This application describes systems and methods for detecting depth in deep trench isolation with semiconductor devices using test key transistors. An method comprises: capturing, by an image sensor, an image; generating a plurality of chrominance channels by converting the image into luminance-chrominance space; performing homogeneous region segmentation on the plurality of chrominance channels to generate one or more regions of interest in the plurality of chrominance channels; and projecting the regions of interest onto eigen-illuminant images to determine gray color pixels on the image, wherein the eigen-illuminant images are generated via performing a machine learning algorithm on a training set of images captured by the image sensor.