First of all, let's talk about what an alpha is. Alphas are used in all kinds of computer graphics software and it's simply a grayscale image, a picture that uses a range of values from black to white. What makes a grayscale image an alpha instead of just an ordinary picture is how it's used. Different softwares can interpret these light and dark values as data rather than as simply a picture. An alpha could be used to define what parts of an image should be transparent; white could be opaque and black could be see-through and every shade of gray could be varying levels of opacity. Or an alpha could be used to define what parts of a model should be shiny and which ones should be dull.
As far as ZBrush goes, alphas are used to define what parts of a brushstroke should get the full effect and which parts should remain unaffected. So let's just test this out. I am going to open up the DemoRhino, and let's change its color to something lighter so it's easier to see. I also want to make it more dense, so let's go into Geometry and click Divide a few times so that we have more polygons to work with. I just want to zoom in a little bit so we can see everything up close. Now, let's go over to the Alpha button over here on the left side and this palette pops up.
This is the set of built-in alphas that comes with ZBrush. Just for demonstration, I'll pick Alpha 17 here. It's kind of a target-looking image. So if we stroke on the model, you can see that the strokes take on this kind of target shape to them. That's because the grayscale image is telling the brush to be more pronounced in the light areas and less so in the dark areas. Let's just undo this. So, alphas really show their potential when the Stroke Style is set to DragRect. Let's change this from Dots to DragRect.
Now we can place a single instance of this alpha on the model wherever we want. You can load in your own alphas that you have made in other programs like Photoshop through the Alpha palette. Just click on the Alpha button and down here at Import. I'm going to scroll down to ZBrush, and inside of this folder is a folder called ZAlphas. Let's grab that Leathery Skin image that we were looking at earlier. So with this opened and we've got DragRect, you can just click and drag. We'll get this kind of rough, organic, leathery texture.
Feel free to experiment with all the different alphas and the way that they can be combined with stroke styles to create all kinds of fun effects. You'll use alphas all the time to stamp on detailed textures and patterns that would be difficult to sculpt by hand.
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