On Thursday, December 23, 2010 10:38:31 Laurent Pinchart wrote:
> Hi Hans,
>
> On Thursday 23 December 2010 10:31:19 Hans Verkuil wrote:
> > On Thursday, December 23, 2010 10:19:38 Laurent Pinchart wrote:
> > > On Wednesday 15 December 2010 11:03:37 Hans Verkuil wrote:
> > > > > On Wednesday 15 December 2010 08:57:29 Hans Verkuil wrote:
> > > > >
> > > > > Hence my question, should we add a way to pass rectangles (basically
> > > > > a structv4l2_rect) through the control ioctls ? It would make sense.
> > > >
> > > > I thought it over and came to the conclusion that we should not do
> > > > that. Instead we can create four separate controls.
> > >
> > > That's not very clean, is it ?
> >
> > Why not? It's perfectly consistent with the way controls work.
>
> Because it multiplies the number of required controls by 4. There's also no
> way for applications to know that the controls are grouped together.
4 values, 4 controls. Simple.
> > > > The problem we run into when adding more complex types is that we can
> > > > no longer communicate min and max values (something that we definitely
> > > > want when dealing with coordinates).
> > >
> > > Why not ? We should still support querying min/max/def values on a struct
> > > v4l2_ctrl. This would of course require an extended queryctrl ioctl.
> >
> > Which will make apps even more complicated. I thought about this as well,
> > but it's a road that will lead to chaos.
>
> Sorry, I don't see why a v4l2_rect type will lead to chaos. That doesn't mean
> we need to support any data structure as control data, just a very limited
> number V4L2 standard ones.
For every new control type applications will have to add support for that.
Especially for GUIs that is actually a lot of work. Do you want to take on the
job of modifying all V4L2 apps whenever you add a new type? I don't. Adding new
controls is easy and cheap, adding new types is hard and difficult.
The STRING and INTEGER64 types were the first types added in a long time (and
I'm sorry I added INTEGER64 since it is actually not used by any driver at the
moment). And it was a lot of work adding string support to v4l2-ctl and qv4l2.
And I think these are still the only apps in existence that support these two
types.
> > > > Another reason is how the control mechanism is designed: they only
> > > > support the basic types (int, bool, string, enum, int64 and a 'button'
> > > > aka action). And the controls are grouped into classes which are named
> > > > through the 'ctrl_class' control.
> > > >
> > > > So effectively controls represent a field in a class (or struct) and
> > > > each class can be presented as a tab page in a control panel.
> > > >
> > > > Simple and straightforward.
> > > >
> > > > If we start to add complex types, then it becomes really hard to define
> > > > the meta data of the control since you are really defining a
> > > > 'mini-class'.
> > >
> > > I agree that arbitrary complex controls might not be a good idea, but
> > > v4l2_rect is a pretty standard data structure in V4L2 and something that
> > > can be useful for different controls.
> > >
> > > > It sounds nice initially, but we really should not do this since I
> > > > believe it will lead to chaos later on. You want complex types, then
> > > > use ioctls, not controls. Or split up the complex type into multiple
> > > > simple types.
> > >
> > > Then let's use an ioctl for focus control. I don't like it.
> >
> > I would vote for 4 controls. Since FOCUS_MODE_AUTO is a control it makes no
> > sense to make an ioctl to set the rectangle. That would be a strange mix.
> >
> > And having 4 controls for the rectangle will actually look good in GUIs,
>
> Applications will display the controls without knowing they are related.
> Having a v4l2_rect type would actually allow applications to display the
> control better.
As I said, are you going to add support for this in all apps? Especially since
many apps just show a list of controls, so if you just ensure that the four
rect controls come one after the other, then they will show up just fine.
> > and with S_EXT_CTRLS you can set all focus-related controls in one call.
>
> Then don't tell me that an extended queryctrl ioctl would make apps even more
> complicated, when you advocate using four separate values to change a single
> control ;-)
???
Most generic apps just enumerate controls, so 1 or 100 controls doesn't matter
to them.
Adding a new type does, though.
Just add four controls for this. It works everywhere and doesn't require any new
APIs and application changes.
Regards,
Hans
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