Nuke - Autocrop

autocrop.py

def getCropRectangle(node):
    width, height = node.width(), node.height()
    xMin = width
    xMax = 0
    yMin = height
    yMax = 0

    for x in range(width):
        for y in range(height):
            if node.sample("alpha", x, y) != 0:
                xMin = min(x, xMin)
                yMin = min(y, yMin)               
                xMax = max(x, xMax)
                yMax = max(y, yMax)

    return xMin, yMin, xMax, yMax

def setAutoCrop(node):
    crop  = nuke.nodes.Crop(box = " ".join(map(str, getCropRectangle(node))), reformat=True)
    crop.setInput(0, node)
    return True

for node in nuke.selectedNodes():
    setAutoCrop(node)