Which vertex of the polygon is clipped first in polygon clipping?

Short answer: There is no universally mandated "first" vertex for polygon clipping. The algorithm processes vertices in the polygon's given order; any vertex can serve as the starting point.

• Many textbook examples start processing from a particular corner (for example, the top-left) simply as a convention to make illustrations consistent.

• In practice, clipping algorithms (such as Sutherland–Hodgman) iterate over the polygon vertices in their stored cyclic order and clip them against each edge of the clipping window.

• Therefore, the result does not depend on choosing the top-left, top-right, bottom-left, or bottom-right vertex as the start, provided the polygon vertex order is preserved.

Sutherland–Hodgman overview:

1. For each edge of the clipping window, create an output vertex list initialized empty.

2. Walk the polygon's vertices in their given cyclic order, treating each pair of consecutive vertices as an edge to be tested against the current clip edge.

3. Based on whether endpoints are inside or outside, add the appropriate intersection and/or vertex to the output list. After processing all polygon edges for that clip edge, the output list becomes the input polygon for the next clip edge.

Conclusion: The question as phrased is ambiguous. A more accurate answer is that clipping begins from the polygon's first listed vertex and proceeds in order; there is no fixed required corner such as "top-left."