2D rendering - algorithm to identify islands and apply "beaches"?

Question

I'm playing with 2D games programming. My toy project is a sailing game with explorable islands and more, but at the moment I'm trying to find the best way to apply "beaches" (i.e. soft ends) to the otherwise fairly rough islands. Tile size is 64x48.

enter image description here

My present solution is to iterate through each tile and look for surrounding tiles, and based on their surroundings, replace the water-tile with the correct texture.

I realise that this is a very flawed approach as it is:

Incredibly inefficient as the surround method is invoked for every tile, even those affected by the change
The order of execution might imply that some tile changes are overwritten

Do you guys have an idea how I might get around and solve this in a better way?

Thank you!

edit

This algorithm is performed when the map is being loaded.

score 2 · Answer 1 · answered Dec 02 '12 at 18:55

At the very least, I would flag the tile so you one have to do this once the first time you render the tile and then reuse the flag value instead of trying to figure it out again.

If you do it at render time, assuming you only show a portion of the map at a time, then it's a lot more efficient than doing it for the entire map all at once.

Other than that, I can't think of another way to make the algorithm more efficient. By definition, a "beach" title is defined as such by the presence of land on the edge of a water tile. So you have no other option than to look at the surrounding tiles...

score 2 · Accepted Answer · answered Dec 02 '12 at 19:20

An algorithm to detect borders (but not corners at first) in a single pass per line / row would be:

for each horizontal line
  previousTag = getTag( first line pixel ) // land or water in our example
  for each pixel of the line
    currentTag = getTag( this pixel )
    if ( previousTag == currentTag ) 
       continue // not a border
    else 
       // We got a vertical border, do what is needed

    previousTag = currentTag
  endforeach
endforeach

Same goes for vertical lines (instead of incrementing x, you increment y. We can here also know if we got a corner instead of a vertical border:

for each vertical line
  previousTag = getTag( first line pixel ) // land or water in our example
  for each pixel of the line
    currentTag = getTag( this pixel )
    if ( previousTag == currentTag ) 
       continue // not a border
    else 
       if ( pixel on the right or on the left is a border )
         // we got a corner
       else
         // we got a horizontal border

    previousTag = currentTag
  endforeach
endforeach

This should be a pre-process, unless your terrain is dynamic. Don't do that each frame anyway!

2D rendering - algorithm to identify islands and apply "beaches"?

2 Answers2