How do you effectively cut out triangles in Photoshop for a web design easily? Is the only way to use the polygonal lasso tool, which always gets me triangles of unequal sizes. I'm trying to achieve something like this:
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Personally speaking, I don't cut out. I create a triangle pattern which I apply to a rectangle. It's a lot easier.
To do this, create a new 6 x 3 (for example) image. Create guides around the bounds and one down the middle. Draw a triangle with the pen tool and make it the required colour. Then I save it as a pattern.
To repeat the graphic, I create a 3px high rectangle (to correspond the the 3, above, change if yours is higher) and apply the pattern using the pattern overlay layer style which repeats it over however wide the rectangle is.
Of course, if you want to, you can make this shape a selection afterwards and delete the selection from the image you want to 'cut'.
Use the Grid, Luke... Turn on the Photoshop grid using Cmd/Ctl-' and use that as your guide. You may have to adjust the grid spacing in Preferences to fit your needs, but that is the simplest way to ensure your triangles are equilateral, right triangles or isosceles (if that's what you need). There's no need to make more than one selection.
You can use hced's suggestion with path selected (second icon from the left in the control bar -- the one with the pen), then press Ctl/Cmd-Enter/Return to turn the path into a selection.
When you have the selections shape you need, delete, then move the selection to a different spot by dragging it with the Lasso tool, delete and repeat. The grid will help you maintain even spacing.
To build on Alan and Scott's answers, use those techniques using a vector mask to achieve your goal. It's a good practice to learn to be non-destructive: should you need to move your serrated edge you simply adjust the mask to increase or decrease the area required.
In this way you don't have to figure out how to 'cut out' your original design, you simply draw the edge you want and off you go.