Requirements for Rasterization


On the right is an example of the input: the polygon outlines are shown overlaid on the pixel grid,


An assignment of pixels to polygons (as shown on the right) that satisfies the following properties:

Shared Boundary:

Sample Point Algorithms

A sample point algorithm determines which polygon should contain a pixel by sampling the polygon at one point. If that one point is contained in the polygon, then the pixel is assigned to that polygon.

In this case, the sample point is the lower left corner of the pixel. The polygon in question lies to the right of the edges pictured The green pixel, pixel(3,7), which covers the region [3,4]X[7,8] has sample point (3.0,7.0). This sample point lies within the polygon, therefore that pixel is assigned to the polygon.

The sample point (2.0,8.0) for the red pixel, pixel(2,8), is not contained by the polygon so that pixel is not assigned to that polygon.

Special cases

When a sample point lies on an edge or vertex of a polygon, we must take care in choosing whether to assign the pixel to that polygon. Since multiple polygons can share an edge or vertex without overlapping, it is possible that we might accidentally assign that pixel to more than one of these polygons.

An example algorithm

You can check out the results of a sample point algorithm for scan conversion implemented in java at
The pseudo-code for that algorithm is also available. 

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Last modified: Wed Mar 18 12:59:36 1998