This is a screen mode where the Amiga could display 64 colors on screen.

The primary limitation was that, despite being able to display 6 bitplanes (2^6 = 64 colors), the Amiga only had 32 color registers to define the color palette used for that sceen. This would have meant that only half of the palette would have been changeable. The other half would have had to be predefined or otherwise overloaded.

Creating shadows

To automatically define the upper 32 colors it was decided that these colors would be identical to the lower 32, only half as bright. This was very easy to calculate for the Amiga by simply right shifting the color values by one bit.

Dec.	Hex.	Bin.
000	00	00000000
001	01	00000001
002	02	00000010
003	03	00000011
004	04	00000100
005	05	00000101
006	06	00000110
007	07	00000111
008	08	00001000
009	09	00001001
010	0A	00001010
011	0B	00001011
012	0C	00001100
013	0D	00001101
014	0E	00001110
015	0F	00001111
016	10	00010000

If you look closely at the numbers it becomes apparent that each time a new bit is included in the number the value doubles. It is therefore very easy to halve a value by simply shifting all bits one bit to the right.

The bit on the right will cannot be shifted further to the right and will be simply dropped. This is actually what would become a fraction in the decimal number base when halving an odd number. In binary there are no fractions, therefore these values are simply rounded down.

For example:

The Amiga uses color values from 0 (0%) to 15 (100%) for red, green and blue.

This yellow color has the rgb values 15, 15, 4.

Full color			Half color
Dec	Hex	Bin	Dec	Hex	Bin
15	0F	00001111	7	07	00000111
15	0F	00001111	7	07	00000111
4	04	00000100	2	02	00000010

The half bright color would be this dark yellow with the rgb values 7, 7, 2.

Note: I formatted the numbers as bytes (8 bits) but the OCS Amigas only used nibbles (4 bits) per red, green and blue value. This only makes a difference when working with the values inside the computer. The basic math principles stay the same.

Uses

This was a great method to create shadow effects as the sixth bitplane would always make everything half as bright as it was without it.

Only few games utilised this feature probably because using six bitplanes was putting quite a strain on the graphics memory and Blitter and it was deemed to expensive for the result that was achieved.

Getting the most out of it

Deluxe Paint IV used this default palette for EHB images. It is clearly visible that the right half of the palette is only half as bright as the left half.

While being nice for shadow effects it was a very different matter if you wanted to get the most out of all 64 colors.

The Deluxe Paint palette is a good example for this. It may not be the prettiest selection, but if we rearrange the colors it becomes clear that it is acutally quite smart.

By placing all bright colors in the lower half of the palette these colors are then available in the upper half of the palette as dark versions. This enables the artist to have full gradient of a color despite only being able to define half of its colors.

This is extremely difficult to pull off because in reality not all palettes can be split up to utilise this feature.

The loading screens in Agony are a prime example of EHB images. The amount of colors and gradients used in the image is simply astonishing and must belong to the best that have been done on OCS Amigas.