import ch.psi.pshell.imaging.Data as Data
class Exposure(Writable):
    def write(self,pos):
        cam.setExposure(pos)
exposure=Exposure()


class Contrast(Readable):
    def read(self):
        data = img.getData()
        roi = Data(data.getRectSelection(500,300,700,600))
        return roi.getGradientVariance()
contrast=Contrast()


#a= lscan(exposure,img.getContrast(), 0.5, 1.0, 0.01, 0.5)
a= lscan(exposure,contrast, 0.2, 0.4, 0.01, 0.7)

(n, m, s) = fit(a.getReadable(0), xdata=a.getPositions(0))
if m is not None:
    print "Setting exposure = ", m
    exposure.write(m)