XeO3: Go faster sprites....

Just had a small brainwave..... Normally 16x16 sprites actually draw 24x24 due to rotation, however when the sprite "fits" perfectly inside a 16x16 (say its coordinate is 0,0 and hence has no actual rotation), you can drop the surrounding characters. While drawing the chars themselves don't take long, they still have to get masked. BUT! becasue +4 sprites are character based, so is my clipping routeing, which means I can drop any individual character from the sprite simply by flagging the character its about to overdraw as $FF. This is how I do clipping off the top and edges, I just have a table of indexes some of which point to $FF characters.

  Normal    NoY      NoX    NoX&Y
  -------------------------------
  ADG       ADG      AD     AD
  BEH       BEH      BE     BE
  CFI                CF

Anyway, what I now do is check to see if the Y coordinate has a fractional component, and if not, I set the bottom row to be clipped! If we still have to draw the full 24x24 sprite (which is most of the time), then this only adds 30 cycles PER sprite(300 over all), which isn't much - really. However, if we find we're dipping below our desired framerate I now have another tool to help speed things up. Take a sprite that simply flys across the screen, well if I make sure this sprite does this on an 8 pixel Y boundary, then its always going to be significantly faster due to the fact it will never draw the lower row.

I could do the same on X where I drop the last row, but because of the scrolling, this won't happen very often... I'll probably try it and see, but I think the Y check is good enough.