QGIS Modeler if/else condition, custom script for Modeler

Question

I want to make a simple if/else function inside my Model and I wanted to know if it's possible or not.

I want this thing to do that if an Input for 'Segment Layer' is selected that it doesn't have to execute the 'Segmentation(meanshift)' and uses 'Segment Layer' instead as an Input for the next Process.

the Inputs for the 'Segmentation(meanshift)'are as follows

Update after trying to implement Joseph's answer: I used the following code and got this error

Update 2, after replacing the parameters I get an error in line 28 this is were 'else:' stands

##Example=name
##BGREN=optional raster
##Segment_Layer=optional vector
##output=output vector

if BGREN is not None:
    processing.runalg('otb:segmentationmeanshift', \
    -BGREN\
    -0\
    -70\
    -70\
    -0.1\
    -100\
    -100\
    -0\
    -0\
    -0\
    -'True'\
    -'False'\
    -1\
    -0.1\
    -'Layer'\
    -'DN'\
    -1024\
    -1\
    -0\
    -output)
else:
    output=Segment_Layer

Update it works :) after a few try's and with Joseph help this code now works like a charm, the final trick was to replace two optional values with 'None'.

##Example=name
##BGREN=optional raster
##Segment_Layer=optional vector
##output=output vector

if BGREN is not None:
    processing.runalg('otb:segmentationmeanshift', \
    BGREN, \
    0, \
    70, \
    70, \
    0.1, \
    100, \
    100, \
    0, \
    0, \
    None, \
    True, \
    False, \
    1, \
    0.1, \
    'Layer', \
    'DN', \
    1024, \
    1, \
    None, \
    output)
else:
    output=Segment_Layer

Answer 1

As mentioned in my comment, a custom script may be the way to go in this situation. You can create one from:

Processing Toolbox > Scripts > Tools > Create new script

The following script sets the input parameters as optional along with an if/else statement depending on which parameter contains a layer. So if a layer was selected for BGREN, it will perform the segmentation tool and sets the output; otherwise the output will be the layer selected from Segment_Layer.

Here is a possible script:

##Example=name
##BGREN=optional raster
##Segment_Layer=optional vector
##output=output vector

if BGREN is not None:
    processing.runalg('otb:segmentationmeanshift', \
    -BGREN, \
    -filter, \
    -filter.meanshift.spatialr, \
    -filter.meanshift.ranger, \
    -filter.meanshift.thres, \
    -filter.meanshift.maxiter, \
    -filter.meanshift.minsize, \
    -mode, \
    -mode.vector.outmode, \
    -mode.vector.inmask, \
    -mode.vector.neighbor, \
    -mode.vector.stitch, \
    -mode.vector.minsize, \
    -mode.vector.simplify, \
    -mode.vector.layername, \
    -mode.vector.fieldname, \
    -mode.vector.tilesize, \
    -mode.vector.startlabel, \
    -mode.vector.ogroptions, \
    -output)
else:
    output=Segment_Layer

The parameters for the otb:segmentationmeanshift look scary! Unfortunately, I do not have Orfeo installed so cannot test this tool. However, the parameters are shown here, you just need to type them in instead of using the GUI. Once done, add the script into your modeler using BGREN and Segment_Layer as input layers:

Also, in your modeler, you may need to change the Required parameter definition of BGREN and Segment_Layer:

Then connect the output of the script to your NDVI tool.

---

I tested this to some extent using other tools which worked depending on which input layer was selected so hopefully it will work for your modeler!

---

---

EDIT:

Here is the code you used slightly modified (you need the commas to separate the parameters, True and False statements don't require quotes around them, the backward slash is just for readability but will include it as a one-liner):

##Example=name
##BGREN=optional raster
##Segment_Layer=optional vector
##output=output vector

if BGREN is not None:
    processing.runalg('otb:segmentationmeanshift', \
    BGREN, \
    0, \
    70, \
    70, \
    0.1, \
    100, \
    100, \
    0, \
    0, \
    0, \
    True, \
    False, \
    1, \
    0.1, \
    'Layer', \
    'DN', \
    1024, \
    1, \
    0, \
    output)
else:
    output=Segment_Layer

Or as a one-liner:

##Example=name
##BGREN=optional raster
##Segment_Layer=optional vector
##output=output vector

if BGREN is not None:
    processing.runalg('otb:segmentationmeanshift',BGREN,0,70,70,0.1,100,100,0,0,0,True,False,1,0.1,'Layer','DN',1024,1,0,output)
else:
    output=Segment_Layer