GIMP Version: 2.8.14
Operating System: Linux
GIMP Experience: New User

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I'm trying to resize by canvas. I Increase the height slightly. The canvas does increase but the newly added space is filled with a checkered black-and white pattern.
I want the new space to be all blank (white).

This question has cropped up a couple of times recently on other forums.

When you open a simple single layer image that sets both the canvas size and the (background) layer size to the same value.

When you resize the canvas, the layer size remains the same.

A couple of fixes.

If you have already resized the canvas: Use Layer -> Layer to Image Size to scale the layer.
Since your image has transparency and you want a white background, add a layer underneath filled with white.

If starting from scratch, use the resize layers drop down menu and set that to all layers.

eg http://i.imgur.com/blhvGJp.jpg the white layer under applies here as well.

Thanks for your reply, I'm trying to follow your suggestion: Layer->Layer to Image Size but nothing happens. I'm am new to gimp so I may be missing something.

Thomas, if you are changing the canvas size, GIMP doesn't resize the existing image along with it. It just changes the parameters of the image canvas. Any existing image or layer will remain its original size, even if you resize the layer to image size. There are two options in this case, the first being easiest if you want the background to remain a solid black. Simply use the bucket tool and refill the canvas with black. If the background has any kind of image within it, you'll need to scale the image or layer to the new canvas size. Keep in mind that scaling the image or layer will result in aspect ratio changes that might affect the appearance of the image/layer from being stretched horizontally and vertically to a new canvas size.

What the extra area is filled with is going to be configurable in GIMP 2.10:
https://plus.google.com/+gimp/posts/67nYGcwd7EP

Kevin

That's promising to know! Thanks, Kevin...

Here is some additional info that might help. I originally imported it from an exiting .pdf that was given to me. I don't have any knowledge on how the pdf was created.
The image appears to have a white background. When you resize it it would not be unreasonable to expect the extra space to be also white.
I have not the slightest clue where the checkered pattern is coming from. It's not visible anywhere on the image.

In Gimp, the checkered pattern represents transparency. ie the extra area is being filled with transparency. You can make a selection of the transparent area and bucket fill with white. Or you can create a white layer below your layer and then merge down.

Also useful to know:
A layer boundary is indicated by a static dotted line. A selection is indicated by a moving dotted line (marching ants).

So I dug a little farther. As an experiment I deleted all the layers he let me delete. In fact, there were only 2, the one I added and the one that was automatically created as a result of the pdf import. What was left was, surprise, surprise, the bare-boned checkered pattern.
But looks like this is not, technically a layer. It's "something else". I keep looking for options to change it but to no avail.

Think of the canvas as a place holder for layers. Layers can be smaller than the canvas. If a layer is larger than the canvas the portion outside of the canvas will not be visible. You cannot draw directly on the canvas; you must draw (edit) on a layer.

If you have deleted all the layers you are left with the canvas which is represented by the checkered pattern. If you add a transparent layer it will also be represented by the checkered pattern but you will be able to draw on it.

To save lots more writing, a quick video demo, if you can spare 2 minutes.

All you need to know.

https://youtu.be/BN1pUxyv9nU

Here is what I came up with:
1. Created a "New" file with the desired size. It came with a "default" blank layer spreading all over the canvas.
2. Copied the existing image.
3. Pasted the image as "new layer".

Thanks everyone for the support.