Check if a given window is full of nodata or 0.

[lionlai1989]

Dear the team of s2p: I would like to propose that discarding a tile full of nodata or 0 during the step of is_this_tile_useful in s2p/initialization.py. It's common that stereo imagery has valid data in the certain AOI and put the unconcerned area as nodata.
E.g., the following two scenes both have nodata pixels over some parts of area. And the current s2p will create tiles across the whole raster, including the area fill with nodata/0.

This PR provides two improvements:

• Speed up the processing time: the discarded tiles will not go through the stereo process (SIFT keypoints finding and matching, rectification, dense matching, triangulation, rasterization), which saves the computation time.
• The output DSM looks nicer and is better for later processes. Here are the DSM comparison between the DSM created before and in this MR.

  Scene1:

• Before this MR:
• In this MR:

Scene2:

• Before this MR:
• In this MR:

Please review it and any feedback is appreciated. Thank you.

[lionlai1989]

Hi @gfacciol: If your schedule permits, could you please look into this PR? I would appreciate your feedback on this matter. Thank you.

Best,

[carlodef]

Hi @lionlai1989, this looks good to me, thanks! I'll merge it as soon as the CI tests have finished

[lionlai1989]

Thanks. :smile_cat: