Apple ][ Graphics – Change is bad (keep moving)

Apparently, moving a (mostly) white object is as easy as I thought it was.  My tool generated the 7 needs frames and they progressed nicely across the screen.  The only tricky part is I need to turn an X value into two different values:

  1. Byte # in row
  2. Frame # to display

This was pretty easy (or so I thought, more on that below).  Take X divide it by 7 and round down to get the byte # in the row.  Take X mod 7 (i.e remainder) and you get the bit offset with in the byte which corresponds to the frame.  I’m worried that math is also too much work to do every movement, so I generated and lookup table for X to Byte/Bit but it’s 2 bytes for each column so that’s another 560 bytes for lookup tables.  Remember we’re working with things on the order of magnitude of 32-48k.  So that’s a total of 944 bytes for lookups, almost a whole K.  I’ll need to figure out which is better doing some testing, for now lookup table it is.

We’re good, right?  For non-white objects, no so much:

Reading further in the book (yes, I end up working ahead when perhaps I shouldn’t), looks like I need to handle odd/even frames for odd/even bytes differently.  Oh, the fun never ends.

Which, when I think about it, makes total sense. Here is the first frame as a bitmap:

Screen Shot 2016-03-18 at 9.07.22 PM

And here is the second:

Screen Shot 2016-03-18 at 9.07.38 PM

The first one is all on green pixels (the G at the bottom) and the second is all on violet pixels (the V). I could just move two bit at a time, the the second (displayed) frame would be:

Screen Shot 2016-03-18 at 9.19.12 PM

So, it’s back to green like I want.  That seems cheap like a 2-bit suit (Ok, I had to).  But, it does feel like cheating.  Maybe that is what I’ll need to do and what games do and we don’t know it, but I don’t think so.

Time to read more and see, looks like the parts I’ve moved through so far at the “easy parts”.  Figures.


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