alpha mask effect in the Effects menu. Remember that you have to have the mask on your clipboard. (CTRL+C, or click Copy)

If you check the Invert Mask checkbox, the mask will invert(black>>white, white>>black). This changes the part which is cut from the image.

In the above image, you can see that after applying the mask to the seamless pattern, the seamless pattern is only visible in the area in which the mask was black. So, the white part of the mask removed what was "underneath" it. What's left is a transparent background.

Inversely, what was underneath the black has been extracted in this image.

So in a nutshell, the mask is like a stencil that hides the parts of the layer you don't want show, removing them from the layer at the same time.

I hope these images and my (poor)explanation helped you.

Edited December 9, 2010 by Kemaru