Or are you going for a more complex shape than an ellipse?

I once wrote a small program that turns bright pink pixels into transparent ones... but I can't find it now.

Basically it'd iterate over the pixels[] array for an image, and assign color(0,0,0,255) to each one that was bright pink.

1. String FILENAME = "water1.PNG";
2. PImage img;
3. PImage newImg;
4. int x;
5. int y;
6. int i;
7. 
   
8. img = loadImage( FILENAME );
9. newImg = createImage( img.width, img.height, ARGB );
10. for( x = 0; x < img.width; x++ ){
11.   for( y = 0; y < img.height; y++ ){
12.     i = ( ( y * img.width ) + x );
13.     if( img.pixels[i] == color( 255, 0, 255 ) ){
14.       newImg.pixels[i] = color( 0, 0, 0, 0 );
15.     } 
16.     else {
17.       newImg.pixels[i] = img.pixels[i];
18.     }
19.   }
20. }
21. newImg.save( FILENAME );