Background Images

Image files can be imported and used as background images in edit windows and image layers in Map Projects.


Image Files

Images can be imported from JPG, PNG and BMP image files. Image files do not contain information about their location and so need to be calibrated within ReefMaster.


Google Earth Overlays

Google Earth ground overlays are image files that have been calibrated as backgrounds within the Google Earth application. Google Earth overlays are stored as either a KML file with an associated image file, or as a single KMZ file. When images are imported via ground overlays, they do not need to be calibrated and cannot be re-calibrated within ReefMaster.


Google Earth overlays are converted into the MBTiles tiled image format as they are imported. Tiling involves splitting the image into many small pieces, that are reconstructed for viewing as required. The advantage to using a tiled image format is that, when displaying images, ReefMaster only needs to hold the visible portion of the image in memory, which provides much better performance and lower memory usage.


Google Earth Overlays are the recommended source of background images and image overlays.


Importing Background Images

To import a new background image, either use the New button in the Images header of the Asset Library or select the Import Image option from the File/New menu.


A file selector window is shown, where a single image file can be selected. Supported file-types are image files of type JPG, PNG or BMP, Google Earth files (containing ground overlays) of type KML or KMZ or files in the MBTiles format. Select the required file, and click Open.


Google Earth Files

If the selected file is a Google Earth KML or KMZ file, then the image is imported and tiled. Tiling can be a time-consuming process, so a progress window is shown whilst this is carried out. Once the tiling is complete, the progress window is closed and the image will be shown in the graphical edit windows in the correct location.


Image Tiling

Images are tiled by being split into a number of small tiles, 256 pixels square, that are then loaded and displayed as required. Image tiles for each imported image are kept in a single tile database file in the MBTiles format. Once an image has been imported once, and an MBTiles files has been created, it is much quicker to import the image into other ReefMaster workspaces using this file. ReefMaster can use the MBTiles file directly and does not have to re-process the image. The MBTiles files for imported images are stored in the same directory as the current workspace.


Calibrating Image Files

When a raw image file is imported, it is given an starting extent centred within the current workspace. The image needs to be calibrated so that it shows in the correct size and in the correct location. Images can be calibrated in two ways; Drag to Calibrate or Point Calibration.


Drag to Calibrate

Images can be graphically calibrated when in Drag to Calibrate mode.



Select the menu item Drag to Calibrate (1) from the context menu of the background image, activated by right-clicking on the image icon at the top left of the image. The image will enter drag to calibrate mode, with grab handles in the centre and each edge and corner.


The square grab handle in the centre of the image is used to drag the entire image.

The handles in the corners stretch an image in two directions.

The handles on the edges of the image stretch the image in a single direction.


Grid lines can assist with calibration, as can background maps; vary the opacity of the imported image using the opacity slider (2) in the context menu, so that items behind can be seen more clearly.


Point Calibration

Images can also be calibrated by entering the precise locations of two or more points within the image. Point calibration must be carried out in the Background Image Edit Window.



Note that images are always assumed to be north facing; ReefMaster does not rotate images, and images are always scaled uniformly. ReefMaster takes the average of all calibration points that are entered.


Select the Calibration mouse mode (circled, only available in the background image edit window).

Click with the left mouse button on the calibration location, and enter the location latitude and longitude into the coordinate edit box.

Once two or more calibration points have been entered, the image will be moved and sized accordingly.

To calibrate using just two points, the points should share neither latitude nor longitude (i.e., they should be on different horizontal and vertical locations within the image).

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