Epoch Objects

Objects have multiple identifiers.

Shape Class: used when creating an object. For example, when spawning a new enemy ship, the code creates a random instance of shape class 2.
Shape Numbers: each class defines 1, 4, or 8 shape slots. A given shape definition can be in more than one slot. The slot is chosen by ANDing a random number with a bit mask, which makes it possible to select even slots only. Shapes referenced more than once are more likely to be selected by the random number generator.
Score Class: used when an enemy ship or base is destroyed. Determines the score, which comes out of a table, and explosion parameters, such as the number of times that the screen flashes. In some places the code simply tests for a nonzero score class.
Unique ID: used in specific situations. For example, colliding with the center of an enemy ship is very different from "colliding" with the center of a time portal. The upper nibble is the same for similar items, e.g. time portal boundaries are all $5x.

In many cases the high nibble of the unique ID is the low nibble of the shape class, but not always. For example, time portals are in shape class $01 so they get created along with enemy ships, but have a $6x unique ID like friendly bases.

Shape definitions begin at $4795 (click link and scroll up for detailed explanation).

Shape Class	Nums	Score Class	Unique ID	Description
$00 (1 entry)
	0	-	$01	Epoch logo
$01 (4 entries)
	1	7	$11	Base/500pts
	0,2,3	6	$12	Base/300pts
$02 (8 entries)
	0	5	$e1	Ship/100pts (two planes)
	1,3	2	$21	Ship/30pts
	6,7	3	$22	Ship/40pts
	2,5	4	$23	Ship/60pts
	4	-	$6f	Time portal
$03 (4 entries, even only)
	0	-	$31	Enemy base projectile (flashing)
	2	-	$32	Enemy ship projectile
$04 (1 entry)
	0	6	$12	Reference to: Base/300pts
$05 (8 entries)
	0	-	$51	Time portal boundary
	1,7	-	$52	Time portal boundary
	2	-	$53	Time portal boundary
	3	-	$54	Time portal boundary
	4	-	$55	Time portal boundary
	5	-	$56	Time portal boundary (odd colors)
	6	-	$57	Time portal boundary
$06 (1 entry)
	0	-	$61	Friendly base
$07 (8 entries)
	0	-	$71	Star (white)
	1,7	-	$72	Star (blue)
	2	-	$73	Star (orange/black)
	3	-	$74	Star (purple)
	4,6	-	$75	Star (green/black)
	5	-	$76	Star (orange, rectangular)
$08 (1 entry)
	0	-	$81	Player projectile
$09 (8 entries)
	0,6	-	$91	Explosion chunk
	1,7	-	$92	Explosion chunk
	2	-	$93	Explosion chunk
	3	-	$94	Explosion chunk
	4	-	$95	Explosion chunk
	5	-	$96	Explosion chunk
$0a (4 entries)
	0	-	$01	Explosion flare
	1	-	$02	Explosion flare
	2	-	$03	Explosion flare
	3	-	$03(!)	Explosion flare

Notes on the visualization images:

The images are individually scaled up to 64x64, which is why the projectile seems larger than the Epoch logo. Widths have been stretched to match screen output.
Flashing colors are presented by dither patterns. For example, the "wings" on the 60-point enemy ship are drawn in black and orange on alternate frames. The game does not use dither patterns, but this approach seemed less distracting than an animated GIF.
Many shapes have a point at the center, used for collision detection, that may be hard to see in-game because it looks like a star or it's drawn in black. So you won't see the black dot at the center of the time portal, but if you look carefully you'll see the blue dot as you fly through the hole in the friendly base.

Each shape has a 24-byte header that defines various parameters, such as unique ID, score class, valid initial Z position, and the number of elements. The header is followed by a series of 16-byte element definitions.

There are four kinds of elements:

$00: point
$01: horizontal line
$80: filled rectangle
$ff: vertical line

Elements are defined by five coordinates: left, top, right, bottom, and depth (Z). All five are signed 16-bit values. When not needed, the right and/or bottom coordinates are ignored.

Each element has one or two colors. Elements with two colors flip back and forth at a rate defined by the element. The colors are:

$00: black
$10: white (single-pixel)
$20: green
$30: purple
$40: orange
$50: blue
$60: white (double-pixel)
$ff: do not draw

Note the low nibble of each byte is zero. The color value is ORed with a pixel position (0-6 for even bytes, 8-14 for odd bytes) and used as an index into a table of pixel patterns.

Elements are drawn in the order in which they are defined.