I can''t get the good disparity image through the stereo_image_proc

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Hi, Everone! I am setting the parameters of the stereo_image_proc to get the well disparity image recently.But I wonder if there exist any methods to ajust the parameters to get the well disparity image?

The raw left image is:

The default disparity map is:

Then i changed some parameters and get the best result and the parameter setting as followed:

The pointclouds2 as this:

You can see, in the disparity map,i can't get the sky and the near road's disparity well, I don't kown why. Is there anyone kown the reson? Ps: I run it in the ubuntu 14.04, and use two image replace the stereo camera for convenient.