i want to add a geotiff image as base layer on openlayers :
- copy my image.tiff in a directory and open it with openlayers directly without using a cartography server.
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Unfortunately you cannot do this.
You can only display those images using OpenLayers which are spported by browsers. That is why using
OpenLayers.Layer.Image you can only display JPEG, PNG & GIF images.
Other Image formats like Erdas Imagine, Tiff, GeoTiff ASCII etc need to be converted to a format which browsers understand, before they can be shown in the browser.
In fact you can turn the GeoTIFF files into an OpenLayers base map with the help of MapTiler software (http://www.maptiler.com/). Once the GeoTIFF file is processed with the software on your desktop - you get a directory with JPEG/PNG tiles together with a sample viewer made using OpenLayers.
Such a viewer and the folder with tiles can be uploaded to any web hosting - so there is no need to install any cartography server...
MapTiler either creates the Standard Mercator tiles - so your GeoTIFF can easily overlay Google Maps or OpenStreetMap, or it can keep the image in the original map coordinates.
If your file is a proper GeoTIFF with the geographical location and coordinate system definition embedded, then the software will load it automatically after you drag&drop it in - as in this video: https://www.youtube.com/watch?v=-rSYFwcWVnc&list=PLGHe6Moaz52PiQd1mO-S9QrCjqSn1v-ay&index=12
If you have a World file, but the coordinate system information is missing you can specify it easily: http://www.maptiler.com/how-to/coordinate-systems/
To turn an ordinary scanned maps without any location info into a map overlay see this video: http://www.maptiler.com/how-to/overlay-image-over-map/