Centering visual elements on the canvas so that your designs aren't distractingly lopsided is an essential part of creating graphics. In Photoshop, there are multiple methods to center something to the canvas.
Centering a Layer
The easiest way to center the contents of a layer on the canvas is to use Photoshop's "Align Layers to Selection" function. Using the Layers pane, select the layer you want to center and ensure it isn't locked. Press "Ctrl-A" to select the entire canvas, press "V" to activate the Move tool and then click the "Align Vertical Centers" and "Align Horizontal Centers" buttons in the Options bar. The same commands are also available in the "Align Layers to Selection" sub-menu of the "Layers" menu.
Working With Multiple Layers
To reposition multiple layers simultaneously, select them in the Layers pane prior to centering them. If the layers aren't connected in any way, Photoshop repositions them individually based on their contents. If the layers are linked, selecting any one of them and then centering repositions all of them. Contrary to what you may expect, the repositioning is still based on each layer's individual contents, so their positions relative to each other change. The only way to center multiple layers to the canvas while maintaining their relative position is to group them together and then center the group.
Centering in Free Transform Mode
Before entering free transform mode, press "Ctrl-Alt-I" to open the Image Size window and make a note of the height and width of your canvas, if you don't know them already. While in free transform mode, ensure the reference point is still set to the center of the object you're transforming using the "Reference Point Location" control on the Options bar. Enter a value that is half the width of your canvas in the "X" field on the Options bar to center the object vertically; enter a value that is half the height of your canvas in the "Y" field on the Options bar to center the object horizontally.
Both methods of centering layers to the canvas work based on the entirety of the layers' contents, not just on what is visible. Because of this, if you are using a layer mask to hide part of a layer's contents, you may not get the results you want when centering that layer. The only solution to this issue is to apply the mask to the layer prior to centering it, deleting the hidden contents. If you think you may want to modify the masking in the future, duplicate the layer, apply the mask to the duplicate, center it, manually move the original layer until it perfectly overlaps the duplicate and then delete the duplicate.
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