Undersample

Undersample Operator

This operator downsamples a real or complex image using sub-sampling method or lowpass filtering method.

Undersampling Method

• Sub-sampling method: The image is downsampled with user specified sub-sampling rates in both range and azimuth directions. Here sub-sampling rates is a positive integer which is the step size in reading row/column pixels from product file. For complex image, the i and q bands in the image are downsampled separately, and the downsampled image is still a complex image.
• Lowpass-filtering method: The image is downsampled with a pre-defined low-pass kernel moving across the image with a step-size determined by the size of the required output image. For complex image, intensity image is computed from the i and q bands before lowpass filtering is applied. The downsampled image is always a real image.

Undersampled Image Size

User can determine the output image size by specifying

• the output image size, or
• the pixel spacings, or
• the downsampling ratios.

Low-Pass Kernel

• The pre-defined low-pass kernel has three dimensions: 3x3, 5x5 and 7x7.
• The elements of the low-pass kernel are all 1's.

Parameters Used

If the sub-sampling method is selected for the downsampling, then the following parameters are used by the operator:

1. Source Band: All bands (real or virtual) of the source product. User can select one or more bands for producing downsampled images. If no bands are selected, then by default all bands are selected.
2. Under-Sampling Method: Sub-Sampling method.
3. Sub-Sampling in X: User provided sub-sampling rate in range.
4. Sub-Sampling in Y: User provided sub-sampling rate in azimuth.

If the Lowpass Filtering method is selected for the downsampling, and the downsampled image is output by image size, then the following parameters are used by the operator:

1. Source Band: All bands (real or virtual) of the source product. User can select one or more bands for producing downsampled images. If no bands are selected, then by default all bands are selected.
2. Under-Sampling Method: Kernel Filtering method.
3. Filter Size: The lowpass filter size.
4. Output Image By: The method for determining output image dimension.
5. Output Image Rows: The row size of the downsampled image.
6. Output Image Columns: The column size of the downsampled image.

If the Lowpass Filtering method is selected for the downsampling, and the downsampled image is output by image dimension ratio, then the following parameters are used by the operator:

1. Source Band: All bands (real or virtual) of the source product. User can select one or more bands for producing downsampled images. If no bands are selected, then by default all bands are selected.
2. Under-Sampling Method: Kernel Filtering method.
3. Filter Size: The lowpass filter size.
4. Output Image By: The method for determining output image dimension.
5. Width Ratio: The ratio of the downsampled image width and the source image width.
6. Height Ratio: The ratio of the downsampled image height and the source image height.

If the Lowpass Filtering method is selected for the downsampling, and the downsampled image is output by pixel spacing, then the following parameters are used by the operator:

1. Source Band: All bands (real or virtual) of the source product. User can select one or more bands for producing downsampled images. If no bands are selected, then by default all bands are selected.
2. Under-Sampling Method: Lowpass Filtering method.
3. Filter Size: The lowpass filter size.
4. Output Image By: The method for determining output image dimension.
5. Range Spacing: The range pixel spacing of the downsampled image in meters.
6. Azimuth Spacing: The azimuth pixel spacing of the downsampled image in meters.