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Stretch Window

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Sam Gillingham edited this page Nov 13, 2019 · 1 revision

Stretch Window

This window controls how the image is displayed. This window can be dragged off the main window to be a separate window. Double click title bar to re-dock. There are a number of controls on how this happens:

Mode Combo Box

There are 4 basic ways in which a image is displayed:

  • RGB (for 3 or more bands - a separate band is chosen for Red, Green and Blue)
  • Color Table (a color table saved in the file is used to color a single band)
  • Greyscale (a single band, the range of values is shown between black and white)
  • PseudoColor (a single band, the range of values is shown in a predefined colour ramp)

Pseudo Color Combo Box

This is to the right of the Mode Combo Box. In Pseudo Color mode this is enabled, and allows the user to choose which color ramp is used to display the data. It is disabled for all other modes. The default colour ramps are taken from Colorbrewer: Color Advice for Maps. Additional color ramps can be loaded using the instructions under: [Using customised colour ramps](Using customised colour ramps).

Bands Combo Box

For the RGB mode all 3 sub - combo boxes are enabled. These are for choosing the 3 bands to colour Red, Green and Blue respectively. For all other modes only the first combo box is enabled - choose the single band to use here.

Stretch Combo Box

This is disabled in Color Table mode. In the other modes this allows you to select what sort of stretch is applied to the data before it is displayed.

  • None (no stretch is applied to the data - usually for data that has already been stretched)
  • Linear (a linear stretch is applied between in min and max values specified in the fields to the right. One or both of these can be from the existing image statistics - 'Statistics Min' and 'Statistics Max')
  • Standard Deviation (a standard deviation stretch is applied. The field to the right allows you to specify how many standard deviations from the mean is used. The mean and standard deviation is collected from the image statistics)
  • Histogram (Histogram stretch - specify the min/max proportion of the histogram used for display in the fields to the right. The histogram data is collected from the image statistics).

Fixed colors

There are 3 so-called 'fixed colors' that can be specified by clicking on the corresponding colour button:

  • No Data. This is the colour that pixel values that are match the GDAL 'no data/ignore' value (if defined) are displayed in.
  • Background. The colour that areas beyond the extent of the image data are shown as.
  • NaN. The colour that NaN values (floating point images only) are displayed in.

Note that the default Alpha value for all of these is zero so images 'underneath' will show through for these values.

Tool Bar

When displayed from the [Main Window](Main Window) or the [Layer Window](Layer Window) there are extra buttons that appear at the top of this window that allow you to interact with the stretch from the current image. These are not available from the [Default Stretch](Default Stretch) window.

Also, when displayed from the Main Window, this sub window is dockable and can be un-docked from the window by dragging.

  • Apply Stretch. The currently defined stretch is applied to the image in the current window and the display updated. Click this when you have finished making changes to the stretch options.
  • Apply Stretch to All. The currently defined stretch is applied all the images open in the current window and the display updated. Click this when you have finished making changes to the stretch options. Note that all images should match in their number of bands etc.
  • Local Stretch. When selecting linear, standard deviation, or histogram stretch; statistics are gathered from the area of the image visible in the viewer window rather than using the whole of image statistics.
  • Save Stretch and Lookup Table. Saves the currently selected stretch to the image file. This stretch is then used when the image is subsequently opened again. This stretch will override the rules defined in the [Default Stretch](Default Stretch) window.
  • Delete stretch. Deletes the saved stretch (if any) from the current image. Next time the image is opened the rules in the [Default Stretch](Default Stretch) window are used.
  • Save stretch and lookup table. This will prompt you for a .stretch file which will then have the stretch and lookup table written into it in JSON format.
  • Import stretch and lookup table from GDAL dataset. If you have a file where the stretch and lookup table has been saved into it you can select it here and the stretch and lookup table from this file will be used to display the current file. Note: the changes are applied immediately. You may want to use this to present a consistent stretch across multiple files.
  • Import stretch and lookup table from a text file. If you have saved a .stretch file previously you can select it here and the stretch and lookup table from this file will be used to display the current file. Note: the changes are applied immediately. You may want to use this to present a consistent stretch across multiple files. Also, you may have changed the contents of the .stretch file to reflect a specialised stretch and this is a way of applying it to an image file.
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