The technical term “clipping” from computer graphics literally means “cutting off”. Basic objects are cut off at the edge of a desired screen section or window. The window can be any polygon, i.e. a polygon that forms a given boundary. All elements inside the window are preserved after clipping, all elements outside are clipped. Clipping is an elementary part of any graphics software. There are three different clipping techniques: The analytical technique, the scissor technique and the stencil technique. In the analytical technique, the visible segments are determined by calculating the intersection points of the output primitives. In the other two techniques, the objects are fully scan-converted. In the scissor technique, only pixels within the scissor box are then written to image memory; in the stencil technique, only pixels within a masked area are written.
Clipping of lines at the rectangle window
When clipping distances, the part that is located within a given rectangular window is to be determined. The two end points of the line are labeled “P” and “Q”. There are four different cases for this calculation:
- Both points P and Q are within the rectangular window. This means that the entire straight line is to be drawn.
- Points P and Q are both outside the rectangle window, which means that the entire straight line is not to be drawn.
- Points P and Q are both outside the rectangle window, but the connecting line passes through the rectangle window. In this case, both intersection points are calculated, and the part of the line between the two intersection points is to be drawn.
- One of the points P and Q is inside the rectangular window, the other one is outside. Here the distance from the point lying inside the window to the intersection point with the window edge is calculated and drawn.
Algorithms by Cohen-Sutherland, Cyrus-Beck and Liang-Barsky are used in these tasks.
The clipping of distances at arbitrary polygons
Again, the portion of a line with endpoints P and Q that lies within a polygon is computed. The intersection points of the line with the polygon window are determined with the help of the so-called “Inside Test” and the “Max-Min Test”. The “Scanline algorithm” is used here, a line-by-line method for calculating 3-D scenes consisting of different polygons.
Polygon clipping is the cutting off a certain part of the polygon with respect to a rectangular window or any polygon. Three cases can occur:
- The edges of the polygon are taken in their entirety, partially, or not at all.
- New edges have to be taken.
- Clipping can create several polygons from one polygon.
With the help of the Souterland-Hodgman algorithm, all three problems are covered.
In the field of media monitoring, the term “clipping” is used for the creation of press reviews. A press clipping thus corresponds to a newspaper clipping or a screenshot in the case of Internet media.
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