[sane-devel] Canon Mx 700 - Scanbuttons

Louis Lagendijk louis at fazant.net
Wed Oct 31 20:51:12 UTC 2012

On Wed, 2012-10-31 at 08:47 -0400, m. allan noah wrote:

> >> If you change the stuff, do you mind also changing the option-type of
> >> button-1/-2 to SANE_TYPE_BUTTON (it is actually SANE_TYPE_INT)?
> >>
> > I am still looking at this. For now I have been able to wrap my brain
> > around how to read a button option. Do I understand you correctly that a
> > get operation on a button returns an integer. The documentation does
> > only state that a button has no value. I see that some backends do
> > return a value for type button, others don't. GIven the docuemntation I
> > am not sure. I will wait for others to comment before I make changes
> > (they would be fairly simple).
> >
> >> Is anyone aware of similar operation (setting one option for querying
> >> another) in other backends (in that case I would consider updating
> >> scanbd to suppport such a schema as well)?
> >>
> > For the pixma backend they are not required anymore....
> >
> >
> > kind regards, Louis
> >
> Sounds like a good change. Unfortunately, the sane standard is a
> little weak on buttons, so backend authors have to guess. I would say
> that button/event reading code should not expect the type to be
> SANE_TYPE_BUTTON, as some sensors will be able to give integer values.
> allan
Hello all,
Strictly speaking accoding to the sane abi doc (sane.ps) buttons do not
have a value (Page 20: An option of this type has no value. Instead,
setting an option of this type has an option-specific side effect.
Page 21: SANE_TYPE_BUTTON, SANE_TYPE_GROUP: the option size is ignored

If a BUTTON can RETURN a value, we need to correct both places:
P20 only talks about SETTING a value, but does not discuss GETTING the
button, but P21 seems to suggest that reading is pointless.

to me this looks as if the button-type is only meant to SET a (boolean)
option and never can be read. But then it has only a subset of
characteristics of the BOOL type (it is actually a boolean with implicit
encoding: leaving it out indicates a false, sending the option, but no
value, means a true, yes I come from a background where ASN.1 is used). 

To me it still looks as if a boolean or integer encoding are  most
appropriate. So I prefer to leave the buttons as integers (although they
are now strictly speaking booleans, as I moved the original and target
parameters to own options


Kind regards, Louis

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