Adobe Photoshop CC comes with a lot of custom shapes, but triangles aren't one of them. With a few clicks, you can quickly create any triangle yourself from a basic square or rectangle, including right-angle, isosceles and a perfect equilateral triangle that would make your high school geometry teacher proud. Because shapes are vectorized images, you can scale your triangle to a billboard size if needed. If you rasterize your rectangle first, creating a triangle is even easier, though it won't scale quite as well.
Add a new layer to a Photoshop canvas. Select the "Rectangle Tool" from the Toolbox. Drag the cursor across the canvas. To form a square so your triangle will have two sides of the same length, hold down the "Shift" key while dragging the cursor. To change the shape's color, click the "Fill" button in the Options bar.
Click and hold the bottom corner of the "Pen Tool" in the Toolbox and select the "Delete Anchor Point Tool."
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Click any of the "Anchor Points" on the corner of the square. The square is instantly transformed into a right-angle triangle.
Click the "Move Tool" in the Toolbox, then select "Free Transform" from the Edit menu. Rotate the triangle to any position by putting the cursor just outside a corner until the cursor looks like a curved double-headed arrow. If you hold down the "Shift" key while rotating, you can easily rotate the triangle so the bottom is parallel to the bottom of the canvas.
Select the "Move Tool" from the Toolbox. Resize the triangle by dragging any corner. Once again, hold down the "Shift" key while dragging to lock the triangle's aspect ratio.
Draw a square using the Rectangle Tool, while making sure the sides are a dimension that is easy to work with. Because measuring will be involved, a square with 10-inch sides is much easier than a square with 9.654-inch sides. Transform the square into right-angle triangle and then rotate it so the longest side is perfectly horizontal.
Make Photoshop's rulers visible by selecting "Rulers" from the View menu. Move the cursor to the small square that joins the two rulers in the upper-right corner and then drag it to the top corner of the rectangle. This centers the ruler on that corner.
Select the "Add Anchor Point Tool" from the Toolbox. Click two points of the triangle's base that will create the size you need. For example, to create an equilateral triangle when the top two sides are 10 inches long, add anchor points at the bottom that are each 5 inches from the center. As you move the cursor a line appears on the ruler, indicating its exact position.
Select the "Convert Point Tool" from the Toolbox and click each anchor point you just added. The anchors you just created make smooth lines. The Convert Point Tool changes them to straight line anchors.
Select the "Delete Anchor Point Tool" from the Toolbox. Click each "Anchor Point" on each of the triangle's bottom corners to remove them. The result is a perfect equilateral triangle.
Resize or scale the triangle as needed by selecting the "Move Tool" from the Toolbox and selecting "Free Transform" from the Edit menu.
Draw a square on the canvas using the Rectangle Tool, as you would for a right-angle triangle. Select "Rasterize" from the Edit menu and select "Shape." This transforms the vectorized shape into pixels, which are much easier to edit.
Select "Transform" from the Edit menu and select "Rotate." Drag a corner while holding down the "Shift" key and rotate the square so it's at a 45-degree angle, resembling a diamond. The angle appears near your cursor while you rotate the shape.
Select the "Rectangular Marquee Tool" from the Toolbox. Drag the cursor across the bottom half of the diamond and press "Delete." The shape becomes a right-angle triangle.
Select "Transform" from the Edit menu and click "Scale." Drag any anchor point to scale the triangle to the shape you need.