====== Orbit Multiresolution Image ======

An Orbit Multiresolution Image (*.omi) is Orbit's native multiresolution image and raster file storage format. \\
The OMI has been developed for encoding and fast viewing of raster and image resource any size on any platform. 

===== Reasons to use =====

  * fast viewing of large datasets (+100 Gb)
  * support for black/white, grey-scale and color images
  * platform independent and can be shared among Windows, Linux and Mac platforms. \\ Unlike other image compression formats : e.g. Mr SID ([[http://en.wikipedia.org/wiki/MrSID|Wikipedia Mister Sid]]) can only be used on Windows

===== How to Create =====

An OMI can be created via the [[111:orbitgis:tools:index|Map 2D Additional tools]] : 
  * [[111:orbitgis:tools:additional:convert_image|]] : Convert one on one, one Image to one OMI (or other image storage file format)
  * [[111:orbitgis:tools:additional:multiresolution_image|]] : Convert multiple Images to one OMI

===== Options and Compression =====

When creating an OMI, flowing options are available to define storage and compression.

{{  :orbitgis:tools:additional:convert_omi_1_versie11.png?nolink|}} 

**Pixelsize** \\
Define the number of bits (1, 8, 24 or 32) to be used to store one pixel, [[111:technologies:resources:index|read more...]]

**Pixelsize 1** \\

A pixel can hold one sample with 2 possible values 0 (black) or 1 (white).

When using pixelsize 1, it will be possible to set a threshold : Light, Medium or Dark. \\
Applying a threshold is a method to simplify an image to sets of pixels with corresponding values. The result is an image containing contiguous regions of the same value. This technique makes it easier to apply analysis.
  * Light will favor light color values.
  * Dark will aggregate dark color values.

**Pixelsize 24** \\

{{  :orbitgis:tools:additional:convert_omi_24_versie11.png?nolink|}}

A pixel can hold 3 samples with a value between 0 .. 255. \\
Using pixelsize 24 enables extra quality parameters. :
  * Lossless “fully recoverable”, no compression
  * Lossy “with losses to quality”
    * Smallest file
    * Standard
    * Quality

Lossy meaning "with losses to quality". Approximate sample values will be stored with sample value compression rate 5:1 up to 20:1 \\
Examples  .jpg, .ecw, .sid 

Lossless always returns the original data. Lossless meaning "fully recoverable". The exact sample values will be stored. \\ 
Examples : .gif, .png, .bmp

It is advised to save images with a wide variety of colors (photos) with lossy compression (jpg). \\ 
Images with a limited variety (computer graphics, icons) should be saved lossless png).

**Pixelsize 8 and 32** have no extra format options.


An OMI stores any image in an intelligent [[111:technologies:resources:image:multiresolution|multi-resolution]] pyramid structure.

When creating an OMI one need to choose the type of compression Lossless or Lossy (Smallest File, Standard or Quality). \\
A Lossy compression will use JPG compression techniques to store the multi-resolution images.

===== Legend and Georeference =====

An OMI file can be accompanied by a, a plain text file used by Orbit to georeference the image. \\
Additionally the OMI can be associated with an [[111:technologies:basic_concepts:ord|ORD]] to define the [[111:technologies:crs:crs_orbit|coordinate reference]] of the image.

The OMI can be accompanied by 
  * an [[111:technologies:resources:other:olg|Orbit Legend file (*.olg)]] to define the legend or presentation of the resource in Orbit.
  * an [[111:technologies:resources:other:ord|Orbit Resource Description file (*.ord)]] or a [[111:technologies:resources:other:prj|projection file (*.prj)]] to define the coordinate reference system of the resource in Orbit.
  * an [[111:technologies:resources:other:oiw|Orbit Image World file (*.oiw)]] to define the spatial position and rotation within the given reference system.