Save Selection in Another Document in Photoshop
I have already presented quick and cool tip on how to manually move selection from one document to another in Photoshop. Today I will present you even cooler, faster and easier tip for the same job. Photoshop has ability to save the active selection in any open document, don't you know? There is also option and to create a new document using active selection, just to inform you about and just in case. And there is one small limitation: document dimensions must match.
Steps to save the active selection in any open documentYou can save (move) selection in any open document, regardless of the color mode and bit depth with requirement that both documents have the same dimensions in pixels. Resolution, usually px/in does not play any role, only dimensions must match.
Step 1 Create selection
Selection in Photoshop can be created in many different ways and I will skip that part. Note: selection must be active or loaded, not necessarily and visible. You can press Ctrl/Cmd + H to hide marching ants. Also keep in mind that painting in Quick Mask mode is not selection unless you press the letter Q one more time to load selection.
Step 2 Save selection and choose in which open document
As mentioned above you can save the active selection in any open document with the requirement that the dimensions are exactly the same. I mean pixel dimensions which you can see in Image > Image Size, by clicking on Status Bar or from the Info panel.
So with the selection active go to Select > Save Selection. Under Destination you will see Document: drop down list. Click to expand list and choose a document name (document name won't be listed if the dimensions are not the same). You can give a name (text field on the right side of Name:) to the selection or to leave blank field in which case it will get a default Alpha name followed by number.
Another option is to use Channel drop down list for more complex things. This option requires that your document in which you want to save (move selection) have already created an Alpha channel in which case it will be listed in the Channel drop down list. After choosing the name of the existing channel in some other document you can use check-boxes to instruct Photoshop what to do: Replace Channel, Add to Channel, Subtract from Channel or to Intersect with Channel.
Step 3 How to ensure that documents have the same pixel dimensions
One of the ways is to create a document with exact same dimensions from File > New > Document Type > choose the name of the existing open document from the drop down list.
Another way is to use the Crop Tool and option: Front Image from the drop down list in the Options bar (Crop Tool must be active). This option is picking up dimensions and resolution of the image in the current tab/window and populating fields in the Options bar. With dimensions picked up from an existing image you need to switch to another tab with the image and to use the Crop Tool which will crop that image to exact same dimensions.
You can ignore Resolution field if you are working for the web, by the way. Resolution set for the image which will be displayed on the web will be simply ignored and there is no point to open discussion on that. Open two images with exact same dimensions in Photoshop. Go to Image > Image Size, uncheck Resample option and change Resolution for one of the images to 10 000 for example. Upload both images on Facebook, Twitter or anywhere else. Do you see any difference? No, there isn't any difference because field with information about resolution will be lost in space and ignored.
Product used in this tutorial:
Adobe Photoshop CC 2015
Photoshop Help / Saving selections and alpha channel masks
Photoshop Help / Crop and straighten photos