Flash MX 2004 Games : Art to ActionScript

Table 6.1 Number of colours
possible with different bit counts
Bits Colours
1 2
4 16
8 256
12 4096
16 65536
24 16777216
Chapter 6: Creating artwork for mobile devices
Figure 6.5 Specifying a colour using hexadecimal
Notice that a 24-bit device has over 16 million different possible colours while a 12-bit
device has only just over 4000, rather than half of 16 million. But we are developing using
a 24-bit environment, so how does this conversion occur? On a 24-bit machine there is 8bit
resolution for the red, green and blue colours. See Chapter 12 for more information on
how colours are specified. Eight bits gives a total of 256 different values for each channel.
On a 12-bit device there is only 4-bit resolution per channel or 16 different options. When
doing the conversion the bottom four bits of each channel of the 24-bit image are effectively
set to 0.
Take a look at Figure 6.5, which shows the Flash MX 2004 colour mixer dialog. In the
bottom left there is an input field where a colour can be specified using numbers and letters.
This combination of numbers and letters is a hexadecimal representation. Each number or letter
corresponds to 4 bits of data. The six different letters or numbers specify the 24-bit image using
the following code ‘#RRGGBB’, starting from the left the first two are the red channel then the
green and finally the blue. The numbers go in the following order 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F
with F being the maximum possible value for 4 bits, i.e. the same as decimal 15. To move from
24-bit colour space to a 12-bit colour space, every second value must be zero. As far as a 12-bit
device is concerned, the colour #876543 is the same as #806040. Try entering some values in
Flash and comparing the difference on a 24- or 32-bit monitor. There is a significant difference.
But the main place where the conversion will be noticeable is when using gradients. All gradients
will appear significantly banded on a 12-bit machine. Take a look at Figure 6.6; here you can see
a Nokia 3650 displaying a banded gradient and a smooth gradient.
The smooth gradient was achieved by converting a 24-bit bitmap of the gradient to 12-bit using
dithering. At this small size the dithering is not noticeable and is a suitable way of converting an
image if a gradient is essential. You are advised, however, to avoid gradients where possible. You
will also find that fades and transparency will affect the performance on the mobile device. Again
it may be better to avoid alpha transparency and to reveal new content using wipes rather than
fades.
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