Today’s Question: I suddenly have a problem [in Photoshop] that I haven’t experienced before when adding extra canvas to an image. When adding extra canvas to a flattened image I‘m getting the transparency chequered board instead of a pure white boarder. The ‘Canvas Extension Color’ at the bottom of the dialogue box is greyed-out (not active). I’m sure there is a very simple solution that I’m overlooking here.
Tim’s Answer: In a way this is two questions in one. Sometimes you might actually want transparent pixels for the additional canvas around an image, and sometimes you might want actual pixels to fill in that area, so it is helpful to understand how this option works. The key is the presence (or lack thereof) of a Background image layer.
If the image includes a Background image layer, the Canvas Size dialog (accessible by choosing Image > Canvas Size from the menu) will include the option to set a color to be used for the new canvas area that will be added to the image. The default option is to use the current background color (white by default) for that canvas area, but you can choose a different option from the “Canvas extension color” popup if you prefer.
If the image does not include a Background image layer, the canvas you add to the image will be transparent, and the “Canvas extension color” popup will be disabled.
This, of course, leads to the question of how to change an image layer into a Background image layer, and how to change a Background image layer to a “normal” image layer. You can make this change by first making sure the layer you want to convert is the active layer on the Layer panel. You can make a layer active by simply clicking on the thumbnail for the layer on the Layers panel.
To change the status of the active layer to or from a Background image layer, you can then first choose Layer > New from the menu. On the submenu that appears you can choose “Background from Layer” if the active layer is not currently a Background image layer, or “Layer from Background” if the layer is currently a Background image layer. These two options are actually one item on the menu, with the specific option shown dependent upon whether the current image contains a Background image layer.
In the case of the example cited in today’s question, the image layer (even though the result of flattening) is not a Background image layer. Therefore, you could choose Layer > New > Background from Layer to convert the layer to a Background image layer, so that you can set the color of the canvas area you add to the image. If you wanted to add transparent canvas area for a different image, you would simply want to make sure the Background image layer is converted to a “normal” layer by choosing Layer > New > Layer from Background from the menu.