CS184: Scan Conversion and Sample Point Algorithms

Scan Conversion

Input:

• A set of polygons each of which is designated by a list of vertices. Each polygon and each vertex may be assigned a color.
• A pixel grid. The pixel designated (x,y) represents the area [x,x+1] by [y,y+1].

Output:

• 
  If two polygons do not overlap, no pixel should be assigned to both polygons.

  On the right is an example of an algorithm that violates this rule.
  

• 
  If a set of polygons completely cover a pixel, that pixel should be assigned to one of the polygons.

  On the right is an example of an algorithm that violates this rule.
  

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 http://www.cs.berkeley.edu/~laura/cs184/scan/ScanTest.html
The pseudo-code for that algorithm is also available.