Digital Image Registration and Comparison
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This page has been written as part of the NIP2 Beginners Guide.
Image registration is the process of ensuring that the matching features of two similar images appear in the same place in each image. If different images have been captured of a single object, at different stages of it's life, for example before, during or after conservation work or with different pieces of equipment, for example visible, infra-red, uv or x-ray, then they may need to be registered to each other before direct digital comparisons are carried out.
Image comparison is the process of checking whether the matching features of two similar images appear in the same place in each image. Comparisons can be used to check the result of the registration procedures or just to check if two similar images match, this can very useful when comparing different versions of the same painting, or even checking whether a drawing or cartoon has been used in the construction of a painting.
There are two basic registration function included in nip2, Linear Match and Perspective Match. Two images need to be selected and then the chosen function can be performed, the second image will be adjusted to match the first based on the positioning of a set of marks.
Figure 8.1: Selection of sample images used to show the results of the image overlay and image registration functions. Image A is the base image that the others starting images, B, C and D want to be resistered to. Images AA, AB, AC and AD are the result of running the overlay function with image A and all of the starting images. Images B2 and C2 show the results of linear matching images B and C with image A and image D2 show the results of perspective matching images C with image A. Images AB2, AC2 and AD2 are the result of running the overlay function with image A and the three matches images.
- [Main Window][Toolkits: Image: Transform: Linear Match]
This functions can be used to correct size, translation and rotation differences between two images. Two marks are placed on each of the original images, which can be moved to indicate the position of two good matching features in the images. In addition to the marks options are provided to lock the image to prevent resizing[#note83 1] and to allow the software to try to improve the position of your marks[#note84 2].
- [Main Window][Toolkits: Image: Transform: Perspective Match]
This functions can be used to correct size, translation, rotation and perspective distortion differences between two images. Four marks are placed on each of the original images, which can be moved to indicate the position of four good matching features in the images.
There are three basic comparison function included in nip2, Overlay, Slider Blend and Along Line, see figure [#fg:reg_comp 8.2]fg:reg_comp. Two images need to be selected and then the chosen function can be performed for the comparison to be performed.
Figure 8.2: Examples of the nip2 objects produced by the three basic comparison functions, Overlay, Slider Blend and Along Line.
- [Main Window][Toolkits: Filter: Overlay]
This function produces a mono version of the original object and then uses them along with a black image to produce a RGB[#note85 3] colour image. By default the first image is used to create the red chanel, the second image is used to create the green chanel and the black is used for the blue channel, this is know as Green over Red. Here the good areas of overlap will appear yellow, where the green and red images match, see figure [#fg:reg_regandcomp 8.1]fg:reg_regandcomp and figure [#fg:reg_comp 8.2]fg:reg_comp. The function object does provide the option to change this image order to produce different colours if it is required.
- [Main Window][Toolkits: Filter: Blend: Slider Blend]
This function effectively places the second image on top of the first and then allows the user to adjust the transparency of the second image by moving a slider back and forward.
- [Main Window][Toolkits: Filter: Blend: Along Line]
This function produces an new image by blending then together down a line. Options are provided for the user to decided if the images are to be blended left-right or top-bottom and to control the position and degree of the blend.
<ref name=nbg_reg_1>This can be good if you are wanting to line two images up but not actually change their size.</ref> <ref name=nbg_reg_2>This process does not always improve your match but can be helpful at times.</ref> <ref name=nbg_reg_3>see section [#sec:colour 3.3.3]sec:colour</ref>