HaloHalo (I am not here refering to the Halo video games!) are undesirable artifacts that can occur when you take an anti-aliased image on one background, and try to change the background. What happens is that a trace ("halo") of the original background may be left around the new image.
I will demonstrate this using by using an anti-aliased image (actually a clip art) on a green background and attempting to change the background from green to blue. However, halo can in fact occur regardless of the original background (even if you start with a white background).
All that needs for halo to occur is:
Let's start by looking at my anti-aliased image on a green background. On the left, the image is shown at actual size, on the right, I have zoomed into a small section of the image so you can see in more detail what is going on.
If you look carefully, at the zoomed in area in the above, you will see that there are some green tinted pixels around the man's picture (on the zoomed in version, most noticeably on his shoulder and the bottom part of his hair), which do not quite match the main green background color. This is the result of anti-aliasing the picture with the background.
Now, what happens if I paste the image on to a new background (setting the original's green background color to be transparent):
You should now clearly be able to see the halo.
The green tinted pixels which were not quite the same color as the main green background (because of anti-aliasing) have been left behind - giving rise to a halo effect. This is because when I turned the main green background transparent, these pixels, having a slightly different greenish color, did not become transparent.
How can we avoid halo?
There are a number of ways:
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