Tutorial on image gradients

[ashwanirathee]

I recently was reviewing ImageCorners.jl and it uses imgradient a lot, and there exist of lot of different types of filter like prewitt, sobel etc that are utilized. I would wanna introduce readers to:

• gradient in general
• gradient for a 2d signal, (grad_x, grad_y)
• gradient orientation and magnitude and help visualize
• gradient operator([grad_x, grad_y]) this is going to be useful later I know
• compare finite differences method and ForwardDiff
• basic edge thresholding

[johnnychen94]

That sounds a good idea; we also have some gradient operators in ImageBase.FiniteDiff