Correlation between Doppler velocity and continuum brightness for solar granulation at the center of the solar disk. The vertical dashed lines mark the thresholds used for the brightness masking to isolate the intergranular lanes and cell interiors, while the horizontal dotted lines mark the corresponding thresholds used for velocity masking. The outermost contour represents the level 1% of the maximum, the innermost contour 90% of the maximum, while the in-between contours are spaced equidistantly in terms of percentage of the maximum.