pyotb 1.2.2

Library to enable easy use of the Orfeo Tool Box (OTB) in Python

pyotb: a pythonic extension of OTB

Installation

Requirements:

• python >= 3.5
• OrfeoToolBox python API installed

pip install pyotb

In case you don't have pip, you can also clone the repository and use python setup.py install instead.

Quickstart: running an OTB app as a oneliner

pyotb has been written so that it is more convenient to run an application in Python.

For example, let's consider one wants to undersample a raster. Using OTB, the code would be like :

import otbApplication

input_path = 'my_image.tif'
resampled = otbApplication.Registry.CreateApplication('RigidTransformResample')
resampled.SetParameterString('in', input_path)
resampled.SetParameterString('interpolator', 'linear')
resampled.SetParameterFloat('transform.type.id.scalex', 0.5)
resampled.SetParameterFloat('transform.type.id.scaley', 0.5)
resampled.SetParameterString('out', 'output.tif')
resampled.ExecuteAndWriteOutput()

Instead, using pyotb, you can pass the parameters as a dictionary :

import pyotb

input_path = 'my_image.tif'
pyotb.RigidTransformResample({'in': input_path, 'interpolator': 'linear', 'out': 'output.tif',
                              'transform.type.id.scaley': 0.5, 'transform.type.id.scalex': 0.5})

Using Python keyword arguments

It is also possible to use the Python keyword arguments notation for passing the parameters:

pyotb.SuperImpose(inr='reference_image.tif', inm='image.tif', out='output.tif')

is equivalent to:

pyotb.SuperImpose({'inr': 'reference_image.tif', 'inm': 'image.tif', 'out': 'output.tif'})

Limitations : for this notation, python doesn't accept the parameter in or any parameter that contains a .. E.g., it is not possible to use pyotb.RigidTransformResample(in=input_path...) or pyotb.VectorDataExtractROI(io.vd=vector_path...).

In-memory connections

The big asset of pyotb is the ease of in-memory connections between apps.

Let's start from our previous example. Consider the case where one wants to apply optical calibration and binary morphological dilatation following the undersampling. Using OTB :

import otbApplication

resampled = otbApplication.Registry.CreateApplication('RigidTransformResample')
resampled.SetParameterString('in', 'my_image.tif')
resampled.SetParameterString('interpolator', 'linear')
resampled.SetParameterFloat('transform.type.id.scalex', 0.5)
resampled.SetParameterFloat('transform.type.id.scaley', 0.5)
resampled.Execute()

calibrated = otbApplication.Registry.CreateApplication('OpticalCalibration')
calibrated.ConnectImage('in', resampled, 'out')
calibrated.SetParameterString('level', 'toa')
calibrated.Execute()

dilated = otbApplication.Registry.CreateApplication('BinaryMorphologicalOperation')
dilated.ConnectImage('in', calibrated, 'out')
dilated.SetParameterString("filter", 'dilatation')
dilated.SetParameterString("structype", 'ball')
dilated.SetParameterInt("xradius", 3)
dilated.SetParameterInt("yradius", 3)
dilated.SetParameterString('out', 'output.tif')
dilated.ExecuteAndWriteOutput()

Using pyotb, you can pass the output of an app as input of another app :

import pyotb

resampled = pyotb.RigidTransformResample({'in': 'my_image.tif', 'interpolator': 'linear', 
                                          'transform.type.id.scaley': 0.5, 'transform.type.id.scalex': 0.5})

calibrated = pyotb.OpticalCalibration({'in': resampled, 'level': 'toa'}) 

pyotb.BinaryMorphologicalOperation({'in': calibrated, 'out': 'output.tif', 'filter': 'dilatation', 
                                    'structype': 'ball', 'xradius': 3, 'yradius': 3})
# equivalent to
# pyotb.BinaryMorphologicalOperation(calibrated, out='output.tif', filter='dilatation', structype='ball',
#                                    xradius=3, yradius=3)

Writing the result of an app

Any pyotb object can be written to disk using the write method, e.g. :

import pyotb

resampled = pyotb.RigidTransformResample({'in': 'my_image.tif', 'interpolator': 'linear',
                                          'transform.type.id.scaley': 0.5, 'transform.type.id.scalex': 0.5})

resampled.write('output.tif', pixel_type='uint16')

Arithmetic operations

Every pyotb object supports arithmetic operations, such as addition, subtraction, comparison... Consider an example where we want to perform the arithmetic operation image1 * image2 - 2*image3

Using OTB, the following code works for 3-bands images :

import otbApplication

bmx = otbApplication.Registry.CreateApplication('BandMathX')
bmx.SetParameterStringList('il', ['image1.tif', 'image2.tif', 'image3.tif'])  # all images are 3-bands
exp = 'im1b1*im2b1 - 2*im3b1; im1b2*im2b2 - 2*im3b2; im1b3*im2b3 - 2*im3b3'
bmx.SetParameterString('exp', exp)
bmx.SetParameterString('out', 'output.tif')
bmx.SetParameterOutputImagePixelType('out', otbApplication.ImagePixelType_uint8)
bmx.ExecuteAndWriteOutput()

With pyotb, the following works with images of any number of bands :

import pyotb

# transforming filepaths to pyotb objects
input1, input2, input3 = pyotb.Input('image1.tif'), pyotb.Input('image2.tif') , pyotb.Input('image3.tif')

res = input1 * input2 - 2 * input2
res.write('output.tif', pixel_type='uint8')

Slicing

pyotb objects support slicing in a Python fashion :

import pyotb

# transforming filepath to pyotb object
input = pyotb.Input('my_image.tif')

input[:, :, :3]  # selecting first 3 bands
input[:, :, [0, 1, 4]]  # selecting bands 1, 2 & 5
input[:1000, :1000]  # selecting 1000x1000 subset

Using OTB only, this would be more laborious :

import otbApplication

# selecting first 3 bands
extracted = otbApplication.Registry.CreateApplication('ExtractROI')
extracted.SetParameterString('in', 'my_image.tif')
extracted.SetParameterStringList('cl', ['Channel1', 'Channel2', 'Channel3'])
extracted.Execute()

# selecting 1000x1000 subset
extracted = otbApplication.Registry.CreateApplication('ExtractROI')
extracted.SetParameterString('in', 'my_image.tif')
extracted.SetParameterString('mode', 'extent')
extracted.SetParameterString('mode.extent.unit', 'pxl')
extracted.SetParameterFloat('mode.extent.ulx', 0)
extracted.SetParameterFloat('mode.extent.uly', 0)
extracted.SetParameterFloat('mode.extent.lrx', 999)
extracted.SetParameterFloat('mode.extent.lry', 999)
extracted.Execute()

Numpy-inspired functions

Some functions have been written, entirely based on OTB, to mimic the behavior of some well-known numpy functions.

pyotb.where

Equivalent of numpy.where. It is the equivalent of the muparser syntax condition ? x : y that can be used in OTB's BandMath.

import pyotb

# transforming filepaths to pyotb objects
labels, image1, image2 = pyotb.Input('labels.tif'), pyotb.Input('image1.tif') , pyotb.Input('image2.tif')

# If labels = 1, returns image1. Else, returns image2 
res = pyotb.where(labels == 1, image1, image2)  # this would also work: pyotb.where(labels == 1, 'image1.tif', 'image2.tif') 

# A more complex example
# If labels = 1, returns image1. If labels = 2, returns image2. If labels = 3, returns 3. Else 0
res = pyotb.where(labels == 1, image1,
                  pyotb.where(labels == 2, image2,
                              pyotb.where(labels == 3, 3, 0)))

pyotb.clip

Equivalent of numpy.clip. Clip (limit) the values in a raster to a range.

import pyotb

pyotb.clip('my_image.tif', 0, 255)  # clips the values between 0 and 255

pyotb.all

Equivalent of numpy.all.

For only one image, this function checks that all bands of the image are True (i.e. !=0) and outputs a singleband boolean raster. For several images, this function checks that all images are True (i.e. !=0) and outputs a boolean raster, with as many bands as the inputs.

pyotb.any

Equivalent of numpy.any.

For only one image, this function checks that at least one band of the image is True (i.e. !=0) and outputs a singleband boolean raster. For several images, this function checks that at least one of the images is True (i.e. !=0) and outputs a boolean raster, with as many bands as the inputs.

Interaction with Numpy

pyotb objects can be transparently used in numpy functions.

import pyotb
import numpy as np

input = pyotb.Input('image.tif')  # this is a pyotb object

# Creating a numpy array of noise
white_noise = np.random.normal(0, 50, size=input.shape)  # this is a numpy object

# Adding the noise to the image
noisy_image = input + white_noise  # magic: this is a pyotb object that has the same georeference as input. 
                                   # `np.add(input, white_noise)` would have worked the same
noisy_image.write('image_plus_noise.tif')

Limitations :

• The whole image is loaded into memory
• The georeference can not be modified. Thus, numpy operations can not change the image or pixel size (e.g. it is not possible to use np.pad)

Interaction with Tensorflow

We saw that numpy operations had some limitations. To bypass those limitations, it is possible to use some Tensorflow operations on pyotb objects.

You need a working installation of OTBTF >=3.0 for this and then the code is like this:

import pyotb

def scalar_product(x1, x2):
    """This is a function composed of tensorflow operations."""
    import tensorflow as tf
    return tf.reduce_sum(tf.multiply(x1, x2), axis=-1)

# Compute the scalar product
res = pyotb.run_tf_function(scalar_product)('image1.tif', 'image2.tif')  # magic: this is a pyotb object
res.write('scalar_product.tif')

For some easy syntax, one can use pyotb.run_tf_function as a function decorator, such as:

import pyotb

@pyotb.run_tf_function  # The decorator enables the use of pyotb objects as inputs/output of the function
def scalar_product(x1, x2):
    import tensorflow as tf
    return tf.reduce_sum(tf.multiply(x1, x2), axis=-1)

res = scalar_product('image1.tif', 'image2.tif')  # magic: this is a pyotb object

Advantages :

• The process supports streaming, hence the whole image is not loaded into memory
• Can be integrated in OTB pipelines

Limitations :

• It is not possible to use the tensorflow python API inside a script where OTBTF is used because of compilation issues between Tensorflow and OTBTF
• It is currently not possible to chain several @pyotb.run_tf_function functions