aperture size for the Sobel operator.
If true then all channels will be processed independently and then joined to build the result. The only exception is when there are 4 channels and in this case, if channels===true, the last 4th channel will be omitted (alpha). If an array of numbers is given then those channels will be processed only. If not given then the operation will behave normally, processing as single channel image.
first threshold for the hysteresis procedure.
Observation: When this has low values flood pass through edges of color similar to the low channel
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if true a more accurate L2 norm will be used to calculate the image gradient magnitude