How to change background colour of an image in GIMP after increasing canvas size

Question

I have expanded an image's canvas in GIMP and I'd like to change the background colour of the new area created by my increase of the canvas size.

I've tried selecting this area but no matter what I do I cannot seem to select this area, only stuff in the area of the original canvas.

I know exporting the image as PNG, closing it and then opening the image again in GIMP will allow me to colour this area, but is there a way I can without closing the image?

Answer 1

You:

• add a new layer (Layer>New layer)
• fill wit with the required color: in the layer creation dialog, some of the choices are the current Foreground or background color, or white. And in any case you can bucket-fill it later with any color.
• move it below your current image, either by dragging it in the Layers list, or by using the Layer>Stack entries (in particular, Lower or Layer to bottom
• If necessary you can merge the layers Image>Merge visible layers

Now, an explanation of your problem: you can only paint within layer boundaries (the fixed dashed lines), but when you enlarge the canvas (Image>Canvas size), the layers remain the same and aren't enlarged by default, so you cannot paint in the added emptiness. You can change this while resizing the canvas using the Resize selector in the dialog, where two options are All layers (usually a bad idea) and Image-sized layers (a good idea). You can also change this at any later time using Layer>Layer to image size to extend the layer with transparency that you can paint on. But in any case, the method at the beginning of this answer is still the quickest/safest to fill the emptiness with color.