What is HAM?

Although --- can be used in any standard Amiga graphics mode including 
Extra Halfbrite, it is primarily a HAM digitizing and paint program.  In most
cases, the maximum number of bit planes that an Amiga screen can use is five,
limiting the maximum number of simultaneous onscreen colors to 32.  --- 
lets you work with 12-bit images such as the RGBN files produced by Turbo 
Silver that on most computers would require an eight-megabyte frame buffer to
display.  It does so by virtue of the special Amiga graphics mode called Hold
And Modify, or HAM for short.

In traditional register-mapped computer displays, the value of a pixel in the
screen bitmap determines the hardware register from which the pixel's color 
value is fetched, so that the colors of an entire image are defined in a few
bytes of RAM.  In HAM displays, each pixel is represented by six bits in the
screen bitmap.  Since Hi-res displays are limited to a maximum of four 
bitplanes, HAM displays must be 320 pixels wide maximum.  In HAMPlus mode, if
the first two bits are both zero, the pixel's color value is obtained from 
one of the 16 color registers. However, if either or both of the first two 
bits is set to one, the interpretation process is quite different.  First the
pixel's value is copied over from the one immediately to its left.  Then one
of the pixel's three color components--red, green, or blue--is set to the 
value of the pixel's lower four bits, a number between 0 and 15.  If the 
first two bits are 01, the blue component is modified.  If they're set to 10,
the red bits of the pixel to the left are replaced.  And if they're set to
11, the green component is altered.

Thus the term Hold and Modify.  Your color selection is extended from the 
normal 32 colors by allowing you to copy (Hold) two of the three color values
used by the preceding pixel and to change (Modify) the other value to the new
value specified by the lower four bits of the pen number.

There are a few limitations to the HAM mode that you should be aware of.  
When you're only using HAM colors on the screen, since you can change only 
one color component per pixel, it make take as many as three pixels to change
completely from one color to another, which can sometimes causes an 
undesirable fringing effect.  However, use of the HAMPlus mode with careful
palette selection can overcome this to a great extent.  Also, since each
pixel's value is usually determined by its neighbor to the left, changing a
pixel can alter all pixels to its right on the scan line.


What is Extra Halfbrite?

Extra Halfbrite is another special graphics mode available with --- for
digitizing and painting that gives you 64 colors to work with.  EHB is an 
extension of the Lo-res mode, and can only be used with Amiga 500's and 
2000's and later models.  Some late-model 1000's are also equipped for this
mode.  If you own an earlier 1000 consult your retailer for an upgrade.

The first half of EHB's 64-color palette is determined the same way Lo-res's
32-color palette.  That is, you're free to choose any and all from the 
Amiga's 4096-color palette.  The second set of 32 colors is created 
automatically by the Amiga, based on your selection of the first 32, each one
at half the brightness of its counterpart in the first set.  If you change a
palette color, its counterpart also changes.  You can use the extra 32 colors
anywhere on the screen, but you can't set them independently.