How to Flatten an Image in Adobe Photoshop CS6
When you flatten layers in Adobe Photoshop Creative Suite 6, the program combines all visible layers into a background. Photoshop deletes hidden layers and fills any transparent areas with white. Flattening is usually reserved for when you’re completely finished editing your image and is a good way to make your file size smaller.
To flatten a Photoshop CS6 image, follow these steps:
Ensure that all the layers you want to retain are visible.
Photoshop CS6 discards all hidden layers.
Choose Layer→Flatten Image or select Flatten Image from the Layers panel menu.
The transparent areas of your flattened image are filled with the background color and appear as a background layer in the Layers panel.
Photoshop CS6 doesn’t prompt you with a warning like, “Are you sure you really want to do this?” But if you mistakenly flatten your image, you can undo the command immediately by choosing Edit→Undo. If you go ahead and perform another action, then undo your mistake by using the History panel. However, if the flattening step is no longer in the History panel, there’s no way to undo the flattening.