**topcua**

OPC Unified Architecture (OPC UA) is a machine to machine communication protocol for industrial automation developed by the OPC Foundation. Refer to https://en.wikipedia.org/wiki/OPC_Unified_Architecture for a detailed overview.
 
A proof-of-concept extension called '''topcua''' provides a Tcl binding to a C based OPC UA implementation from https://open62541.org/ and can be found in https://www.androwish.org/index.html/dir?name=jni/topcua

The code is very portable and might run on all Tcl supported platforms provided that the platform's C compiler supports C99.

The source tree contains an example script implementing a webcam in a few lines of code. The most interesting piece is how variables/data/things can be mapped between Tcl and OPC/UA domains as shown in the '''_lastimg''' and '''_setparm''' procedures and how the corresponding items in the OPC/UA address space are created in the '''opcua add ...''' invocations. This allows to connect from an OPC/UA client tool like https://www.unified-automation.com/products/development-tools/uaexpert.html%|%UAExpert%|% or https://www.prosysopc.com/products/opc-ua-client%|%OPC UA Client%|% and to display the camera image. 

But be aware that this is all early alpha quality and may contain memory leaks and other kinds of serious bugs.

======
# A little OPC/UA webcam in about 100 LOC
#
# Requires Linux, MacOSX, or FreeBSD, due to tcluvc support,
# but can be easily modified for Windows to use tclwmf instead.

package require Tk
package require topcua
package require tcluvc

# hide Tk toplevel
wm withdraw .

# get first available camera
set cam [lindex [uvc devices] 0]
if {$cam eq {}} {
    puts stderr "no camera found"
    exit 1
}

# open camera
if {[catch {uvc open $cam capture} cam]} {
    puts stderr "open failed: $cam"
    exit 1
}

# set format to 320x240
foreach {i fmt} [uvc listformats $cam] {
    if {[dict get $fmt "frame-size"] eq "320x240"} {
        uvc format $cam $i 1
        break
    }
}

# photo image for capture
set img [image create photo]

# image capture callback
proc capture {cam} {
    # limit frame rate, otherwise it consumes too much CPU for
    # image processing and the OPC/UA server part starves
    lassign [uvc counters $cam] all done dropped
    if {$all % 20 == 0} {
        uvc image $cam $::img
        set ::png [$::img data -format png]
    }
}

# create OPC/UA server
opcua new server 4840 S

# implementation of OPC/UA data sources
namespace eval ::opcua::S {
    # data source callback
    proc _lastimg {node op {value {}}} {
        if {$op eq "read"} {
            return [list ByteString $::png]
        }
        # hey, this is a camera, not a screen
        return -code error "write shouldn't happen"
    }
    # data source callback
    proc _setparm {name node op {value {}}} {
        if {$op eq "read"} {
            array set p [uvc parameter $::cam]
            set v 0
            if {[info exists p($name)]} {
                set v $p($name)
            }
            return [list Int32 $v]
        }
        set v [dict get $value "value"]
        catch {uvc parameter $::cam $name $v}
        return {}
    }
}

# create our namespace in OPC/UA land
set ns [opcua add S Namespace LilWebCam]

# get Objects folder
set OF [lindex [opcua translate S [opcua root] / Objects] 0]

# create an object in our namespace in Objects folder
set obj [opcua add S Object "ns=$ns;s=LilWebCam" $OF Organizes "$ns:LilWebCam"]

# create some variables in our folder to deal with camera settings
set att [opcua attrs default VariableAttributes]
dict set att dataType [opcua types nodeid Int32]
dict set att accessLevel 3        ;# writable
foreach name {brightness contrast gain gamma hue saturation} {
    opcua add S Variable "ns=$ns;s=[string totitle $name]" $obj HasComponent "$ns:[string totitle $name]" {} $att [list ::opcua::S::_setparm $name]
}

# get node identifier of Image data type, a subtype of ByteString
set IT [lindex [opcua translate S [opcua root] / Types / DataTypes / BaseDataType / ByteString / Image] 0]

# create variable in our folder to return last photo image
set att [opcua attrs default VariableAttributes]
dict set att dataType $IT        ;# Image data type
dict set att valueRank -1        ;# 1-dimensional array
opcua add S Variable "ns=$ns;s=Image" $obj HasComponent "$ns:Image" {} $att ::opcua::S::_lastimg

# start server using Tk's event loop
opcua start S

# start camera
uvc start $cam 
======


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