Extra Half Bright mode
The colour of every point on the screen is governed by a value held in one of the Amiga's colour
registers. Each register can be loaded from a selection of 4096 different colours.
There is no point in wasting the computer's memory with dozens of available colours, if only two of them are going to be employed for some simple text. On the other hand, there is no point being restricted to 16 or 32 colours if images need to be as realistic as possible. There are two special screen modes that change the number of colours for use, Extra Half Bright mode (EHB), and Hold And Modify mode (HAM).
Extra Half Bright mode doubles the number of available colours to 64. This is achieved by creating two colours from each of the Amiga's 32 colour registers. Colour numbers 0 to 31 are loaded straight from one of the colour registers, as normal. But the EHB mode creates an extra set of colours alongside the originals, by looking at their values and dividing them in half. This makes the new set of colours exactly half as bright as the originals. The new set of colours uses index numbers from 32 to 63.
Obviously, you can take full advantage of EHB by loading the 32 colour registers with the brightest colours available, so that pastel shades are generated automatically. Alternatively, if you needed to create specialised graphics, like an old-fashioned photograph for example, you might want to restrict the 32 colour registers to reds, greys and browns.
Using EHB mode makes no difference at all to any other parts of your programming, and EHB screens are treated in exactly the same way as the default screen. It is also possible to create Bobs in this mode. Here is a simple example of EHB colours.
E> Screen Close 0
Screen Open 2,320,167,64,Lowres : Flash Off
For C=1 To 32
Ink C
Bar 0,(C-1)*5 To 160,(2+C-1)*5
Ink C+32
Bar 160,(C-1)*5 To 319,(2+C-1)*5
Next C
Hold And Modify mode
For an artist to carry around 4096 tubes of different coloured paint would be expensive and
stupid, so an artist makes use of common colours, and mixes them together to create the exact
shade needed. Computers use exactly the same process, allowing the programmer to hold on to
an existing colour and modify it very slightly, time and time again. This is the theory behind the
Amiga's Hold And Modify (HAM) mode.