[sane-devel] test backend: 1 bit RGB data for grid pattern

[Henning Meier-Geinitz]

Hi,

On Tue, Sep 24, 2002 at 11:58:41PM +0200, abel deuring wrote:
> while playing with the test backend, I noticed that the "scan data" 
> produced for the grid pattern in 1 bit RGB mode differs from the output 
> for 1 bit grayscale mode. The data for gray scale mode is like:

Argh, I hate this 1-Bit RGB mode. Nobody needs it and it provides that
many opportunities to make mistakes. Bit-order, meaning of 1 and 0,
byte-interleaved versus bit-interleaved. The easiest way would be to
forbid 1 bit RGB in the standard :-)

> 
> 	... 00 00 00 0f ff ff ff f0 00 00 00 ...
> 
> while the data for 1 bit RGB mode is:
> 
> 	... 00 00 00 f0 ff ff ff 0f 00 00 00 ...
> 
> (where one number in the line above means actually "means" three bytes 
> for the three colors.)

If I remember correctly, I made the difference in bit-order
intentionally. The reason was simple: that's the way it was
implemented in xscanimage. And that's the only definition I found. I
think there was some discussion on the list about this but we camr to
no conclusion. Probably because nobody really needed that mode :-)

> I'm too tired to check the Sane API doc, instead a question: Am I right 
> that this is a bug, or do we have different bit orders for 1 bit gray 
> and RGB mode ?

It's not logical, you are right. As far as I understsand, the SANE
standard does NOT state how the bit-order for 1 Bit modes is. There is
an image for 8-bit modes that could be interpreted like "bit 7 is the
first pixel" but it's not written explicitely. That's also the order
used in pbm files. The confusion is the reason why I have added an
entry "Add some text about the meaning of bits in 1-bit modes." to the
TODO list.

> I don't think so; the fix would be to change line 173 in test-picture.c 
> from:
> 
> 	SANE_Word xfull = x * 8 / 3 + x1;
> 
> to:
> 
> 	SANE_Word xfull = x * 8 / 3 + (7 - x1);

Ok. But then we should at least also change xscanimage. If I remember
correctly, quiteinsane was also able to display these modes. Xsane
doesn't like them at least when using the preview which is ok.
However, it prints that it can't display 16 bits/color :-)

Bye,
  Henning