How to draw precisely in photoshop

Question

I am very new in Photoshop. I drew one 400 px x 400 px square. Now I want to cut that from corner to corner so that I get two triangles. How can I do it? It must fit within that 400 px by 400 px. No gap can be there.

What I did was I took 400 px x400 px square. Then took pen tool to draw one triangle inside it. copy that layer and rotate to make another. But when repeat the process I can see there are gaps which means my method is wrong. I wish pen tool snaps with guides.

Please help. Thank you.

Answer 1

Go to Illustrator or other vector drawing program. In Photoshop you can draw plain paths with no stroke and area fill as you like, but filled areas and stroked paths get rendered to plusminus one pixel accuracy:

The image is only 16 x 16 pixels zoomed in. It was divided to 2 equal triangles by drawing a path as you tried. The path looked perfect (from corner to corner) but the filled area became as shown above. There's no anti-aliasing.

Drawing to the corner is possible only if one has so high zoom in that pixels become well visible. There's no snaps nor smart guides like in Illustrator.

With Anti-alias ON you'll get a half transparent zone along the tilted edge:

The inaccuracy and anti-aliasing make well visible gaps if you have only 400 x 400 pixels. You'll meet the same also in vector programs, because perfect shapes are also there rendered to pixels. But there you can easily add a stroke or make the shapes otherwise to overlap.

In Photoshop you can as a workaround have 2 layers per a square: the top layer contains a triangle and the bottom layer contains the full square. But that's not at all handy, because you cannot easily draw perfectly without snaps.

Answer 2

In Photoshop:

Draw the pieces in separate 400x400 pixels image. There you can well succeed to make a perfect triangle from corner to corner. Here it's made by making a polygonal lasso selection and filling the area. Polygonal lasso snaps to the guides which snapped to image edges.

It's a good idea to draw half of the triangles as squares. It makes the tilted seam watertight because there's no seam that anti-aliasing could make transparent.

Drag or paste whole 400x400 layers to your bigger image. The pieces snap perfectly if you have snaps ON.

Opposite to what's said in another answer: Photoshop CC has also smart guides if you turn them ON in View > Extras. But remember to turn also the snapping ON.