[sane-devel] SANE2 proposal

[Nick Lamb]

On Mon, Apr 15, 2002 at 11:45:41PM +0200, abel deuring wrote:
> I would understand "gray:12" as meaning "gray scale data with 12 bit
> values", but we allow only 1 and n*8 as depth. Or should "gray:12" mean
> the real bit depth of the scanner's ADC?

It took me a moment to understand this too, but the intention here (which
may need more supporting text) is to report the true dynamic range in
bits, so as you said, a scanner with a 12-bit ADC would set this to
"gray:12" even though it would be sending 16 bits per sample.

Frontends can ignore this parameter and get excellent results, but if
for some reason (e.g. file formats) they care about the real depth of
the data they can discover it in this manner.

e.g. an appropriate use for this information is to write it into the
PNG significant bits structure 'sBIT' when saving a PNG.

That reminds me, the SANE 2 specification needs a "note to developers"
that mentions the need to copy high bits down when converting samples
from a lower bit depth to a higher one e.g.

12 bit  >  16 bit

0x000      0x0000
0x012      0x0120
0x5FE      0x5FE5
0xFED      0xFEDF
0xFFF      0xFFFF

This was explained previously on the SANE list but we can't expect
every backend author to go back and read the entire history of the
list, so a mention in the documentation seems appropriate.

Nick.