BitmapData#getFirstPixel()

Scans the BitmapData, pixel by pixel, until it encounters a pixel that isn't transparent (i.e. has an alpha value > 0).
It then stops scanning and returns an object containing the color of the pixel in r, g and b properties and the location in the x and y properties.

The direction parameter controls from which direction it should start the scan:

0 = top to bottom
1 = bottom to top
2 = left to right
3 = right to left