VGI -- VMW Retro Vector Graphics Interface

This was designed mostly as a proof-of-concept to drawing some Myst
scenes.  Might not work well for general graphics.

The Apple II Hi-res screen is 280x192 and 6 colors with lots of limitations
that are a bit complex to get into here.  Some of the limitations in the
interface here are working around Apple II issues.


Apple II Hi-res Colors:
~~~~~~~~~~~~~~~~~~~~~~~
	0 = black (for purple/green)
	1 = green
	2 = purple
	3 = white (for purple/green)
	4 = black (for blue/orange)
	5 = orange
	6 = blue
	7 = white (for blue/orange)


	Note Apple II uses NTSC artifact colors, so weird things happens when
	you try to put colors next to each other:
  
	+ Colors need to be two pixels wide, if you draw single width
		they may not appear depending on even/odd column
	+ White lines drawn single width will be colored instead
	+ Two colors of same group next to each other will have white or black
		border between
	+ Two colors of different group next to each other will have odd effects
		as can only change group every 3.5 pixels.


The VGI Commands
~~~~~~~~~~~~~~~~

===================
VGI Clearscreen (0)

	CLS	color

	clears screen using ROM routine using color

=================
VGI Rectangle (1)

	RECT	color1 color2 x1 y1 x2 y2

	draw filled rectangle from (x1,y1) to (x2,y2)

	color1 and color2 are drawn as alternating colors (horizontal stripes)

	x1 must be < 255.  relative drawing is used so x2 can go to
	the edge of the screen (279)

	Unlike the other routines, this one uses custon horizontal-line
	routines for speed so much faster than the ROM line routine

==============
VGI Circle (2)

	CIRC	color cx cy r

	Draw a circle of color with center (cx,cy) and radius r

	Circles with radius 0 will give an interesting non-circle effect.
	Bresenham circles are drawn and so certain sizes don't look the
		best.  Also if the radius is too big things might break
		as the 8-bit unsigned int oveflows

=====================
VGI Filled Circle (3)

	FCIRC	color cx cy r

	Draw a filled circle of color at location (cx,cy) with radius r

=============
VGI Point (4)

	POINT	color x y

	Draws a point of color at (x,y)

	Note, X can be up to 279 here (the color is overloaded behind
		the scenes to support this)

	The last point you draw is the starting point for LINETO

==============
VGI Lineto (5)

	LINETO	x y

	Draws a line to (x,y).  The starting point and color come from
	the last POINT command.

	Note X cannot be > 255 so it can be trouble drawing at the right
	edge of the screen.  POINT can go up to 279, so you might be able
	to get what you want by drawing from right to left

==========================
VGI Dithered Rectangle (6)

	DRECT	c1 c2 x1 y1 x2 y2

	Draws a rectangle, but this time the two colors are raw bitmaps
	for the output colors, not one of the Apple II colors.  This lets
	you do fancier dithered color patterns, but it's really hard
	getting dither patterns that look nice.


=========================
VGI Vertical Triangle (7)

	VTRI	color vx vy xl xr yb
	VTRISK	color vx vy xl xr yb skip

	Draw a flat bottom filled triangle of color.
	The top vertex is at (vx,vy)
	The bottom goes from (xl,yb) to (xr,yb)

	If your triangle is too tall you might get moire effects.

	This was much simpler than a proper filled triangle routine.

	The skip variant adds SKIP as an X increment instead of 1
	In memoy the skip is encoded in the high nibble of color

===========================
VGI Horizontal Triangle (8)

	HTRI	color vx vy yt yb xr

	Draw a flat-sided filled triangle of color.
	The side vertex is at (vx,vy)
	The opposite side goes from (xr,yt) to (xr,yb)


==================================
VGI Vertical Striped Rectangle (9)

	VSTRP	c1 c2 x1 y1 x2 y2

	Draws a rectangle, but this time the two colors are raw bitmaps
	for the output colors, but to use even/odd columns.
	On apple II this allows repeating pattern, but since it's a multiple
	of 7 you can't get true stripes (for that we'd need a 4-byte
	repeating pattern)

=============
VGI Line (10)

	LINE	color x1 y1 x2 y2

	Draws a line from (x1,y1)

	Note, X1 can be up to 279 here (the color is overloaded behind
		the scenes to support this)

	The last point you draw is the starting point for LINETO

============
VGI End (15)

	END

	this tells the drawing routine that we are done


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Size comparison:

	VGI data	Captured+Compressed	compressed autoconvert
	========	===================	======================
clock	385B		1228B
rocket	623B
door	189B
book	212B
firep	486B