Sample points are covered in chapter 5.5 of the online GIMP Users
Manual. The text strings will appear as messages in the status bar
of the image window.
The "Add Sample Point" message appears when you hold down the CTRL key
and click on either the horizontal or vertical ruler, and then drag
the mouse onto the canvas.
The "Move Sample Point" message appears when the Color Picker Tool is
active and you click on a previously added sample point and drag it
around the image canvas. This message will change to "Remove Sample
Point" if you drag the sample point over either the horizontal or
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