Image 1 is the original scanned image, while in Image 2 the threshold value is changed into 56. This means that pixels in the image with brightness level below 56 will all be cut into black, and those with brightness level above 56 are all turned into white. Therefore, you will find that by setting a lower threshold value, some details are lost in the new image. However, if there are many unwanted dots in an image, you can eliminate them.
When you have finished with the threshold adjustment, you can click OK to confirm, or click Cancel to abort and exit the dialog box. Otherwise, you can click Reset button and the settings will return to the default 128.
Invert button
You can click the Invert button to get an inverted or negative image:
Original image
Image after Invert is applied
Mirror button
You can click the Mirror button to get a mirror reflection effect on the original as if you were viewing the image in a mirror:
Original image
Image after Mirror is applied