diff --git a/python/plugins/processing/algs/gdal/GdalAlgorithmDialog.py b/python/plugins/processing/algs/gdal/GdalAlgorithmDialog.py
index a3093d76c77..6097ef8edff 100644
--- a/python/plugins/processing/algs/gdal/GdalAlgorithmDialog.py
+++ b/python/plugins/processing/algs/gdal/GdalAlgorithmDialog.py
@@ -78,7 +78,8 @@ class GdalParametersPanel(ParametersPanel):
         self.parametersHaveChanged()
 
     def connectParameterSignals(self):
-        for w in list(self.widgets.values()):
+        for wrapper in list(self.wrappers.values()):
+            w = wrapper.widget
             if isinstance(w, QLineEdit):
                 w.textChanged.connect(self.parametersHaveChanged)
             elif isinstance(w, QComboBox):
diff --git a/python/plugins/processing/core/GeoAlgorithm.py b/python/plugins/processing/core/GeoAlgorithm.py
index f7bd2360490..8350d11e964 100644
--- a/python/plugins/processing/core/GeoAlgorithm.py
+++ b/python/plugins/processing/core/GeoAlgorithm.py
@@ -16,9 +16,6 @@
 *                                                                         *
 ***************************************************************************
 """
-from builtins import str
-from builtins import object
-
 
 __author__ = 'Victor Olaya'
 __date__ = 'August 2012'
@@ -38,6 +35,9 @@ from qgis.PyQt.QtCore import QCoreApplication, QSettings
 
 from qgis.core import QgsVectorLayer, QgsRasterLayer
 
+from builtins import str
+from builtins import object
+from processing.gui.ParametersPanel import ParametersPanel
 from processing.core.ProcessingLog import ProcessingLog
 from processing.core.ProcessingConfig import ProcessingConfig
 from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
@@ -46,10 +46,8 @@ from processing.core.parameters import ParameterRaster, ParameterVector, Paramet
 from processing.core.outputs import OutputVector, OutputRaster, OutputTable, OutputHTML, Output
 from processing.algs.gdal.GdalUtils import GdalUtils
 from processing.tools import dataobjects, vector
-from processing.tools.system import setTempOutput
 from processing.algs.help import shortHelp
 
-
 class GeoAlgorithm(object):
 
     def __init__(self):
@@ -134,6 +132,10 @@ class GeoAlgorithm(object):
         """
         pass
 
+
+    def getParametersPanel(self, parent):
+        return ParametersPanel(parent, self) 
+    
     def getCustomParametersDialog(self):
         """If the algorithm has a custom parameters dialog, it should
         be returned here, ready to be executed.
@@ -183,6 +185,12 @@ class GeoAlgorithm(object):
         calling from the console.
         """
         return None
+    
+    def processBeforeAddingToModeler(self, alg, model):
+        """Add here any task that has to be performed before adding an algorithm
+        to a model, such as changing the value of a parameter depending on value
+        of another one""" 
+        pass 
 
     # =========================================================
 
diff --git a/python/plugins/processing/core/parameters.py b/python/plugins/processing/core/parameters.py
index 8f822e3af80..68e3ffd03df 100644
--- a/python/plugins/processing/core/parameters.py
+++ b/python/plugins/processing/core/parameters.py
@@ -123,7 +123,6 @@ class Parameter(object):
 
         self.optional = parseBool(optional)
 
-        # TODO: make deep copy and deep update
         self.metadata = deepcopy(self.default_metadata)
         self.metadata.update(deepcopy(metadata))
 
@@ -179,7 +178,7 @@ class Parameter(object):
     def wrapper(self, dialog, row=0, col=0):
         wrapper = self.metadata.get('widget_wrapper', None)
         # wrapper metadata should be a class path
-        if isinstance(wrapper, str):
+        if isinstance(wrapper, basestring):
             tokens = wrapper.split('.')
             mod = __import__('.'.join(tokens[:-1]), fromlist=[tokens[-1]])
             wrapper = getattr(mod, tokens[-1])
@@ -362,7 +361,10 @@ class ParameterExtent(Parameter):
             return False
 
     def getValueAsCommandLineParameter(self):
-        return '"' + str(self.value) + '"'
+        if self.value is not None:
+            return '"' + str(self.value) + '"'
+        else:
+            return str(None)
 
     def getAsScriptCode(self):
         param_type = ''
@@ -582,8 +584,8 @@ class ParameterMultipleInput(ParameterDataObject):
 
     exported = None
 
-    def __init__(self, name='', description='', datatype=-1, optional=False):
-        ParameterDataObject.__init__(self, name, description, None, optional)
+    def __init__(self, name='', description='', datatype=-1, optional=False, metadata={}):
+        ParameterDataObject.__init__(self, name, description, None, optional, metadata=metadata)
         self.datatype = int(float(datatype))
         self.exported = None
         self.minNumInputs = 0
@@ -1133,8 +1135,8 @@ class ParameterString(Parameter):
     ESCAPED_NEWLINE = '\\n'
 
     def __init__(self, name='', description='', default=None, multiline=False,
-                 optional=False, evaluateExpressions=False):
-        Parameter.__init__(self, name, description, default, optional)
+                 optional=False, evaluateExpressions=False, metadata={}):
+        Parameter.__init__(self, name, description, default, optional, metadata)
         self.multiline = parseBool(multiline)
         self.evaluateExpressions = parseBool(evaluateExpressions)
 
diff --git a/python/plugins/processing/gui/AlgorithmDialog.py b/python/plugins/processing/gui/AlgorithmDialog.py
index 052b6e2cdfa..01915df38ba 100644
--- a/python/plugins/processing/gui/AlgorithmDialog.py
+++ b/python/plugins/processing/gui/AlgorithmDialog.py
@@ -39,7 +39,6 @@ from processing.core.ProcessingLog import ProcessingLog
 from processing.core.ProcessingConfig import ProcessingConfig
 
 from processing.gui.BatchAlgorithmDialog import BatchAlgorithmDialog
-from processing.gui.ParametersPanel import ParametersPanel
 from processing.gui.AlgorithmDialogBase import AlgorithmDialogBase
 from processing.gui.AlgorithmExecutor import runalg, runalgIterating
 from processing.gui.Postprocessing import handleAlgorithmResults
@@ -63,7 +62,7 @@ class AlgorithmDialog(AlgorithmDialogBase):
 
         self.alg = alg
 
-        self.setMainWidget(ParametersPanel(self, alg))
+        self.setMainWidget(alg.getParametersPanel(self))
 
         self.bar = QgsMessageBar()
         self.bar.setSizePolicy(QSizePolicy.Minimum, QSizePolicy.Fixed)
@@ -99,14 +98,17 @@ class AlgorithmDialog(AlgorithmDialogBase):
         for output in outputs:
             if output.hidden:
                 continue
-            output.value = self.mainWidget.valueItems[output.name].getValue()
+            output.value = self.mainWidget.outputWidgets[output.name].getValue()
             if isinstance(output, (OutputRaster, OutputVector, OutputTable)):
                 output.open = self.mainWidget.checkBoxes[output.name].isChecked()
 
         return True
 
-    def setParamValue(self, param, wrapper, alg=None):
-        return param.setValue(wrapper.value())
+    def setParamValue(self, param, wrapper):
+        if wrapper.widget:
+            return param.setValue(wrapper.value())
+        else:
+            return True
 
     def checkExtentCRS(self):
         unmatchingCRS = False
@@ -131,7 +133,7 @@ class AlgorithmDialog(AlgorithmDialogBase):
                             if p.crs() != projectCRS:
                                 unmatchingCRS = True
             if isinstance(param, ParameterExtent):
-                value = self.mainWidget.valueItems[param.name].leText.text().strip()
+                value = self.mainWidget.wrappers[param.name].widget.leText.text().strip()
                 if value:
                     hasExtent = True
 
diff --git a/python/plugins/processing/gui/ExtentSelectionPanel.py b/python/plugins/processing/gui/ExtentSelectionPanel.py
index 6ec0d34208a..4ff826716c4 100644
--- a/python/plugins/processing/gui/ExtentSelectionPanel.py
+++ b/python/plugins/processing/gui/ExtentSelectionPanel.py
@@ -59,7 +59,7 @@ class ExtentSelectionPanel(BASE, WIDGET):
         if self.param.optional:
             if hasattr(self.leText, 'setPlaceholderText'):
                 self.leText.setPlaceholderText(
-                    self.tr('[Use "auto" to use min covering extent]'))
+                    self.tr('[Leave blank to use min covering extent]'))
 
         self.btnSelect.clicked.connect(self.selectExtent)
 
@@ -104,7 +104,7 @@ class ExtentSelectionPanel(BASE, WIDGET):
         popupmenu.exec_(QCursor.pos())
 
     def useMinCoveringExtent(self):
-        self.leText.setText('auto')
+        self.leText.setText('')
 
     def useLayerExtent(self):
         CANVAS_KEY = 'Use canvas extent'
diff --git a/python/plugins/processing/gui/ParametersPanel.py b/python/plugins/processing/gui/ParametersPanel.py
index 20ed8b3e493..8bd9b6c0190 100644
--- a/python/plugins/processing/gui/ParametersPanel.py
+++ b/python/plugins/processing/gui/ParametersPanel.py
@@ -67,10 +67,9 @@ class ParametersPanel(BASE, WIDGET):
 
         self.parent = parent
         self.alg = alg
-        self.valueItems = {}
         self.wrappers = {}
+        self.outputWidgets = {}
         self.labels = {}
-        self.widgets = {}
         self.checkBoxes = {}
         self.dependentItems = {}
         self.iterateButtons = {}
@@ -102,41 +101,39 @@ class ParametersPanel(BASE, WIDGET):
 
             wrapper = self.getWidgetWrapperFromParameter(param)
             self.wrappers[param.name] = wrapper
-            self.valueItems[param.name] = wrapper.widget
             widget = wrapper.widget
 
-            if isinstance(param, ParameterVector):
-                layout = QHBoxLayout()
-                layout.setSpacing(2)
-                layout.setMargin(0)
-                layout.addWidget(widget)
-                button = QToolButton()
-                icon = QIcon(os.path.join(pluginPath, 'images', 'iterate.png'))
-                button.setIcon(icon)
-                button.setToolTip(self.tr('Iterate over this layer'))
-                button.setCheckable(True)
-                layout.addWidget(button)
-                self.iterateButtons[param.name] = button
-                button.toggled.connect(self.buttonToggled)
-                widget = QWidget()
-                widget.setLayout(layout)
-
-            tooltips = self.alg.getParameterDescriptions()
-            widget.setToolTip(tooltips.get(param.name, param.description))
-
-            label = QLabel(desc)
-            # label.setToolTip(tooltip)
-            self.labels[param.name] = label
-            if param.isAdvanced:
-                self.layoutAdvanced.addWidget(label)
-                self.layoutAdvanced.addWidget(widget)
-            else:
-                self.layoutMain.insertWidget(
-                    self.layoutMain.count() - 2, label)
-                self.layoutMain.insertWidget(
-                    self.layoutMain.count() - 2, widget)
-
-            self.widgets[param.name] = widget
+            if widget is not None:
+                if isinstance(param, ParameterVector):
+                    layout = QHBoxLayout()
+                    layout.setSpacing(2)
+                    layout.setMargin(0)
+                    layout.addWidget(widget)
+                    button = QToolButton()
+                    icon = QIcon(os.path.join(pluginPath, 'images', 'iterate.png'))
+                    button.setIcon(icon)
+                    button.setToolTip(self.tr('Iterate over this layer'))
+                    button.setCheckable(True)
+                    layout.addWidget(button)
+                    self.iterateButtons[param.name] = button
+                    button.toggled.connect(self.buttonToggled)
+                    widget = QWidget()
+                    widget.setLayout(layout)
+    
+                tooltips = self.alg.getParameterDescriptions()
+                widget.setToolTip(tooltips.get(param.name, param.description))
+    
+                label = QLabel(desc)
+                # label.setToolTip(tooltip)
+                self.labels[param.name] = label
+                if param.isAdvanced:
+                    self.layoutAdvanced.addWidget(label)
+                    self.layoutAdvanced.addWidget(widget)
+                else:
+                    self.layoutMain.insertWidget(
+                        self.layoutMain.count() - 2, label)
+                    self.layoutMain.insertWidget(
+                        self.layoutMain.count() - 2, widget)
 
         for output in self.alg.outputs:
             if output.hidden:
@@ -152,8 +149,7 @@ class ParametersPanel(BASE, WIDGET):
                 check.setChecked(True)
                 self.layoutMain.insertWidget(self.layoutMain.count() - 1, check)
                 self.checkBoxes[output.name] = check
-            self.valueItems[output.name] = widget
-
+            self.outputWidgets[output.name] = widget
         for wrapper in list(self.wrappers.values()):
             wrapper.postInitialize(list(self.wrappers.values()))
 
diff --git a/python/plugins/processing/gui/wrappers.py b/python/plugins/processing/gui/wrappers.py
index 7cdff13a281..e4309e02380 100644
--- a/python/plugins/processing/gui/wrappers.py
+++ b/python/plugins/processing/gui/wrappers.py
@@ -831,7 +831,7 @@ class StringWidgetWrapper(WidgetWrapper):
             else:
                 text = self.widget.getValue()
             return text
-        if self.dialogType == DIALOG_BATCH:
+        elif self.dialogType == DIALOG_BATCH:
             return self.widget.text()
         else:
             if self.param.multiline:
diff --git a/python/plugins/processing/modeler/ModelerParametersDialog.py b/python/plugins/processing/modeler/ModelerParametersDialog.py
index 0c4df307f36..d5dc14f2c27 100644
--- a/python/plugins/processing/modeler/ModelerParametersDialog.py
+++ b/python/plugins/processing/modeler/ModelerParametersDialog.py
@@ -146,20 +146,21 @@ class ModelerParametersDialog(QDialog):
             self.wrappers[param.name] = wrapper
 
             widget = wrapper.widget
-            self.valueItems[param.name] = widget
-            if param.name in list(tooltips.keys()):
-                tooltip = tooltips[param.name]
-            else:
-                tooltip = param.description
-            label.setToolTip(tooltip)
-            widget.setToolTip(tooltip)
-            if param.isAdvanced:
-                label.setVisible(self.showAdvanced)
-                widget.setVisible(self.showAdvanced)
-                self.widgets[param.name] = widget
-
-            self.verticalLayout.addWidget(label)
-            self.verticalLayout.addWidget(widget)
+            if widget:
+                self.valueItems[param.name] = widget
+                if param.name in list(tooltips.keys()):
+                    tooltip = tooltips[param.name]
+                else:
+                    tooltip = param.description
+                label.setToolTip(tooltip)
+                widget.setToolTip(tooltip)
+                if param.isAdvanced:
+                    label.setVisible(self.showAdvanced)
+                    widget.setVisible(self.showAdvanced)
+                    self.widgets[param.name] = widget
+    
+                self.verticalLayout.addWidget(label)
+                self.verticalLayout.addWidget(widget)
 
         for output in self._alg.outputs:
             if output.hidden:
@@ -301,34 +302,6 @@ class ModelerParametersDialog(QDialog):
             alg = self.model.algs[value.alg]
             return self.tr("'%s' from algorithm '%s'") % (alg.algorithm.getOutputFromName(value.output).description, alg.description)
 
-    def setTableContent(self):
-        params = self._alg.parameters
-        outputs = self._alg.outputs
-        visibleParams = [p for p in params if not p.hidden]
-        visibleOutputs = [p for o in outputs if not o.hidden]
-        self.tableWidget.setRowCount(len(visibleParams) + len(visibleOutputs))
-
-        for i, param in visibleParams:
-            item = QTableWidgetItem(param.description)
-            item.setFlags(Qt.ItemIsEnabled)
-            self.tableWidget.setItem(i, 0, item)
-            item = self.getWidgetFromParameter(param)
-            self.valueItems[param.name] = item
-            self.tableWidget.setCellWidget(i, 1, item)
-            self.tableWidget.setRowHeight(i, 22)
-
-        for i, output in visibleOutputs:
-            item = QTableWidgetItem(output.description + '<'
-                                    + output.__module__.split('.')[-1] + '>')
-            item.setFlags(Qt.ItemIsEnabled)
-            self.tableWidget.setItem(i, 0, item)
-            item = QLineEdit()
-            if hasattr(item, 'setPlaceholderText'):
-                item.setPlaceholderText(ModelerParametersDialog.ENTER_NAME)
-            self.valueItems[output.name] = item
-            self.tableWidget.setCellWidget(i, 1, item)
-            self.tableWidget.setRowHeight(i, 22)
-
     def setPreviousValues(self):
         if self._algName is not None:
             alg = self.model.algs[self._algName]
@@ -376,12 +349,14 @@ class ModelerParametersDialog(QDialog):
         for selected in selectedOptions:
             alg.dependencies.append(availableDependencies[selected].name)
 
+        self._alg.processBeforeAddingToModeler(alg, self.model)
         return alg
 
     def setParamValue(self, alg, param, wrapper):
         try:
-            value = wrapper.value()
-            alg.params[param.name] = value
+            if wrapper.widget:
+                value = wrapper.value()
+                alg.params[param.name] = value
             return True
         except InvalidParameterValue:
             return False