Parameter op takes one of the strings “set”, “reset”, “intersect”, or “null”.
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* '''reset''' sets the clipping rectangle to include the entire window. The remaining parameters is ignored and can be left out.
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* '''set''' sets the clipping rectangle to the x, y coordinates with the specified width and height. The remaining parameters default to the system window location and size.
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* '''intersect''' sets the clipping rectangle to the intersection of the current clipping rectangle with the rectangle specified in the routine parameters.
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* '''null''' sets the clipping rectangle to empty. All subsequent graphics commands will be ignored.
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Typically the “set” operation is called before drawing, say, a text string. It is important to call the “reset” operation after drawing the last clipped graphic so that you don’t leave a lingering clipping rectangle as a side effect.