Re: shape table code question

["Michael J. Mahon" <mjmahon@aol.com>]

Jonno Downes wrote:
> Howdy
>
> Can anyone point me at an explanation of how the vector bytes in  a
> shape table are interpreted?
>
> I've seen various explanations of how to create shape tables, and they
> all talk about creating vectors, then coding that vector into 3 bit
> sections where
> first bit of 3 : 0=dont plot, 1=plot
> remaining 2 bits of 3: 00=move up, 01=move right, 02=move down,
> 11=move left
>
> then you take 2 of these 3 bit codes, prepend with "00" and you have
> an 8 bit byte to put in your shape table.

Actually, you try to put *three* vectors in each byte, with
the proviso that the third must be a "move" and not a "plot"

> Which all makes sense, except I am currently looking at the Beage Bros
> "Apple Mechanic" manual (http://beagle.applearchives.com/the_software/
> vintage_beagle_bros_softwar/apple_mechanic.html ) and there is a table
> on page 46 "Apple Shape Table Vectors" that shows the high 2 bits can
> be nonzero, and all 8 bits in the byte are significant.
>
> Can anyone explain exactly how the vectors are decoded?

The two lower-order 3-bit groups are used as-is, and the third,
two-bit vector is used with a high-order 0 prepended.  It is often
possible to code a shape so that all three vectors in a byte can
be used, though the "top" one is restricted to a non-plotting move.

Shape coding is constrained even more if it is desired that the
shape be able to be used with SCALE > 1 and still look right.

But as long as SCALE will always be 1, and each vector either does
or does not plot a point, there is considerable freedom in choosing
the order of the moves and plots, to minimize the total size of the
shape table entry.

One should also arrange to plot each point exactly once, so that
and XDRAW will properly erase the shape.

It is also worth noting that the total number of vectors is a prime
determinant of the time required to DRAW a shape, so lots of redundant
moves can slow things down.

-michael

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Home page: http://home.comcast.net/~mjmahon

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