Step1: Sky Detection 

Apply algorithm to detect a border that separates the sky region from ground region

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Step2: Image filtering

 1) Apply bilateral filter to smooth out the input image while preserving its edges. 

 2) Increase brightness and saturation of the image

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Step3: Sky replacement

Replace the sky region in the input image with the sky from some source image by choosing one candidate sky image from the sky image set, resizing the sky region from source image to match the sky region of targeted image and then copying pixels to fill the sky region of targeted image.

 

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