What is new with OTB 8.0

OSSIM is used for geometric sensor modelling and metadata parsing. It has been a dependency of the OTB since its beginning. Then adapter classes have been added to hide OSSIM headers from OTB public API. With the version 8 of OTB, it is time to remove this dependency, whose development cycle is difficult to follow. Until OTB v7, only a small portion of OSSIM was used anyway.

A new Metadata framework

Reading and writing the metadata used to be handled by OSSIM. With OTB 8.0, GDAL is in charge for this task. Like with OSSIM, this framework has a mechanism to parse the metadata read by GDAL and make them accessible to the applications. The list of supported sensors didn’t change from the previous versions. Sensors not in this list can still be used with OTB, but the metadata won’t be accessible.

Formosat

Fomosat products are based on the DIMAP file format. The user can provide either the DIMAP file or the tiff file as input of an OTB application.

Ikonos

Ikonos products use the TIFF file format. To use an Ikonos product, the user should provide one of them as input for an OTB application.

Pleiades

Pleiades products are based on the DIMAP file format. The user can provide either the DIMAP file or the tiff/jpeg2000 file as input of an OTB application. We recommend the use of the tiff/jp2000 file. Indeed, when reading the DIMAP file we noticed that the image is shifted by 0.5 pixel (this is a documented bug in GDAL).

QuickBird

QuickBird products use the TIFF file format. To use a QuickBird product, the user should provide one of them as input for an OTB application.

Spot 5, 6 & 7

Spot products are based on the DIMAP file format. The user can provide either the DIMAP file or the tiff/jpeg2000 file as input of an OTB application.

WorldView 2

WordView 2 products use the TIFF file format. To use a WordView 2 product, the user should provide one of them as input for an OTB application.

CosmoSkyMed

CosmoSkyMed products contain either HDF5 files or Tiff files. One Tiff file per polarization, or one HDF5 file containing all the data. When reading a HDF dataset, one needs to select the right subdataset using the Extended Filename &sdataidx=<(int)idx>. For example, in some CosmoSkyMed products, the first subDataset is a quicklook, and the actual product is the second subdataset:

-in “<PRODUCT_NAME>.h5?&sdataidx=2”

Radarsat 2

Radarsat2 products uses the TIFF file format. To use a Radarsat product, the user should provide one of them as input for an OTB application.

Sentinel 1

Sentinel 1 products use the TIFF file format. To use a Sentinel 1 product, the user should provide one of them as input for an OTB application.

TerraSarX

TerraSarX products contains COS files. To use a TerraSarX product, the user should provide one of them as input for an OTB application.

A new organization for the Sensor Models

OSSIM oversaw the forward and inverse transforms. It would use either an RPC model, a SAR model, or a sensor specific model.

We now use GDAL’s RPC model in OTB. After running some tests, we estimated that it was more accurate than OSSIM’s. Some localization gaps of the order of 0.01 pixel are to be expected.

We implemented a new SAR model for OTB, based on OSSIM’s. The unique, significant, difference is the way it deals with time. OSSIM uses a double precision to record times. The time needs to be accurate at the microsecond, and a double doesn’t allow such accuracy. When working on one line, this is not a problem, but the accumulation of errors on multiple lines generates a significant error which can lead to the selection of the wrong line. To fix this, we used another time library that is more precise. This means the new version of OTB provides results slightly better: a few lines will have a different value.

A new way to handle DEM

Once again, OSSIM is replaced by GDAL for this task. Many OTB application use an elevation model as input, usually with the parameter “-elev”. This parameter accepts any raster file supported by GDAL, or a directory containing such files. In this second case, all rasters from the input directory will be opened by GDAL, so it could be a good idea to use a VRT. It is the same for the geoid file, but it is important to note that in previous OTB versions (using OSSIM) it was common to use the egm96.grd file as geoid. This file cannot be opened by GDAL. However, it is still possible to use it with a little trick described in the documentation.

Status of the tests

The continuous integration platform executed all the tests on the release 8.0.0-alpha2, and they all exited with success.

Beside the automatic, non-regression tests, we also run functional tests with products and application commonly used. We were particularly cautious in testing applications related to sensor models and DEM, like the orthorectification. To perform the tests, we used Pleiades products to perform some treatments (calibration and orthorectification). We also used multiple TerraSAR-X and Sentinel-1 products to perform various treatments (deburst, calibration and orthorectification) with different parameters. This test helped us identify some variations with the previous version: the results are more accurate, but they take more computation time. We will describe these differences in the following sections.

Numerical gaps with the previous version

Optical Calibration

No difference is to be expected, except for Pleiade products. Indeed, we improved the calibration for the Pleiade products by changing the position of the pixel used to take the sun angles from top center to center. See this thread on the forum for more details. This leads to a numerical gap of the order of 10e-4 in absolute.

Optical Orthorectification

We studied the disparity between an orthorectification generated with OTB 7.4 and an orthorectification generated with OTB 8.0 with a Pleiades product. The analyses have shown a disparity of 0.01 pixel along the row and 0.02 pixel along the columns. These disparities are negligible compared to Pleiade’s localization precision.

SAR Deburst

Our analyses have shown that with OTB 8.0, the problem of line selection due to nanosecond error accumulation is solved. Thus, the lines that were miss selected by OSSIM are now correctly selected. This means that a couple of lines will be different with the new version of OTB. Except for those lines, no difference was assessed.

SAR Calibration

This application doesn’t produce numerical gaps. However, if the calibration is directly processed after deburst, the lines mis selected during the deburst are still different.

SAR Orthorectification

We studied the disparity between an orthorectification generated with OTB 7.4 and an orthorectification generated with OTB 8.0 with a Sentinel1 SLC product. The analyses have shown a disparity of 0.07 pixel along the row and 0.025 pixel along the columns. This is within the precision of the measurements.

Difference of computation time

Calibration

We noticed a gain of processing time (about 15% for optical calibration and about 66% for SAR calibration). This is due to a better use of the calibration LUT.

Orthorectification

There is a loss of processing time (about 40%) for this application. We are currently investigating the cause. We think it comes from the new time library.

Removed functionalities

DownloardSRTMTiles

The DownloadSRTMTiles application was not functional anymore because the official API used to retrieve the tiles is not usable without authentication anymore. After some discussions, the decision was taken to remove this application from OTB. The task performed by this application can easily be done by a simple python script, there is no need for an OTB application.

What is missing for the release of OTB 8.0?

The version OTB-8.0.0-alpha2 was released on November 8th, 2021. We need more testing to release the final version 8.0. Please, don’t hesitate to use OTB 8.0 alpha2, and tell us if you have some issues with the application you usually use.

In the meantime, we will work on the optimization of the computation time for Orthorectification. We will also run some more tests.

The release of OTB 8.0 will be largely discussed during the OTB User Days that will be held in Toulouse, France from November 29th to December 1st.