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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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/
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.