A method and apparatus for biogas extraction. The method includes obtaining a set of images that includes a field of view encompassing at least a portion of the flexible roof and a background; generating a sample region having a first end overlapping only the flexible roof and a second end overlapping only the background; determining an average pixel color of an end slice located at the first end or the second end of the sample region; determining, based on the average pixel color of the end slice of the sample region, a fill percentage of the sample region attributed to an object overlapped by the end slice; estimating an amount of inflation of the flexible roof based on the fill percentage and a correlation between fill percentages of the sample region and predetermined amounts of inflation included in calibration data; and controlling extraction of the biogas based on the estimation.