When you decide to mosaic images together, you probably want to use Image Dodging, Illumination Equalization, Color Balancing, or Histogram Matching to give the finished mosaicked image a smoother look devoid of bright patches or shadowy areas that can appear on images. Many of the color differentiations are caused by camera angle or cloud cover. Before applying any of those features, you can use Exclude Areas feature to mark any types of areas you do not want to be taken into account during a Color Balancing, Image Dodging, or Histogram Matching process. Areas like dark water or bright urban areas can be excluded so as not to throw off the process.
Exclude Areas function works on the principal of defining an AOI (area of interest) in a particular image, and excluding that area if you wish. The feature makes it very easy to pinpoint and draw a polygon around specific areas by featuring two viewers, one with a shot of the entire image, and one zoomed to the AOI you have selected with Geographic Link cursor.
If you right-click your mouse while your cursor is in the viewer, you notice several options offered to help you better view your images by fitting the image to the viewer window, changing the Link cursor color, zooming in or out, rotating the image, changing band combinations, and so on.
At the bottom of Set Exclude Areas dialog, there is a toolbar containing options for creating a polygon for your AOI, using Region Growing tool for your AOI, selecting multiple AOIs, displaying AOI styles, and finding and removing similar areas to your chosen AOI.
Use Image Dodging to radiometrically balance images before you mosaic them. Image Dodging uses an algorithm to correct radiometric irregularities (such as hotspots and vignetting) in an image or group of images. Image Dodging and Color Balancing are similar; Color Balancing applies the correction to an image by modeling the shape of the problem (plane, conic, and so forth) while Image Dodging uses grids to localize the problem areas within an image.
Image Dodging corrects brightness and contrast imbalances due to several image inconsistencies.
- Dark corners caused by lens vignetting
- Different film types in the group of images
- Different scanners used to collect the images (or different scanner settings)
- Varying sun positions among the images
- Hot spots caused by the sun position in relation to the imaging lens
The following tips show when to use the various parameters to correct specific inconsistencies.
Adjust color images only for brightness and contrast
Deselect the Band Independent parameter (Image Dodging dialog). This way you can control the brightness and contrast across all color bands while preserving individual color ratios of red to green to blue.
Enhance Shadowed Areas
To pull detail out of shadow areas or low-contrast areas, set Grid Size (Image Dodging dialog) to a larger number so that the process uses many small grids. You can also set Max. Contrast parameter (Set Dodging Correction Parameters dialog) to a higher number such as 6. Another method is to set the Max. Grey Shift (Set Dodging Correction Parameters dialog) to a larger number such as 50 (default = 35).
Reduce contrast or increase contrast
If your dodged images have too much contrast or not enough contrast try the following.
- Change Max. Grey Shift (Set Dodging Correction Parameters dialog). Start at 50 and reduce slightly if you see clipping effects at black or white.
- Change Max Contrast (Set Dodging Correction Parameters dialog). You may see a major change when you change the Max Grey Shift, and if you need to make a more subtle change, decrease the contrast by 0.1 to 0.3.
Balance groups of images with different inconsistencies
If you have a large mosaic project and the images have different inconsistencies, then group the images with the same inconsistency and dodge that group with the parameters specific to that inconsistency. For example, if you have images from multiple flight lines that have hot spots in different areas and/or of different intensity, run dodging on the images that have similar hotspots.
If you have images with dominant water bodies and coastline, use a Grid Size (Image Dodging dialog) of 6 or 8. This smaller grid size works well because of the difference between bright land and dark water. This will reduce vignetting and hotspots without adversely affecting the appearance of the water. Also uncheck Band Independent (Image Dodging dialog) since a large area of dark water can corrupt the color interpretation of the bright land area.
Image Dodging dialog contains options for Current Image, Options for All Images, Display Setting, a viewer area displaying the image, and preview of the dodged image. If you want to skip dodging for a certain image, you can skip to the next image you want to mosaic.
In Statistics Collection, you can change the Grid Size, Skip Factor X, and Skip Factor Y. You can apply changes to a specific number of images.
In Options For All Images, choose whether the image should be dodged by each band or as one. Then decide if dodging is performed across all of the images you intend to mosaic or just one image. This is helpful if you have a set of images that all look smooth except for one that may show a shadow or bright spot. If you click Edit Correction Settings, then compute settings first. After settings are computed, Set Dodging Correction Parameters dialog opens. In this dialog, change and reset the brightness and contrast and constraints of the image.
Use Display Setting to choose either a RGB image or a Single Band image. If using an RGB image, you can change those bands to whatever combination you wish. After you compute the settings a final time, preview the dodged image in the dialog viewer so you know if you need to do anything further to it before mosaicking.
Use Mosaic Color Balancing to balance any color disparities in your images before mosaicking them together into one large image. When you choose to use Color Balancing in Color Corrections dialog, you can color balance your images automatically or manually.
For more control over how the images are color balanced, choose the manual color balancing option. In manual mode, use the Mosaic Color Balancing tool to choose different surface methods, display options, and surface settings for color balancing your images.
When choosing a surface method you should concentrate on how the light abnormality in your image is dispersed. Depending on the shape of the bright or shadowed area you want to correct, you choose one of the following:
- Parabolic - Color difference is elliptical and does not darken at an equal rate on all sides.
- Conic - Color difference will peak in brightness in the center and darken at an equal rate on all sides.
- Linear - Color difference is graduated across the image.
- Exponential - Color difference is very bright in the center and slowly, but not always evenly, darkens on all sides.
It may be necessary to experiment when trying to decide what surface method to use. It can sometimes be particularly difficult to tell the difference right away between parabolic, conic, and exponential.
Conic is usually best for hot spots found in aerial photography although linear may be necessary in those situations due to the correction of flight line variations.
Linear method is also useful for images with a large fall off in illumination along the look direction, especially with SAR images, and also with off-nadir viewing sensors.
When you select the Common center for all layers option, all layers in the current image have their center points set to that of the current layer. Whenever the selector is moved, the text box updated, or Reset button clicked, all of the layers are updated. If you move the center point, and you wish to bring it back to the middle of the image, you can click Reset Center Point in the Surface Method area.
Use Display Setting area of Mosaic Color Balancing tool to choose between RGB images and Single Band images. You can also alter which layer in an RGB image is the red, green, or blue.
When you choose a Surface Method, the Surface Settings become the parameters used in that method’s formula. The parameters define the surface, and the surface is then used to flatten the brightness variation throughout the image. You can change the following Surface Settings:
- Center X
- Center Y
- Axis Ratio
As you change the settings, you can see the Image Profile graph change. You can preview the color balanced image also. This is helpful because you can change any disparities that still exist in the image.
Histogram Matching is used in other facets of IMAGINE, but it is particularly useful to the mosaicking process. Use the Histogram Matching option to match data of the same or adjacent scenes that was captured on different days, or data that is slightly different because of sun or atmospheric effects.
By choosing Histogram Matching through the Color Corrections dialog in Mosaic Pro, you can choose the Matching Method, the Histogram Type, and whether or not to use an external reference file. For Matching Method, decide if you want your images to be matched according to all the other images you want to mosaic or just matched to the overlapping areas between the images. For Histogram Type you can choose to match images band by band or by the intensity (RGB) of the images.
If you check Use external reference, you can use an image file or parameters as your Histogram Source. If you have an image that contains the characteristics you would like to see in the image you are processing through Histogram Matching, then you should use it.