Tutorial: persistent camera settings (was Re: Checking, no rush, on my open questions)

[Julien Letessier]

On 6 Sep 2004, at 20:50, Hank Roberts wrote:

> Unibrain cameras:
>   Their support (finally) told me their  "fire-i" camera settings last 
> only as long as the application changing them is open.  Unibrain makes 
> a Windows app that supposedly saves and reloads settings, but again 
> the settings are lost when that app closes.  No other app can save 
> settings and reapply them.

Hi all,

This seems to be a recurrent question, and the aforementioned lack of 
persistence is true for several other cameras in addition to the fire-i 
(e.g. some Sonys and Marlins, and at least one of my ToUcams).

So here's a rough sketch the solution I use for persistence of camera 
settings.

I'm not saying that this is the one way to go, as this obviously works 
only if you're using the SG (as I don't really dig VDigs), like in the 
minimung, mungrab, sonofmungrab examples.

* saving settings:

	- call SGSettingsDialog() or anything you use to modify the settings
	- use SGGetChannelSettings() to put all settings into a UserData
	- use PutUserDataIntoHandle() to serialize the UserData into a Handle
	- use CFDataCreate() to transform you memory chunk into a CFDataRef
	- use CFPreferencesSetAppValue(theKey, theCFSettings, 
kCFPreferencesCurrentApplication)
	  to associate theKey to your settings [1], for the current application
	- don't forget to CFPreferencesAppSynchronize()
	  to save to the .plist file

	The tasks of locking/unlocking handles, and CFRelease'ing the right 
stuff, if left as an exercice to the reader ;)

	Once all's done settings are stored in the dictionary entry you 
specified (theKey), in the application's [2] preferences file (that's 
something like $HOME/Library/Preferences/com.gizmo.MyNiftyApp.plist)

* restoring settings

	Basically it's the same, the other way around.

	- CFPreferencesCopyAppValue(theKey, kCFPreferencesCurrentApplication)
	  fetches back your binary data from the .plist preferences file
	- PtrToHand(CFDataGetBytePtr((CFDataRef)theCFSettings)
	  turns it back into a handle
	- NewUserDataFromHandle() gives you the UserData
	- finally, SGSetChannelSettings() restores the settings

	This should be done pretty soon after calling SGNewChannel(), while 
the SG isn't previewing/recording (yet).

[1] I recommend you use a key built from the DeviceName and the 
InputName. Otherwise you're going to obtain very weird, mixed up 
settings when plugging in a different camera, possible even ruining a 
sensitive CCD. Don't laugh.

[2] This means each { user, application, camera } combination gets 
unique persistent settings. This may or may not be what you're looking 
for; in the latter case, tinkering with the options of 
CFPreferencesSetAppValue() / CFPreferencesCopyAppValue() may provide 
other combinations (e.g. user / host / camera).

G'd night,
-- 
Julien T. Letessier
PhD Intern                   /\/\
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