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2001.03.29 16:57 "Re: Bit depth", by Chris Hanson
Martí Maria wrote:
> > What is required is a left shift of 5 places which is equivalent to
> > multiplication by 65536/2048=32.
>
> Nope, think it carefully... try those values
>
> Max value of 11 bits: 2047 = 0x7FF, if you apply a
> left shif of 5 you got 65504=0xFFe0, while you should
> obtain maximum 16 bit value: 0xFFFF
>
> This is a frequent mistake when converting from 8 to 16 bits,
> many implementations I've seen does simply shift left 8, while
> they should multiply by 257
Marti is basically right about preserving the upper range.
A trick that was suggested in the Amiga IFF documentation
when converting to higher bit depths, was to replicate the
higher-order bits into the unused low-order bits.
If your original value was 0x0fcb (12-bits lower used)
you could convert this to either 0xfcbb or 0xfcbf, by grabbing
either the highest nibble (4-bits) or the lowest nibble in
the original value. In this way, as the original value
approaches 0 or 0xfff, the 'fluff' bits that you are adding to
complete the range will also properly scale to 0 or f.
I'm not really clear on whether using the highest or lowest
nibble is better. Probably depends on the characteristics of
your original data.
This operation can be done efficiently with masks and shifts,
no floating-point multiplication or division is necessary.
Chris - Xenon
--
Chris Hanson | Xenon@3DNature.com | I've got friends in low latitudes!
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