Inspired by jfrazierjr's tutorial on masks in GIMP and decided that the few PS users around here might need a little clearing up and to clear away some of the misty confusion behind masks. I suggest you read through the above tutorial on the principals behind what a mask is and why it works. I don't feel I need to redefine what he's already explained.

But any of you PS users that have gone through a gimp tutorial with PS sooner or later got to a section where masks left you confused and frustrated. So if the principal of masks are the same, then why can you follow a tutorial for GIMP with PS and vice versa? Because the way the programs use the masks are a little bit different. How they are manipulated and so forth. Luckily and hopefully I'll try to clear some of it up and explain why these powerful little tools should be in every PS users arsenal for making maps. I don't know GIMP all that well, so if I make any references that are wrong, hopefully RobA or jfrazierjr will come along and clear that up.

Photoshop uses three different kinds of masks. Layer Masks (rasterized), Vector Masks (non-rasterized), and Clipping Masks. Outlined below:
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Since all I want to really do for this first lesson is give you a simple demonstration of what a mask is and what it does (or can do) This little mini-tut is directly ripped off from jfrazierjr's "hit and run" tut in Torq's thread about masks. So simple even I could understand it and how it could be powerful little tool. It was such a fine and simple demonstration, I can't help but present one in similar fashion.

What the mask does here is allows mask to let the layer below it blend in through it. giving a very unique and (if well done) great looking ground effect. Try it with multiple layers of differieng textures with some set to overlay, hardlight and multiply.

This exercise we'll be using plain and simple, just so you can really get a good look at what a mask does. I started out with a 3"x3" 150dpi RGB white background file. We go to Edit>Fill>Pattern and choose a nice rocky pattern, I went for a nice gray rocky area and not worrying about seamless textures at the moment. Then I make a new layer (SHIFT+CTRL+N) and then Edit>Fill>Pattern and choose a nice big green grass looking texture. Which immediately covers the rocky area I put in before.
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So.... Let's make them get together and blend, Make sure you have the Grass Layer highlighted and Either go to the menu and choose Layer>Layer Mask>Hide all
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or (highlighted in the first picture) hit the little Mask Icon at the bottom of the layer palette (square with a circle in the middle) with an ALT + Left Click (**Not CTRL Click, which will add a Vector Mask**) and all your grass disappears at the same time a big black thumbnail appears next to the grass thumbnail with a linked icon in between them:
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Now take a nice bristle brush like a stipple ( I like to space it out so its stlil touching, but there is some space between the bristles), Make sureyou click on the black mask thumbnail, set your palette color to WHITE and drop down the brush opacity to about 20% and start going arond the map and brushing that grass in until you get the look you want. I kinda exaggerated some things i this one below... but you get the jist of it. We'll be going into all kinds of things a little later on, including selections, channels and paths. and how powerful they all can be in a mapper arsenal. But work with blending in multiple texture levels for land (AND water!!!) effects...<ITS FUN!!!!>

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Stay tuned for more about masks