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### 5.4 Image

This menu groups operations which apply only to images.

New
  Makes a new image. Region on Image makes a new region, arrow, guide or mark on an image. It’s usually easier to open a viewer on an image and Ctrl-drag.
Convert to Image
  Try to make an image out of anything.
Format
  Switch between the various precisions.
Header
  Try to change or examine the image header in various ways.
Cache
  This caches an image in RAM. Use this to save the results of a long computation.
Levels
  Various tools that change the levels in an image. Tone Curve is the only complex one: it lets you adjust the image levels with a set of sliders.
Transform
  Various tools that change the geometry of an image.

  To use Rotate / Straighten, mark an arrow on an image (Ctrl-drag up and left in an image view window) along a near-horizontal or near-vertical edge. When you click on Rotate / Straighten, nip2 will rotate the image by the smallest amount that makes that edge exactly horizontal or vertical.

  Linear Match takes two images and rotates and scales the second so that the images can be superimposed. Drag the tie-=points to mark common features. Use Filter / Overlay or Filter / Colourize to actually superimpose them.

  Rubber Sheet is useful for fixing things like lens distortion. You give Find two images, a reference and a distorted version of that reference, and it automatically finds a transform which will map the distorted image back on to the reference image. Use Apply to apply the discovered transform to another image.

Band
  Extract/insert/delete image bands. Use To Dimension to change image bands into a horizontal or vertical dimension. Use To Bands to compress the horizontal or vertical dimension into bands (small images only!).
Crop
  Crops an image. It’s often easier to drag out a region. This menu item is only really useful for cropping large groups of images.
Insert
  This takes two images and pastes the smaller into the centre of the larger. The two images have to have the same number of bands. If you open an image viewer on the large image, you’ll see an area which you can drag around to set the exact insert point.
Select
  Draw elipses and polygons on an image. Useful for selecting defined areas.
Join
  Use to join two images together bandwise, left/right or up/down. Array joins a list of lists of images together into a single large image.
Tile
  Repeat an image horizontally and vertically to make a larger image, or chop an image into a set of tiles.
Patterns
  These items all make useful images for you, from checkerboards to gaussian masks. XY Image is the most useful: you can use it to build other patterns.
Test Images
  These items make a variety of useful testcharts for evaluating spatial response and colour.

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