That leaves the bottom shadow. For that, we’ll need to re-use two of the sprites we’d previously been using for cell shadows. This will impose the tightest timing requirements upon us of anything in the design. Because we are altering the sprite graphics, the magnification, and the X coordinate, we cannot reconfigure the sprite registers until after we have finished rendering the last cell, and we’ll only have a few scanlines of space to do that work. As we’ll see once we get into the nuts and bolts, we’ll normally have dozens.
And some things we genuinely don’t know:
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The article is in Chinese—here are some quotes translated via Google Translate: