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The Color Balance Tool

Use GeoExpress to perform color balancing operations for raster images when you select MG3, MG4, or JPEG 2000 as the output format. You can color balance 8-bit and 16-bit unsigned grayscale and RGB images. You can also color balance mosaics and multiple images at a time. There are three types of corrections that you can apply: uniform corrections, non-uniform corrections, and automatic corrections.

For information about color balancing and mosaics, see Performing Color Balancing for a Mosaic. For information about color balancing many images at a time, see Performing Batch Color Balancing.

Uniform Corrections

You can apply uniform corrections to single images and mosaics. Uniform corrections affect the brightness and contrast of all pixels in the same way, so light spots will still be light and dark spots will still be dark regardless of the corrections that you apply.

In the Uniform Corrections tab, select the bands for which you want to perform corrections and use the sliders to adjust gamma, brightness, and contrast values.

Non-Uniform Corrections

You can apply non-uniform corrections to mosaics only. Non-uniform corrections adjust the color values of specific portions of each mosaic tile. Use non-uniform corrections when each tile in a mosaic has similar flaws. For example, when one side of each tile is light and the other side is dark. When you combine multiple images with similar imperfections, the flaws become very noticeable. To correct these patterns across a mosaic, use non-uniform corrections.

The following list describes the three types of non-uniform corrections:

• Vignette correction. Corrects color inconsistencies caused by a camera lens. Use vignette correction when your images display a lower light intensity around their edges. The correction automatically normalizes the highlights and dark spots across tiles to create a mosaic that appears more unified. Vignette correction affects all the tiles in the mosaic.
• Tilt correction. Corrects bidirectional reflectance. Bidirectional reflectance describes the effect of the angle of the sun on an image. Use tilt correction when the images in a mosaic were all taken under similar lighting conditions. For example, your images may be darker on one side than the other. When you apply tilt correction to all the tiles in a mosaic, GeoExpress matches the color levels of each tile in the mosaic. Tilt correction affects all the selected tiles in a mosaic.
• Seam line correction. Corrects discrepancies in the edges of tiles in a mosaic. Use seam line correction to smooth the transitions between tiles. Depending on the geometry of a given mosaic, sometimes seam lines cannot be generated. In that case, the Apply Seam Lines option is unavailable. Seam line correction affects all the tiles in a mosaic.

NOTE: Tilt correction is computationally intensive. If there are many tiles in the mosaic, both the preview and the mosaic job may take more time to process.

Automatic Corrections

Automatic corrections adjust the color values of multiple images or mosaic tiles to match another image.

You can apply automatic corrections to single images and mosaics. If the corrections are going to be applied to multiple images, all the images must have the same number of output bands and the same data type. If you select Duplicate job parameters, this also applies to the image whose color corrections you want to match.

The following list describes the automatic correction methods that you can use:

• Match tiles to each other. Mosaics only. Sets the intensity of each tile in a mosaic to match the average of all tiles.
• Match to a selected tile. Mosaics only. Sets the intensity of the tiles that you select to match the color levels of a source tile that you specify. To specify a source tile, select a tile in the tile list and click Set Selected Tile As Source Tile in the Automatic Corrections panel. Then, select the tiles in the tile list to which you want to apply the corrections and click Apply Corrections.
• Duplicate job parameters. Matches the color balancing corrections applied to another image in the Job List. If you have not applied color balancing corrections to another job yet, GeoExpress displays a blank pane. Use this option to quickly apply color balancing corrections to many images at a time.
• Match to histogram. Matches the histogram values of another image that you select. GeoExpress calculates histogram values for the image that you select, including brightness, contrast, gamma, and more, then GeoExpress attempts to apply those values to the current image.