You can step through a stack one slice at a time, or you can animate it like a repeating movie loop. Lower numbers represent areas of less reflectance. This reflected energy is measured in Watts per square meter (W/m^2 - the amount of energy per square meter).
The range can be as little as 3% for water with light shining on it to as high as 95% for fresh snow cover. It is the percentage of solar (shortwave or ultraviolet) radiation reflected by a given surface on Earth. Albedo is derived from the Latin word "albus" for white. The images show how much sunlight Earth reflects (its albedo) during the course of a year. These images were downloaded from the NASA Earth Observations (NEO) site that you visited in Week 1. Stacks can be saved in several formats, including animated gif for web display and, with the appropriate software installed, QuickTime movie format. Also, processes such as thresholding apply to all of the slices of the stack. This guarantees that you are measuring the exact same part of the image in every slice. An advantage of stacking images to make measurements is that when you select an area to measure on one slice of the stack, that selection automatically applies to all slices in the stack. Stacking these images helped you to visualize changes in the lake over time and to make measurements. They represent data collected for the same region but at different times. The Lake Mead satellite images that you stacked and animated in Week 2 are an example of a time series data set. Using spectral data, you can use ImageJ to create both natural and false color views of a scene. Stacks of spatial data can be animated and measured, but you can also use ImageJ to construct entirely new views of features in the images. Stacking temporal images allows you to animate them to rapidly display them in sequence making changes over time easier to see and understand, and allowing you to precisely measure the same regions of the image over time. Stacks are used to display and analyze images that are related to each other in some way, such as by time (temporal), space (spatial), or color (spectral). The number and size of the images you can stack depend on the amount of memory in your computer. To stack a set of images, they must all be the same width, height, and bit depth. Many operations, such as selecting, filtering, thresholding, and contrast enhancement can be applied to all slices in a stack.
Stack windows have a scroll bar across the bottom to cycle through the slices, and you can animate the images at a speeds from one frame every 10 seconds to over 1000 frames per second. The images or layers that make up a stack are called slices. ImageJ can display two or more images in a single window, as a stack. Create a stack from a sequence of imagesĮxplore More if You Have Time: Create a Stack from a Montage.
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