algorithm to detect different colored areas in bitmap?

[lozina]

I am not a graphics programmer so I have no idea what terms to use to search for this... but it should be a pretty common algorithm.

I have a bitmap image which has several irregular shaped areas of solid color. Imagine a map of the united states where each state was filled in with a solid unique color. I need to scan such an image and create polygons out of the states.

The ultimate goal is that the program will have a bunch of polygons (or just arrays of vertexes?) which represent each state in the map, so If I display that map on a screen and the user clicks on the map I can figure out what state the user clicked on by testing in which polygon the mouse coordinates fall.

I know there is an easier way to do this, by dynamically getting the pixel at the coordinate and then looking up the pixel's color against a table to match the country but I can't do this for reasons I'll only confuse you with.

 

[Markbnj]

Search under image processing topics for edge detection algorithms.

Edit: should add, I can't tell if the state thing is just an example, or is your real problem. I also don't know if the application is web or thick client. If your data is really as stable as political boundaries, then it would be a heck of a lot simpler to hand-define a set of polys to encompass the right areas (essentially what a web imagemap is).

 

[lozina]

It is for a 2D application written in C# using XNA.

The state thing is just an example but my real situation is exactly the same. I need to define clickable areas that are accurate so I'd rather not try to hand define polygons for it. Besides the "map" is large so it would take a long time doing it by hand.

 

[Ken g6]

A vectorization program might help you, if you can read the vector drawing data. Here's a program to convert BMP to SVG (Scalable Vector Graphics).

Edit: This one might be better.

 

[GoatMonkey]

Does each irregular shape have its own unique color, or are there duplicate colors with lines defining the edges?

 

[lozina]

Here is an article that discusses the exact algorithms I'm looking for, but unfortunately it just has a vague general description of how they actually work and post no code. http://www.geovista.psu.edu/si...99/Gc99/076/gc_076.htm

If you scroll halfway down the page you see a colored map of china, each province in a solid color. Then you see these feint lines traced around each blocky province which represents the line the algorithm traced around them. This is exactly what I need to do. Except its not a geographic map in my case but that doesn't matter.