How Earth Blox determines contrast settings

Modified on Thu, 11 Jan 2024 at 03:44 PM

Automatically determining contrast values can be very computationally expensive, and therefore slow. Earth Blox uses two methods for doing this, designed to provide a high level of accuracy, whilst not significantly slowing the workflow run.

Contrast settings for image collections (time series)

Contrast settings for a collection of images in Earth Blox are calculated via a random sample of 1000 pixels per image. This has two potential consequences:

  1. If a workflow is re-run, a different sample would be taken, and the contrast values may change slightly
  2. The contrast values are not guaranteed to be the universal values for an image as they are based on sampling. So an image may have pixel values ranging from -1.00 to +1.00, but selecting the "Dataset Min and Max" option might scale the data as -0.98 to +0.99. If you know the min and max pixel values (such as with an NDVI), then these can be set via the 'Custom' option.

Contrast settings for a single image

For a single image, a different approach is taken to calculate contrast values. Earth Blox will use all pixels in the image, at the highest resolution possible that keeps the total number of pixels below 10,000.

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