For anyone interested, I squeezed a couple more ms out of the 2D loop by replacing the boolean with an increment/decrement variable and using strict equals. It’s about 30ms faster on a 2000×2000 image.

while(true){
color = bitmapData.getPixel(x, y);
x += i;
if(x === width){
if(++y === height) break;
i = -1;
} else if (x === 0){
if(++y === height) break;
i = 1;
}
}

but i agree that try catch should not be used for situations where a check against null could also be set. i think in this cases a test against null is clearer then try/catch.

here are my test result:
// s != null so try block is executed; n= 1000000:
// with try catch: 305
// test with null: 292

// s == null so catch block is executed; n= 100000:
// with try catch: 1623
// test with null: 10

var i:int;
var j:int;
arr[i+j]; // the expression returns a Number, which is used to as an array index
arr[int(i+j)]; // the expression returns an int, without any runtime casting. Fastest.
arr[(i+j) as int]; // the expression returns a Number, but then is cast at runtime to int. This version is significantly slower, because it’s the only one that casts at runtime.

If, however, we use an array index that is unambiguously a Number, casting it to int before using it isn’t any faster.

var n: Number;
arr[n]; // The array internally knows how to cast the Number to int.
arr[int(n)]; // This casts the same as the above line, except explicitly.
arr[n as int]; // This is a slightly slower way cast from Number to int for some reason.

What I said in my previous comment about casting objects AFTER they are retrieved from an array are still valid.