Cropping an Image

GeoExpress can encode a defined (cropped) region from any image whether it is a single image, an image mosaic, or an individual image inside a mosaic. With the proper switches set you can target an area to crop on encode by specifying geographic coordinate pairs; by specifying a center geographic coordinate and width and height; or by specifying the same options using image pixel values. You can combine image cropping with any encoding function such as updating images, creating mosaics, optimizing using higher compression, resampling, etc. Cropping via command line also means that you can batch process crop functions for a highly automated image production system.

This feature allows you to encode a subsection of an image without first having to export the subsection to a separate file. It also is useful for removing transparency areas of an image before encoding. By cropping away the edges containing transparency, the MrSID image, in some instances, can be made without encoding any background color into the output image.

NOTE: The default for DOQ images is to automatically crop to Primary Corner Crosses to achieve a seamless mosaic.

When an image is cropped, the following attributes of the image are changed:

• width of the output MrSID image = input width (left pixel offset + right pixel offset)
• height of the output MrSID image = input height (top pixel offset + bottom pixel offset)
• file size = (input file size * width of the output MrSID image * height of the output MrSID image) / (input height * input width)

Using a Text File for Cropping

To crop multiple images, you may want to use a text file to list files and cropping values. The text file must have a .mos extension.

For example, if you want to crop three images to remove 10 pixels from the top, 20 pixels from the left, 30 pixels from the bottom, and 40 pixels from the right, you might write the following text file:

foo1.tif 10 20 30 40
foo2.tif 10 20 30 40
foo3.tif 10 20 30 40

To run the cropping job, use the following syntax:

mrsidgeoencoder -i mycrop.mos -tiff -cmos -o mycropped.sid

Like other text file lists supported by GeoExpress, similar rules apply to cropping:

• All four cropping values must exist for each file, even if no cropping is desired (e.g. 0 0 0 0).
• Only one file per line.
• Cropping values must be in top, left, bottom, right order.
• Cropping values must be positive integers.

Cropping by Vector Overlay

To crop an image with a vector overlay file, use the -cropvectoroverlay/-cropvo and -cropvectorlayer/-cropvol switches.

The -cropvectoroverlay switch specifies the name of the vector overlay file. The -cropvectorlayer switch specifies which layer to use. Layers are zero-based (0 is the first layer, 1 is the second, etc.). If a layer isn't specified, layer 0 is used by default. For example:

 mrsidgeoencoder -i foo.tif -o foo.sid -cropvo overlay.xml -cropvol 0

When the image is encoded, the output file will have the dimensions of the smallest rectangle that contains the shape, and the areas outside of the shape will be filled in with black.

NOTE: Vector overlays with multiple polygons are not supported. If you have a vector overlay that has multiple polygons you can use your favorite GIS package to extract a single polygon to a new vector overlay and then import the new vector overlay into GeoExpress.

Cropping Switches

The cropping switches are listed in the following table.