Trims off excess blank background from your image, making it smaller. Results will vary depending on the image.
The best method to see if this works for your current image is to try the function. You can click Edit /Undo (or the Undo tool in the Quick Access toolbar) if the image is not trimmed for your needs.
Choose Auto-detect or Use current background color. (Auto-detect is the default and usually the best to use in most situations.
Allows you to decide how closely the bitmap pixels of the new color value must match the "old color" value.
§ Move the slider all the way to the left (0%) for an exact match. Only the pixels with the exact specified values of Red, Green and Blue components will be replaced.
§ Move the slider to right, selecting higher % values, and more and more "close color" pixels will be replaced.
Check this box to see the effect on the image immediately, as you trim it.