For all the typesetters and scan cleaners out there:
Back when I did some type setting, I developed a process to (almost) automate the process of cleaning up speech bubbles.
So, here's a little tutorial on how to do it.
I used Photoshop, but the basic concept should also work with other photo editing software such as GIMP.
First, you select the white space in all the speech bubbles you want to clean up with the magic wand tool (selection is highlighted red):
You don't need to select the spaces inside of letters such as O or D.
Next you go to "Select > Modify > contract" and decrease the size of the selection by 3 or so pixels, this number might change depending on the resolution you're working with.
This is to avoid overlap between the background and the borders of the bubbles later in the process.
Now, create a Layer from your selection (Ctrl+J or right-click on the selection and click "New Layer via copy"). Turn off visibility for the background layer, you should now see something like this:
Now select the empty space with the Magic wand tool:
Next, reverse the selection (Select > Inverse):
Now make a new "Solid Color" layer in the same color as the speech bubbles.
The solid color layer should look like this:
Now make the background layer visible again and that's it, you now have empty speech bubbles, ready for whatever you wanna put in there.
Now you might wonder why on earth you would go through all these steps if you can just use the brush tool and do it manually in like half the time, and the answer is:
YOU CAN RECORD THIS WHOLE PROCESS AS A PHOTOSHOP ACTION AND PUT IT ON A HOTKEY
Something that is not possible with the brush tool. You still have to manually select all the whites in the speech bubbles, of course, but doing it this way allowed me to clean up 600 pages in like half an hour.
Another benefit is that no information gets lost, the original text from speech bubbles is still there, just under 2 new layers
I really hope this can be of use for some typesetters out there, you guys are doing god's work.