TITLE:  Extra Half Bright mode

There are lots of fun things you can do in 6 bit planes on the Amiga,
including Hold and Modify color displays and dual-playfield mode with two
separately scroll-able, completely independent, 8 color playfields with
overlay and transparency.

But tonight I'm going to discuss something you don't already know about
your Amiga -- even if you've read all the tech manuals cover to cover.

The specs for the Amiga say that in a completely static, low-resolution
display, with nothing fancy going on (like Hold and Modify), you can
display up to 32 colors simultaneously out of a pallette of 4096. Well not
quite.  You can actually display up to 64 distinct colors out of that
pallette of 4096! Have I got your attention?  Welcome to Extra-Half-Brite
mode.

The Amiga display hardware automatically enters Extra-Half-Brite mode
whenever you select a 6 plane display (which must be low res due to the
hardware) and have NOT selected either Hold and Modify or Dual Playfield
mode.

The graphics kernel software further enforces the rule that you must set
the EXTRA_HALFBRITE flag in the ViewModes for the ViewPort (Screen in
Intution lingo) or it will automatically trim your screen down to 5 planes.
This is done both for backward compatability and to insure support in
future Amiga architectures.

Consider the playfield data bits for a given pixel.  The bits from the
first five planes form a color register selector for that pixel, allowing
you to choose among the 32 color registers in the Amiga.  The bit from the
sixth plane is interpreted as follows:

  0 -- Use the color in the selected color register just as specified.

  1 -- Take the color in the selected color register, shift each of its R,
G, and B components right one bit, and use the new color value thus formed.

The net result is as if you had 64 color registers where the colors in the
top 32 were "half-intensity" counterparts of the colors in the bottom 32!

Of course, that means there is a dependency between the choice of colors in
the 32 real registers and the resulting colors in the 32 psuedo-registers.
Nevertheless, I assert we have as much right to claim 64 colors on screen
as IBM has to claim 16 colors from a monitor that is physically capable of
producing only 8 colors at 2 intensities!  At least we can select our 32
colors out of a pallatte of 4096!

Note also that, since fractional color components have no meaning in the
hardware, there are several distinct real colors that produce the same
extra color.  For instance (in hex):

  888 --> 444,    988 --> 444,   898 --> 444,   999 --> 444,  etc.

Despite all this quibbling, with a little thought it's easy to see how you
can choose a set of 32 real colors to make sure all 64 real plus extra
colors are distinct.  And they are every-pixel-addressable!

Why have we kept this little jewel a secret?  No, it's not that we were
planning to lull the competition into complacency and then spring an
instant double of the Amiga's color capacity on them.

Not all Amiga 1000s have Extra Half Bright mode.  All Amiga 500s and 2000s
have it.

This revision of the Amiga chipset occurred after the Amiga Hardware manual
was written, so it is not discussed there.  Note that you can't hurt
anything by running such a program on an older machine -- the values in the
6th bit plane will simply be ignored.  Color printing of Extra Half Bright
mode should work on AmigaDOS 1.1 and 1.2.

This sample program shows Extra-Half-Brite mode in operation.

 *  Dale Luck    -- October, 1985
 *  Bob Pariseau -- November 8, 1985  (Editorial changes)
 *  John Foust   -- June 29, 1987 (more edits and updates)