Bullfrog's Fusion showing semi transparent drop shadows underneath all objects.
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.
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.
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.
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|
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.
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.
Psygnosis' Agony loading screens featured the awesome art of Franck Sauer. The EHB colors are marked in the right version of the image.
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.